NEW INVADERS OF THE SOUTHWEST

Rachel Winston, Wendy DesCamp, Jennifer Andreas, 

Carol Bell Randall, Joseph Milan, and Mark Schwarzländer

United States 

Department of Agriculture

TECHNOLOGY

TRANSFER

Forest Health Technology

Enterprise Team

University of Idaho

Extension

FHTET-2014-13

December 2014
The Forest Health Technology Enterprise Team (FHTET) was created in 

1995 by the Deputy Chief for State and Private Forestry, USDA, Forest 

Service, to develop and deliver technologies to protect and improve the health of 

American forests. This book was published by FHTET as part of the technology 

transfer series. 

http://www.fs.fed.us/foresthealth/technology/

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and 

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orientation, or marital or family status. (Not all prohibited bases apply to all programs.) Persons with 

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print, audiotape, etc.) should contact USDA’s TARGET Center at 202-720-2600 (voice and TDD).

To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326-W, 

Whitten Building, 1400 Independence Avenue, SW, Washington, D.C. 20250-9410, or call 202-

720-5964 (voice and TDD). USDA is an equal opportunity provider and employer.

The use of trade, firm, or corporation names in this publication is for the information and convenience 

of the reader. Such use does not constitute an official endorsement or approval by the U.S. Department 

of Agriculture or the Forest Service of any product or service to the exclusion of others that may be 

suitable.

Federal Recycling Program

Printed on Recycled Paper

Cover photos (clockwise from top left): Bermuda buttercup (MathKnight), Mexican fan palm (Forest 

& Kim Starr), carob tree (Riu Chixoy), red sesbania (Chris Evans, Illinois Wildlife Action Plan, www.

bugwood.org), sea fig (Joaquim Alves Gaspar)
NEW INVADERS OF THE SOUTHWEST

Rachel L. Winston

Environmental Consultant

MIA Consulting, LLC

Sandpoint, ID

rachel@getmia.net

Wendy DesCamp

Education Specialist

Washington State Noxious Weed Control Board

Olympia, WA

wdescamp@agr.wa.gov

www.nwcb.wa.gov/

Jennifer E. Andreas

Integrated Weed Control Project

Washington State University Extension

Puyallup, WA

jandreas@wsu.edu

www.invasives.wsu.edu

Carol Bell Randall

Entomologist, Forest Health Protection

USDA Forest Service

Coeur d’Alene, ID

crandall@fs.fed.us

Joseph Milan

Biological Control Specialist

Bureau of Land Management

Boise, ID

jmilan@blm.gov

Mark Schwarzländer

Associate Professor for Entomology

University of Idaho, PSES Despartment

Moscow, ID

markschw@uidaho.edu

For additional copies of this publication, contact: 

Mark Schwarzländer

University of Idaho

PSES Department

Moscow, ID 83844

208.885.9319

markschw@uidaho.edu

Richard Reardon

USDA Forest Service

180 Canfield Street

Morgantown, WV 26505

304.285.1566

rreardon@fs.fed.us

Acknowledgments

Numerous state and regional weed experts and land managers contributed their time and 

expertise to create the species list utilized in this guide. The distribution maps were compiled 

by Michelle Lewis. The layout was designed by Rachel Winston, with assistance from Wendy 

W. Harding. We would like to thank all of the photographers who granted permission for the 

use of photos. We also extend our gratitude to Richard Reardon (Forest Service-Forest Health 

Technology Enterprise Team or FHTET) for producing this guide.
Table of Contents 2 New Invaders of the Southwest

About This Field Guide 4

Green Flowers 8

Arabian schismus, Schismus arabicus 

Blue panicgrass, Panicum antidotale

Buffelgrass, Pennisetum ciliare

Carnation spurge, Euphorbia terracina

Carob tree, Ceratonia siliqua

Devil’s thorn, Emex spinosa

Edible fig, Ficus carica

Evergreen ash, Fraxinus uhdei

Fountain grass, Pennisetum setaceum

Jubata grass, Cortaderia jubata

Long-flowered veldtgrass, Ehrharta longiflora

Mediterraneangrass, Schismus barbatus

Pampas grass, Cortaderia selloana

Panic veldtgrass, Ehrharta erecta

Perennial veldtgrass, Ehrharta calycina

Puna grass, Amelichloa brachychaeta

Ravenna grass, Saccharum ravennae

Tree spurge, Euphorbia dendroides

8

10

12

24

30

28

32

34

12

14

208

14

20

20

16

18

26

Pinkish-Purple Flowers 36 

Camelthorn, Alhagi maurorum

Highway iceplant, Carpobrotus edulis

Iberian starthistle, Centaurea iberica 

Purple starthistle, Centaurea calcitrapa

Russian salt tree, Halimodendron halodendron

42

40

36

36

44

Sea fig, Carpobrotus chilensis

Three-lobed morning glory, Ipomoea triloba

40

38

Red Flowers 46 

Red sesbania, Sesbania punicea 46

White Flowers 48 

African rue, Peganum harmala

Brazilian peppertree, Schinus terebinthifolius

58

66
New Invaders of the Southwest 3

Bridal veil broom, Retama monosperma

Canary Island knapweed, Volutaria canariensis

Carolina horsenettle, Solanum carolinense

60

48

50

Croftonweed, Ageratina adenophora

Crystalline iceplant, Mesembryanthemum crystallinum

Greater swinecress, Lepidium coronopus

Mexican fan palm, Washingtonia robusta

Onionweed, Asphodelus fistulosus

Syrian beancaper, Zygophyllum fabago

62

52

54

68

56

64

Yellow Flowers 70 

Austrian yellow cress, Rorippa austriaca

Bermuda buttercup, Oxalis pes-caprae

Canary Island St. Johnswort, Hypericum canariense

Capeweed, Arctotheca calendula

Creeping yellow cress, Rorippa sylvestris

Crown daisy, Glebionis coronaria

Dudaim melon, Cucumis melo var. dudaim

70

92

98

78

70

80

96

Fennel, Foeniculum vulgare

Globe chamomile, Oncosiphon piluliferum

Rush skeletonweed, Chondrilla juncea

Sahara mustard, Brassica tournefortii

94

82

84

72

Shortpod mustard, Hirschfeldia incana

Sicilian starthistle, Centaurea sulphurea

Smooth distaff thistle, Carthamus creticus

74

86

88

Stinkwort, Dittrichia graveolens

Wall rocket, Diplotaxis tenuifolia

Woman’s tongue tree, Albizia lebbeck

Woolly distaff thistle, Carthamus lanatus

90

76

100

88

Glossary 102

Selected References 106
4 New Invaders of the Southwest IntroductionAbout This Field Guide

Invasive plants are a major concern worldwide. They displace native species, 

decrease forage and agricultural production, alter soil nutrients and water cycling, 

and lower the aesthetic value of natural areas. With the increase of world travel, 

exotic plant introductions are on the rise. 

Not all newly introduced species become invasive. Those which do often follow 

an invasion pattern; they remain at low levels for several years (lag phase) and 

then enter a phase where they increase dramatically. Attempting to control large 

weed infestations is a costly endeavor; it is much more cost effective to allocate 

resources toward weed prevention or the rapid treatment of new introductions. 

Unfortunately, the process of adding newly introduced species to watch lists or 

control lists is frequently tedious and lengthy. By the time many invasive species 

are on the radar, they have become widely established. The purpose of this guide is 

to help land users recognize new invasive plants, so they can be treated rapidly and 

eradicated rather than becoming large and expensive problems. 

This guide focuses on hot and arid regions of southwestern USA, including portions 

of: Arizona, California, Nevada, New Mexico, and Utah. The cool and temperate 

regions of northwestern North America are the focus of the sister guide, New 

Invaders of the Northwest. There is overlap between species and regions, especially 

in areas bordering our superficial boundary. 

The species in this guide were selected by:

1. Combining state and provincial noxious weed, watch, and new invader 

lists/alerts 

2. Adding species with high ecological impact ratings as assigned by the 

California Invasive Plant Council and NatureServe 

3. Identifying those species not yet widespread throughout southwestern 

USA as first-round candidates

4. Combining the opinions of numerous state and regional weed experts to 

narrow the candidate list to those present in this guide

It was not possible to include all new species of concern, but this manual will hopefully 

serve as a good starting point. The plants included herein are arranged first by flower 

color, and then grouped with related species progressing from grasses through forbs, 

vines, shrubs, and trees. Select definitions for plant terms are included in the glossary. 

For more in-depth explanations of plant parts and life cycles, or for more help with 

plant identification, please see the references listed at the end of this guide.

Each plant is represented by multiple photos and descriptions emphasizing key 
New Invaders of the Southwest 5

identification traits and ways to distinguish it from look-alike species. Attempts 

were made to utilize the most current scientific names for all weeds. The USDA 

GRIN database (Germplasm Resources Information Network) was followed in 

this regard. Please note the scientific names for many species have changed since 

previous weed publications. 

Plant distribution data is presented in a map for each species. The sources used to 

compile current distribution information are included in a table below. Distribution 

information was also provided by individuals and recent weed alert reports. 

Weed Distribution Sources

Consortium of California Herbaria ucjeps.berkeley.edu/consortium/

EDDMapS eddmaps.org

Southwest Environmental Information Network swbiodiversity.org/portal/index.php

USDA-NRCS PLANTS Database plants.usda.gov/java/

Where information was available, counties where 

the weed has been documented are represented by 

a solid gray fill. Some documented populations of 

weeds have since been eradicated. The locations 

of these populations are still included in the 

distribution maps because it is possible some 

plants, seeds, or propagules survived.

Weed spread is often rapid. Even if a weed is not depicted as occurring in a specific 

region, it could have spread into that region since the collection of distribution 

information presented herein. Particular care should be taken searching for species 

in regions surrounding known infestations, as weed spread into nearby areas is likely. 

All states where a weed is listed as noxious are included in a special section. 

Some states have sub-levels for noxious weed designations. These are included in 

abbreviated form where applicable, and are explained in greater detail in a table on 

the following pages. 

If you find a new invader

Should you find one of the species listed in this manual in a new region (or a species 

you believe to be a new invader), notify your local weed authority immediately, and 

devise a treatment plan to eradicate the infestation as promptly as possible.
6 New Invaders of the Southwest Introduction

State Noxious Weed Classification 

Arizona (AZ)

Regulated 

Prohibited 

Restricted 

California (CA) No sub-categories

Nevada (NV)

A 

B

C

New Mexico (NM)

A 

B 

C 

Watch List

Utah (UT)

A

B

C
New Invaders of the Southwest 7

Definition

Species (including plants, stolons, rhizomes, cuttings, and seed) that, if found within the state, may 

be controlled or quarantined to prevent further infestation or contamination.

Species (including plants, stolons, rhizomes, cuttings, and seed) that are prohibited from entry into 

the state.

Species (including plants, stolons, rhizomes, cuttings, and seed) that, if found within the state, shall 

be quarantined to prevent further infestation or contamination.

Weeds not found or limited in distribution throughout the state; actively excluded from the state 

and actively eradicated wherever found.

Weeds established in scattered populations in some counties of the state; actively excluded where 

possible, actively eradicated from nursery stock dealer premises; control required by the state in 

areas where populations are not well established or previously unknown to occur.

Weeds currently established and generally widespread in many counties of the state; actively 

eradicated from nursery stock dealer premises; abatement at the discretion of the state quarantine 

officer.

Species are currently not present in New Mexico, or have limited distribution. Preventing new 

infestations of these species and eradicating existing infestations is the highest priority.

Species are limited to portions of the state. In areas with severe infestations, management should be 

designed to contain the infestation and stop any further spread.

Species are wide-spread in the state. Management decisions for these species should be determined 

at the local level, based on feasibility of control and level of infestation.

These species are of concern in the state and have the potential to become problematic. More data 

is needed to determine if these species should be listed; when these species are encountered please 

document their location and contact appropriate authorities.

Species declared as “Early Detection and Rapid Response (EDRR)”. These are not native to the 

state of Utah, pose a serious threat to the state, have a relatively low population size within the state 

and are of the highest priority for control.

Species declared as "Control". These are not native to the state of Utah, pose a threat to the state, 

have a moderate population throughout the state, and generally are thought to be controllable in 

most areas.

Species declared as "Containment". These are not native to the state of Utah, pose a threat to the 

agricultural industry and agricultural products, are found extensively in the state, and are thought 

to be beyond control. Statewide efforts are generally towards containment of smaller infestations. 
Arabian schismus & Mediterraneangrass

Schismus arabicus Nees & S. barbatus (L.) Thell.

8 New Invaders of the Southwest GREEN FLOWERS

Arabian schismus: a) plant, b) leaves (a,b Western New Mexico University Department of Natural 

Sciences & Dale A. Zimmerman Herbarium), c) inflorescence (Patrick Alexander, swbiodiversity.

org), d) floret (D. Walters and C. Southwick, USDA, www.bugwood.org)

Mediterraneangrass: e) plant (Max Licher, swbiodiversity.org), f) leaves, g) inflorescences (f,g Joseph 

M. DiTomaso, University of California - Davis), h) floret (D. Walters and C. Southwick, USDA) 

(f-h www.bugwood.org)

e

b

a c d

f

g h
Family Poaceae

New Invaders of the Southwest 9

SYNONYMS: Arabian schismus (AS): Arabian grass, camel grass; 

Mediterraneangrass (MG): common Mediterranean grass, old han schismus, 

Festuca barbata L.

ORIGIN: northern Africa, Mediterranean, Middle East (both species)

GROWTH TRAITS: Both species are very similar and differentiated largely by 

details in the florets. They are annual bunchgrasses growing up to 8” tall (20 

cm) from extensive root systems near the soil surface. Stems are round in crosssection and can be erect or sprawling. Leaves are usually inrolled and thread-like, 

typically up to 1 mm wide, and less than 8” long (20 cm). Ligules are rings of 

hairs. Flowering occurs in spring. Florets occur in dense clusters up to 2” long (5 

cm). Florets are enclosed by 2 bracts. The outermost bract (lemma) has 2 lobes 

while the innermost bract (palea) does not. AS does not have an awn between 

the lemma lobes, and the palea is much shorter than the lemma lobes. MG has a 

short awn, and the palea extends nearly as long as the lemma lobes. All parts of 

the plants are green when young, turning purple with maturity. 

REPRODUCTION: Both species spread by seed only. Seeds may remain viable 

in the soil for multiple years.

 

HABITAT: Both species thrive in arid to semi-arid regions, growing in between 

shrubs where they avoid extensive shading. They can be found in disturbed and 

undisturbed ground.

LOOK-ALIKES: Most bunchgrasses that resemble these two species are perennials 

and typically grow larger than Arabian schismus and Mediterraneangrass. The 

native six-weeks fescue (Vulpia octoflora) and exotic hardgrass (Schlerochloa dura) 

are annuals, low-growing, and have similar inflorescences. Six-weeks fescue has 

obvious awns 0.1-1” long (3-25 mm) while hardgrass has a short inflorescence 

(<1”, 2 cm) with flattened florets clustered on one side of the flowering stem.

NOXIOUS WEED LISTINGS: Neither species is listed as noxious in any 

southwestern state.

NOTES: Though not 

listed as noxious, 

their potential 

ecological impact and 

distribution are both high. Arabian schismus Mediterraneangrass
Blue panicgrass

Panicum antidotale Retz.

10 New Invaders of the Southwest GREEN FLOWERS SYNONYMS: blue panicum, Panicum miliare Lam.

ORIGIN: Asia

GROWTH TRAITS: Perennial, sod-forming grass growing 8-10’ tall (21⁄2-3 

m) from a rhizomatous root system. Older plants are typically bush-like with 

multiple branches and a tough, knotty base. Leaves are light blue-green with 

swollen nodes. Leaf blades are 6-12” long (15-30 cm), smooth (without hair), 

and with obvious midveins obvious on the underside. Ligules have tiny hairs. 

The inflorescence is open and erect, but slightly droopy at maturity. It occurs on 

stalk tips from April to December. Inflorescences are typically up to 1’ long (30 

cm). Flowers are yellow-green and often shiny. Mature seeds are brown and shiny.

REPRODUCTION: By seed and vegetatively through rhizomes. Seeds remain 

viable in the soil for many years.

Blue panicgrass: a) recovering plants (©2001 CDFA), b) infestation (Forest and Kim Starr, Starr 

Environmental, www.bugwood.org)

a b
Family Poaceae

New Invaders of the Southwest 11

HABITAT: Found in open, disturbed sites including 

roadsides, fields, irrigated pastures, and along 

irrigation waterways.

LOOK-ALIKES: Several native and exotic grasses 

may resemble blue panicgrass, the most similar being 

other species in the genus Panicum, including the 

native fall panicgrass (Panicum dichotomiflorum). 

Those overlapping the range of blue panicgrass 

differ by typically being summer annuals rather 

than sod-forming perennials. They can be further 

differentiated from blue panicgrass and from each 

other by differences in their inflorescence size and 

structure, as well as the size or presence of hairs on 

leaves or leaf parts.

NOXIOUS WEED LISTINGS: CA

NOTES: This species is a nutritious forage 

grass intentionally planted in Arizona and 

Texas, though it is escaping pastures and 

becoming weedy in some regions.

Blue panicgrass: c) leaves, sheaths, and stem d) young inflorescence (Forest and Kim Starr, Starr 

Environmental, www.bugwood.org), e) mature inflorescence (c,e ©2001 CDFA)

Look-alike: fall panicgrass (John 

Hilty, swbiodiversity.org)

c d e
Buffelgrass & Fountain grass

Pennisetum ciliare (L.) Link & P. setaceum (Forssk.) Chiov.

12 New Invaders of the Southwest GREEN FLOWERS

Buffelgrass: a) plant, b) leaf attachment and stalk (Joseph M. DiTomaso, University of California - 

Davis, www.bugwood.org), c) inflorescence (a,c Forest & Kim Starr)

Fountain grass: d) plant, e) leaf attachment and stalk (Joseph M. DiTomaso, University of 

California - Davis, www.bugwood.org), f) inflorescence (d,f Forest & Kim Starr)

c

b

a

e

d f
Family Poaceae

New Invaders of the Southwest 13

SYNONYMS: Buffelgrass (BG): African foxtail grass, Cenchrus ciliaris L.; Fountain 

grass (FG): crimson fountain grass Cenchrus setaceus (Forssk.) Morrone

ORIGIN: BG: Africa, Middle East, India; FG: Africa, Middle East

GROWTH TRAITS: Both species are tufted perennials typically growing 16-

59” tall (40-150 cm) with round stems. Leaves have prominent midveins. 

Ligules are a fringe of white hairs. Inflorescences are long, narrow spikes with 

numerous long, purplish bristles. BG has rhizomes and stolons, allowing it to 

form dense mats. Leaves are flat, up to 20” long (51 cm), and up to 1⁄2” wide 

(13 mm). Flowering occurs from spring through fall. Inflorescences are 0.8-8” 

long (2-20 cm), 0.4-1” wide (1-21⁄2 cm), and erect. Bristles are stiff, wavy, and 

usually less than 1⁄2” long (12 mm). FG has a fibrous root system. Leaves are 

rolled or strongly folded, 8-25” long (20-65 cm), 0.08-0.14” wide (2-31⁄2 mm), 

and slightly hairy. Flowering occurs nearly year-round. Inflorescences are 3-12” 

long (8-30 cm), up to 2” wide (5 cm), and tend to droop. Bristles are straight 

and unequal in length, with some reaching 0.7-1.4” long (18-35 mm). 

REPRODUCTION: BG: By seed and vegetatively with rhizomes and stolons; 

FG: By seed. Seeds may remain viable in the soil for six years or more. Fertilization 

is not required for either species to produce viable seeds.

 

HABITAT: Both species prefer dry conditions and can be found in deserts, coastal 

dry forests, grasslands, thornscrub communities, and along roadsides.

LOOK-ALIKES: Many other species of Pennisetum (both native and exotic) occur 

in the SW; most can be differentiated by growing much shorter and groundhugging, having non-spiky inflorescences, and/or having whitish bristles and 

florets. The native satintail (Imperata brevifolia) has a similar size and rhizomes, 

but its spike-like inflorescence is whitish and densely silky-hairy. Its ligules are 

membranous, flat, and hairy.

NOXIOUS WEED LISTINGS: BG: AZ (Prohibited); FG: NV (A), NM 

(Watch List)

NOTES: BG was 

intentionally 

introduced for forage, 

FG has been widely 

used as an ornamental. Buffelgrass Fountain grass
Jubata & Pampas grass

Cortaderia jubata (Lemoine ex Carrière) Stapf & C. selloana (Schult. & Schult. f.) Asch. & Graebn.

14 New Invaders of the Southwest GREEN FLOWERS

Jubata grass: a) plant (Gordon Leppig & Andrea J. Pickart), b) leaf blade, c) inflorescence 

(b,c Forest & Kim Starr)

Pampas grass: d) plant (Wouter Hagens, taken in the Kijktuinen Nunspeet), e) leaves of cultivated 

plant (Kenpei), f) inflorescences (Kenneth M. Gale, www.bugwood.org)

c

b

a

f

e

d
Family Poaceae

New Invaders of the Southwest 15

SYNONYMS: Jubata grass (JG): Andean pampas grass, purple pampas grass, 

pampas grass; Pampas grass (PG): white pampas grass, Uruguayan pampas grass

ORIGIN: South America (both species)

GROWTH TRAITS: Both species are large, densely tufted, perennial grasses 

growing from dense fibrous roots with shallow, short rhizomes. Leaves are very 

long and have sharply serrated margins. Ligules are dense rings of hairs 0.08-

0.12” long (2-3 mm). Dense flower plumes appear summer through fall and 

are 1-3’ long (30-90 cm). JG are all-female plants growing 6-23’ tall (1.8-7 m) 

with the stems at least 2 times longer than the spreading tussock. Leaves are 3-5’ 

long (0.9-1.5 m) and flat or slightly v-shaped in cross-section. Plumes are deep 

violet, turning pinkish then gold at maturity. Awns are very short. PG grows 

6-13’ tall (1.8-4 m). Plants are either all-female or hermaphroditic. Stems of 

hermaphroditic plants are 2 times longer than the tussock; female plant stems 

are equal to or slightly longer than the tussock. Leaves are up to 6’ long (1.8 m), 

v-shaped in cross-section, and with bristly, curled tips. Plumes are light pink to 

silver white; awns are long. Tussocks are more fountain-like than jubata grass.

REPRODUCTION: Both species spread by seed and fragmented tillers. Seeds 

are believed to remain viable in the soil for less than six months.

 

HABITAT: Both species prefer coastal, disturbed sites such as roadsides, bluffs, 

steep cliffs, riverbanks, and shrubland. However, both can be invasive at inland 

locations. PG can tolerate standing water.

LOOK-ALIKES: Several native and exotic grasses resemble jubata and pampas 

grass. The bunched tussocks help differentiate jubata and pampas grass from 

the rhizomatous and linear-growing giant and common reeds (Arundo donax

and Phragmites australis, respectively). Ravenna grass (Saccharum ravennae) is a 

similar-looking bunchgrass, but its leaf and stem bases are densely hairy.

NOXIOUS WEED LISTINGS: JG: Not listed in any SW state; PG: NM 

(Watch List)

NOTES: Both 

species are escaped 

ornamentals.

Jubata grass Pampas grass
Puna grass

Amelichloa brachychaeta (Godr.) Arriaga & Barkworth

16 New Invaders of the Southwest GREEN FLOWERS SYNONYMS: puna needlegrass, espartillo, Argentine needlegrass, Achnatherum 

brachychaetum (Godr.) Barkworth, Stipa brachychaeta Godr.

ORIGIN: South America

GROWTH TRAITS: Perennial bunchgrass with upright stems growing 16-39” 

tall (0.4-1 m) from a fibrous root system, sometimes with very short rhizomes. 

Leaf sheaths and blades have tiny hairs along margins. Leaf blades are 3-14” 

long (8-35 cm), rough, and typically rolled outwards (making them appear 

wiry). Ligules are short, flat membranes with tiny hairs at their sides. Florets 

are produced in two places. The more obvious inflorescence occurs at stem tips 

in spring and is 4-10” long (10-25 cm). Florets are sparse, open, and have awns 

0.4-0.7” long (10-18 mm) that are often bent twice. The second set of florets 

develop in basal leaf sheaths from self-fertilization. These florets remain in the 

plant until the leaf sheaths are disturbed or senesce. Seeds are typically teardropshaped, tan, and up to 0.1” long (3 mm).

Puna grass: a) plant (Joseph M. DiTomaso, University of California - Davis, www.bugwood.org), 

b) infestation (©2001 CDFA)

a b
Family Poaceae

New Invaders of the Southwest 17

REPRODUCTION: By seed only. Seeds may 

remain viable in the soil for up to two years.

HABITAT: Capitalizes on disturbance and can be 

found in a variety of habitats, including roadsides, 

grasslands, pastures, open hillsides, streambanks, 

and waste areas. Often associated with alfalfa fields.

LOOK-ALIKES: Several needlegrass species 

(Achnatherum, Nassella, and Stipa spp.) resemble 

puna grass with their narrow leaves and bunched 

growth. However, they have narrow inflorescences 

with longer awns, and none have self-fertilized 

seeds develop in basal leaf sheaths. 

NOXIOUS WEED LISTINGS: AZ (Prohibited), CA

NOTES: This species quickly becomes 

problematic in rangeland and grassland 

settings as the sharp, pointed leaves of 

older plants are unpalatable to grazing 

animals.

Puna grass: c) florets within leaf sheaths, d) terminal inflorescence (c,d Joseph M. DiTomaso, 

University of California - Davis, www.bugwood.org), e) seeds (©2001 CDFA)

Look-alike: green needlegrass, 

Nassella viridula (Matt Lavin)

c d e
Ravenna grass

Saccharum ravennae (L.) L.

18 New Invaders of the Southwest GREEN FLOWERS SYNONYMS: hardy pampas grass, elephant grass, plume grass, Erianthus 

ravennae (L.) Beauv., Andropogon ravennae L., Ripidium ravennae (L.) Trin., 

Tripidium ravennae (L.) H. Scholz

ORIGIN: Eurasia, Mediterranean

GROWTH TRAITS: Perennial bunchgrass growing 8-13’ tall (2.4-4 m) from 

a densely fibrous root system. Leaves are basal and also distributed up the stem 

to the base of the inflorescence. Leaves have serrated margins, a thick, white 

midvein on upper sides of blades, and can be 20-40” long (0.5-1 m) and 1⁄2” 

wide (1.2 cm). Leaf bases are unlobed and are densely covered with long, fuzzy 

hairs that typically hide the ligule. Dense flower plumes are purplish (maturing 

to silver to tan), up to 2’ long (0.6 m), and bloom from July through October. 

Florets are covered in tufts of silky hairs, giving the inflorescence an overall fluffy 

appearance.

Ravenna grass: a) plant (Jennifer Andreas, Washington State University Extension), b) infestation 

(The Nature Conservancy Archive, The Nature Conservancy, www.bugwood.org)

a b
Family Poaceae

New Invaders of the Southwest 19

REPRODUCTION: By seed only. Seed viability is 

unknown.

HABITAT: Found in open, disturbed locations 

at both moist and dry sites, including marshes, 

ditchbanks, seeps, floodplains, rocky hillsides, 

gravel roadsides, and escaped from yards. Tolerant 

of cold weather.

LOOK-ALIKES: Several native and exotic grasses 

resemble Ravenna grass. The clumped bunches help 

differentiate Ravenna grass from the rhizomatous 

and linear-growing giant and common reeds 

(Arundo donax and Phragmites australis, respectively). Jubata and pampas 

grass (Cortaderia jubata and C. selloana) are similar-looking bunchgrasses, but 

Ravenna grass leaf and stem bases are densely hairy, its leaf margins are not as 

sharp, its leaf midveins are thick white on the upper sides of the blades, and its 

flowering stems can have red coloring.

NOXIOUS WEED LISTINGS: NM (A)

NOTES: Frequently used for soil erosion control 

and as an ornamental, but often escapes 

cultivation. Spreading rapidly in several 

parts of North America.

Ravenna grass: c) leaf blades with serrated margins and white midveins, d) hairy leaf base, 

e) inflorescences (c-e Wendy DesCamp, Washington State Noxious Weed Control Board)

Look-alike: jubata grass (Gordon 

Leppig & Andrea J. Pickart)

c d e
Veldtgrasses

20 New Invaders of the Southwest GREEN FLOWERS Trait

Long-Flowered

Ehrharta longiflora

Panic

Ehrharta erecta

Perennial

Ehrharta calycina

Life

History

Annual Perennial Perennial

Preferred

Habitat

Disturbed, sandy, 

open

Moist or shaded 

initially, spreading to 

dry, variety of soils

Disturbed, sandy, 

sometimes savannahs or 

woodlands

Growth

Form

Erect to sprawling 

stems

Spreading bases, 

stems curving 

to become erect, 

produces tillers

Erect stems, plant tussockforming

Height 8-31” (20-80 cm) 12-24” (30-60 cm) 12-30” (30-75 cm)

Leaves

≤8” (20 cm) long, 

flat, wrinkled partway 

along margin, tinged 

with purple

2-6” (5-15 cm) long, 

flat

3-8” (7-20 cm) long, 

flat, wrinkled partway 

along margin, tinged with 

purple

Florets

Green, awns 0.08-

0.8” long (2-20 mm), 

subtended by 2 purple 

bracts

Green, no awns Green, turning purple 

with age, no awns

Long-flowered veldtgrass: a) plants, b) leaf blade and inflorescence (a,b ©2005 Tom Chester, http://

tchester.org/plants/analysis/ehrharta/longiflora.html), c) inflorescence (Harry Rose)

c

b

a
Family Poaceae

New Invaders of the Southwest 21

Panic veldtgrass: d) plant (©2011 Vernon Smith), e) leaf attachment and ligule (©2008 Zoya 

Akulova), f) inflorescence (Matt Lavin, Bozeman, MT)

Perennial veldtgrass: g) plant , h) leaf base pulled back to expose ligule (g,h Harry Rose),

 i) inflorescence (Jerry Kirkhart) 

i

h

g

f

e

d
Veldtgrasses (continued)

22 New Invaders of the Southwest GREEN FLOWERS NAMES: Long-flowered veldtgrass, Ehrharta longiflora Sm. (LV)

 Panic veldtgrass, Ehrharta erecta Lam. (PanV)

 Perennial veldtgrass, Ehrharta calycina Sm. (PerV)

SYNONYMS: LV: annual veldtgrass, tall ehrharta; PanV: erect veldtgrass, 

Ehrharta panicea Sm.; PerV: purple veldtgrass

ORIGIN: southern Africa (all species)

GROWTH TRAITS:

Long-flowered veldtgrass (LV): Annual growing upright to somewhat sprawling 

and typically 8-31” tall (20-80 cm) from a fibrous root system. Leaves are flat, 

up to 8” long (20 cm), 0.4” wide (1 cm), wrinkled partway along the margin, 

and often tinged with purple. The ligule is membranous and toothed. Flowering 

occurs in spring. Inflorescences are 4-8” long (10–20 cm), often spreading, and 

consist of numerous drooping clusters of 3 green florets subtended by 2 purplish 

bracts. The lower 2 florets are sterile with long awns 0.08-0.8” long (2-20 mm). 

The upper floret is fertile.

Panic veldtgrass (PanV): Sprawling perennial with stems curving upward from 

their prostrate bases. Stems typically grow 12-24” tall (30-60 cm) from a fibrous 

root system and can produce tillers that root from leaf nodes growing on or 

near the ground. Leaves are flat, 2-6” long (5-15 cm), and 0.16-0.6” wide (4-

15 mm). The ligule is membranous and toothed. Flowering occurs in spring to 

early summer. Inflorescences are open, 2-8” long (6-20 cm), branched, and with 

clusters of 3 green florets. The lower 2 florets are sterile while the upper is fertile. 

All florets have no awns. 

Perennial veldtgrass (PerV): Erect, tussock-forming perennial with stems growing 

12-30” tall (30-75 cm) and forming dense tussocks. Roots are fibrous and 

occasionally rhizomatous. Leaves are flat, often purple-tinged, 3-8” long (7-20 

cm), 0.08-0.3” wide (2-7 mm), and wrinkled partway along the margin. The 

ligule is membranous and toothed. Flowering occurs spring through summer. 

Inflorescences are 4-6” long (10-15 cm), open, and with clusters of 3 green 

florets that turn reddish-purple with age. The lower 2 florets are sterile while the 

upper is fertile. All florets have no awns. 

REPRODUCTION: LV: By seed only. PanV: By both seed and vegetatively 

with tillers. PerV: Largely by seed, but very occasionally by rhizomes. Seeds of 

all three species are believed to remain viable in the soil for more than one year.

 
Family Poaceae

New Invaders of the Southwest 23

HABITAT: LV and PerV both capitalize on disturbance and are most frequently 

found in sandy soil. PerV has also been found in savannahs and woodlands.

PanV typically occurs in more moist or partially shaded areas including urban, 

irrigated, riparian, and coastal habitats (especially stabilized dunes) in a wide 

variety of soils. Once established, PanV can expand into more dry and open 

areas.

LOOK-ALIKES: Grasses can be very difficult to 

differentiate, and several native and exotic grass 

species resemble veldtgrasses by sharing numerous 

features. Bromes (Bromus spp.) are quite similar 

with their stiff and often drooping inflorescences 

and their leafy bases. Many brome species have 

ligules similar to veldtgrasses as well as flat leaf 

blades of the same size and shape as those of 

veltgrasses. Bromes can be differentiated by having 

leaf sheaths that are closed for most of their length, 

awns that are typically inserted just beneath bract 

tips, usually more than 3 florets (3-30), and having 

hair on the female part (ovary) of a floret. 

NOXIOUS WEED LISTINGS: None of these species are listed as noxious in 

any southwestern state.

NOTES: Though not listed as noxious in any southwestern state, the potential 

ecological impact and distribution of all three species are high. 

Look-alike: closed leaf sheaths 

of soft brome (Matt Lavin, 

Bozeman, MT)

Long-flowered veldtgrass Panic veldtgrass Perennial veldtgrass
Carnation spurge

Euphorbia terracina L.

24 New Invaders of the Southwest GREEN FLOWERS SYNONYMS: Geraldton carnation spurge, false caper, terracina spurge

ORIGIN: Mediterranean

GROWTH TRAITS: Herbaceous perennial growing up to 3.3’ tall (1 m) from a 

robust vertical taproot with horizontal lateral roots. All parts of the plant exude 

a white latex sap when torn. Stems are green to reddish, hairless, slender, and 

unbranched at first but dividing into ≤5 primary flower stems. Leaves are bright 

green, alternate, oblong, lance-shaped, 1⁄2-2” long (11⁄2-5 cm), and have rounded 

tips and no leaf stems. Flowers occur in an umbel-like inflorescence subtended 

by large, wide bracts. The umbel is divided, with branches forking into 2 

flowers. All forks are subtended by the large, wide bracts. Individual flowers are 

inconspicuous and yellow and surrounded by green, showy bracts. Flowering 

occurs in early spring for early germinating plants. Plants germinate throughout 

the growing season, followed quickly by flowering. Fruits are smooth, 3-lobed 

capsules. Seeds are tan, oval, and have a fleshy projection. Seeds can be propelled 

several feet from the plant upon drying. 

Carnation spurge: a) plant (©2007 Luigi Rignanese), b) infestation (©2010 Michael O’Brien)

a b
Family Euphorbiaceae

New Invaders of the Southwest 25

REPRODUCTION: By seed and buds from the 

damaged root crown. Seeds remain viable in the 

soil for 3-5 years. 

HABITAT: Capitalizes on disturbance to invade 

a variety of habitats including coastal sage scrub, 

grasslands, oak woodlands, riparian areas, and 

chaparral edges. Survives in high and low light and 

in both cool/moist and hot/dry climates.

LOOK-ALIKES: Other species of Euphorbia

resemble carnation spurge with their showy green 

bracts, lance-shaped and stemless leaves, and the 

production of latex sap when torn. The majority of 

those which occur in the Southwest are exotic weeds. Most can be differentiated 

by having rhizomatous or fibrous roots and having leaves with smooth margins. 

Eggleaf spurge (Euphorbia oblongata) is very similar but has a densely hairy stem.

NOXIOUS WEED LISTINGS: CA

NOTES: Spreading rapidly in southern 

California. The latex sap is toxic and should 

be handled with care.

Carnation spurge: c) leaf (Sunphlo), d) inflorescence and bracts (Sarah Gregg), e) seeds (Carole 

Ritchie, USDA-NRCS PLANTS Database)

Look-alike: eggleaf spurge 

(Marty Hudson, Klickitat 

County NWCB)

c d e
Tree spurge

Euphorbia dendroides L.

26 New Invaders of the Southwest GREEN FLOWERS SYNONYMS: woody spurge

ORIGIN: Mediterranean

GROWTH TRAITS: Perennial, rounded shrub or small tree growing up to 

10’ tall (3 m) from a fleshy root system. All parts of the plant exude a white 

latex sap when torn. Stems and branches are smooth, hairless, and have scars 

remaining from fallen leaves. Leaves are alternate, oblong lance-shaped with no 

leaf stems, and up to 3” long (71⁄2 cm) with a single prominent midvein. Flowers 

are inconspicuous, yellow, and surrounded by yellowish-green, showy bracts. 

Flowering occurs in early spring. Fruits and foliage turn red in summer, and 

leaves fall to limit transpiration. Seeds are brown, round, smooth, and can be 

propelled several feet from the plant upon drying. 

REPRODUCTION: By seed and buds from the damaged root crown. Seeds are 

typically short-lived but can be viable for approximately five years. 

Tree spurge: a) plant (Tato Grasso), b) infestation (Frank Vincentz)

a b
Family Euphorbiaceae

New Invaders of the Southwest 27

HABITAT: Found in semi-arid and Mediterranean 

climates, preferring sunny hillsides and open, 

coastal areas without frost.

LOOK-ALIKES: Other species of Euphorbia

resemble tree spurge with their showy green 

bracts, lance-shaped and stemless leaves, and the 

production of latex sap when torn. The majority 

of those which occur in the Southwest are lowgrowing exotic weeds, such as leafy spurge 

(Euphorbia esula), which also may turn red in dry 

conditions during summer. Tree spurge can be 

differentiated by its combination of a much greater 

overall size, denser leaves, prominent leaf scars on 

stems, and the red color that fruits and leaves turn 

in summer.

NOXIOUS WEED LISTINGS: Not listed as 

noxious in any southwestern state.

NOTES: Though not listed as noxious in any 

southwestern state, its potential ecological 

impact and distribution are both high. 

The plant is an escaped ornamental and 

is spreading in Santa Barbara County, 

California. The latex sap is toxic and 

should be handled with care.

Tree spurge: c) leaves (Frank Vincentz), d) flowers, bracts, and immature fruits (Tato Grasso), 

e) mature fruits and foliage (Hans Hillewaert)

Look-alike: leafy spurge 

(Jennifer Andreas, Washington 

State University)

c d e
Devil’s thorn

Emex spinosa (L.) Campd.

28 New Invaders of the Southwest GREEN FLOWERS SYNONYMS: spiny emex, spiny three-corner jack, Rumex spinosus L.

ORIGIN: Mediterranean

GROWTH TRAITS: Herbaceous annual growing 12-24” tall (30-60 cm) from a 

long, thick taproot. Stems are round, ridged, sometimes reddish, often somewhat 

sprawling, and branch periodically. Leaves are alternate, smooth, triangular to 

egg-shaped, 2-43⁄4” long (5-12 cm), and have slightly wavy margins. Leaf stalks 

are long, hairless, and with membranous sheaths at their bases. Flowering occurs 

from early summer to winter with separate male and female flowers occurring on 

the same plant. Male flowers are small, inconspicuous, green, and occur in short 

clusters on stalks. Female flowers are spiny, without stalks, and occur in clusters 

around leaf axils. Fruits are produced in two places. Those occurring aerially on 

stems are triangular and green when young, turning reddish-brown, hardened, 

and with 3 sharp spines at maturity. Spines are up to 0.11” long (3 mm). Larger 

but less spiny fruits also form at the root crown. These are yellowish-red but turn 

Devil’s thorn: a) plant, b) infestation (a,b Forest & Kim Starr)

a b
Family Polygonaceae

New Invaders of the Southwest 29

brown with maturity. Each fruit contains a single 

glossy seed. When the plant dies back, its drying 

root pulls the crown seeds into the soil.

REPRODUCTION: By seed only. Seeds remain 

viable in the soil for at least eight years.

HABITAT: Tolerates a wide variety of conditions 

but does best with disturbance. Frequently found 

along roadsides, railways, flood zones, field edges, 

pastures, grasslands, and other dry or sandy 

locations.

LOOK-ALIKES: While various native and exotic species have similar triangular 

wavy leaves, non-showy male flower clusters, or spiny female flowers clustered 

around leaf axils, the combination of all of these traits helps differentiate Emex

species from possible look-alikes. There are only two species in the genus. The 

exotic southern three-corner jack (Emex australis) is very similar but has larger 

spiny fruits, smaller leaves, and tends to grow more prostrate than devil’s thorn.

NOXIOUS WEED LISTINGS: Not listed as noxious in any southwestern state.

NOTES: Though not listed as noxious in any 

southwestern state, its potential ecological 

impact and distribution are both high.

Devil’s thorn: c) leaves, d) male flowers (c,d Forest & Kim Starr), e) fruits (Rolf Engstrand)

Look-alike: southern threecorner jack (Kevin Thiele)

c d e
Carob tree

Ceratonia siliqua L.

30 New Invaders of the Southwest GREEN FLOWERS SYNONYMS: St. John’s bread, locust bean

ORIGIN: Mediterranean, Middle East

GROWTH TRAITS: Tree typically growing up to 33’ tall (10 m) from a deep and 

extensive root system. The thick trunk has rough bark and spreads into several 

sturdy branches forming a broad, semi-spherical crown. Leaves are alternate 

and divided into 6-10 opposite leaflets. Leaflets are rounded-elliptic, shiny 

green, leathery, 11⁄2-3” long (4-8 cm), and have wavy margins and prominent 

yellow midveins. Male and female flowers (both non-showy and lacking petals) 

typically appear on separate trees from July to November. Male flowers are long 

clusters of reddish-stamens while female flowers are greenish-yellow and are in 

long clusters. The fruit is a long pod 4-12” long (10-30 cm) that can be straight 

or strongly curved and is dark brown and leathery at maturity. Pods contain 

5-15 small, flattened, brown seeds. Male flowers do not produce fruit.

REPRODUCTION: By seed only. Seeds may remain viable for up to five years.

Carob tree: a) tree (Giancarlo Dessì), b) bark (©2007 Luigi Rignanese)

a b
Family Fabaceae

New Invaders of the Southwest 31

HABITAT: Can withstand a variety of conditions, 

but prefers full sun and dry, hot conditions without 

frost or acidic soil. Extremely drought tolerant and 

escapes cultivation to colonize disturbed sites.

LOOK-ALIKES: Though many trees may resemble 

carob tree by having either a half-spherical canopy, 

long clustered and non-showy flowers, compound 

leaves, or long seed pods, very few species have the 

full combination of all those traits. Other trees 

in the Fabaceae, such as honey locust (Gleditsia 

triacanthos), have similar flowers and fruits, but 

their canopy is more irregular, their leaflets are 

narrower and more numerous, and they often have 

thorns.

NOXIOUS WEED LISTINGS: Not listed as 

noxious in any southwestern state.

NOTES: Though not listed as noxious in any 

southwestern state, its potential ecological 

impact and distribution are both high. It 

is frequently cultivated, but has recently 

been observed escaping into nearby 

disturbed habitats.

Carob tree: c) leaf with leaflets (Frank Vincentz), d) female flower (Rickjpelleg), e) male flower 

(Seminsky), f) mature fruit (Riu Chixoy)

Look-alike: honey locust (Chris 

Evans, Illinois Wildlife Action 

Plan, www.bugwood.org)

c d e f
Edible fig

Ficus carica L.

32 New Invaders of the Southwest GREEN FLOWERS SYNONYMS: common fig, Ficus caprificus Risso

ORIGIN: Middle East, southwestern Asia

GROWTH TRAITS: Tree growing up to 30’ tall (9.1 m) from an extensive root 

system. New plants can sprout from root fragments, and fallen limbs can root 

and create new plants. Mature trees have a sprawling canopy, multiple trunks, 

and several branches with smooth, light gray-brown bark. Leaves are shaped like 

the palms of hands with multiple, rounded lobes. Leaves are alternate, 2-10” 

long (5-25 cm), 2-8” wide (5-20 cm), and have lighter-colored petioles 2-4” 

long (5-10 cm). The plant has two forms: the “caprifig” produces male flowers 

and female flowers with only short styles while the “fig” produces only female 

flowers with long styles. All flowers lack petals and occur within a pear-shaped 

fruit. Caprifig fruit is dry and unpalatable while fig fruit is fleshy and savored by 

many animals, including humans. Figs are 11⁄2-3” long (4-8 cm) and vary from 

dark purple to greenish-yellow to pinkish-green. Caprifigs produce 3 crops per 

year while fig crops produce 2, each ripening in summer or late fall.

Edible fig: a) small wild tree, b) infestation (a,b John M. Randall, The Nature Conservancy, www.

bugwood.org)

a b
Family Moraceae

New Invaders of the Southwest 33

REPRODUCTION: By seed and vegetatively 

through root sprouts or rooting limbs. It is unknown 

how long seeds may remain viable in the soil.

 

HABITAT: Typically invade riparian forests, 

streamside habitats, levees, and canalbanks.

LOOK-ALIKES: While other species may have 

similar leaves and a growth form that overall 

resembles edible fig, most look-alikes do not have 

flowers that occur completely within the fleshy 

fruit. The exotic Punjab fig (Ficus palmata) grows 

to a similar height, has lobed leaves, and edible fruit varying in color similar 

to edible fig. Punjab fig leaves are not as lobed as edible fig and have serrated 

margins. Punjab fig fruits are also smaller (1” or 21⁄2 cm in diameter). 

NOXIOUS WEED LISTINGS: Not listed as noxious in any southwestern state.

NOTES: Though not listed as noxious in 

the SW, its potential ecological impact and 

distribution are both high. Regularly 

cultivated in California, but has escaped 

cultivation in areas where soil stays moist. 

Successful fig production is dependent on 

pollination by the wasp Blastophaga psenes. 

Edible fig: c) twigs and shoots (Robert Vidéki, Doronicum Kft.), d) leaves (Forest & Kim Starr), 

e) mature fruit, cut and whole (David Karp) (c,e www.bugwood.org)

Look-alike: Punjab fig (Liné1)

c d e
Evergreen ash

Fraxinus uhdei (Wenz.) Lingelsh.

34 New Invaders of the Southwest GREEN FLOWERS SYNONYMS: shamel ash, tropical ash

ORIGIN: southern Mexico, Central America

GROWTH TRAITS: Fast-growing, upright tree reaching 60-80’ tall (18-25 m) 

from a shallow root system. Trees are multi-branched with rounded canopies 

when growing solitary, but are less-branched and narrow in dense infestations. 

Bark is fissured and gray (often mottled). Leaves are opposite and divided into 

5-9 leaflets. Leaflets are opposite but with a single terminal leaflet. Leaflets are 

glossy, dark green, lance-shaped, and have toothed margins. Trees are evergreen 

in warm climates. Flowers are inconspicuous and appear from winter through 

spring. Trees are either male or female with female plants producing clusters of 

fruits in spring and summer. Fruits are papery, winged, narrow, and 1⁄2-11⁄2” long 

(11⁄4-4 cm).

REPRODUCTION: By seed. Seeds remain viable in the soil for up to eight years.

Evergreen ash: a) trees, b) infestation (a,b Forest & Kim Starr)

a b
Family Oleaceae

New Invaders of the Southwest 35

HABITAT: Grows best in moist, well-drained soils 

including oak woodlands, riparian areas, and 

irrigated urban areas. Shade tolerant.

LOOK-ALIKES: The opposite leaves divided into 

5-9 leaflets help differentiate evergreen ash from 

non-ash species. Several species of ash (both native 

and exotic) are established in North America, 

though the majority occur in the East. Evergreen 

ash can be differentiated from ash species in the 

Southwest by its combination of: growing as a 

tree rather than a shrub, having compound leaves, 

having smooth shoots (as compared to velvety), 

and having inconspicuous flowers rather than the 

obvious white flowers of other species. Evergreen ash leaflets are also green on 

the underside (as opposed to white), have pointed tips, always have serrated 

margins, and are more numerous (up to 9) compared to look-alike ashes.

NOXIOUS WEED LISTINGS: Not listed as noxious in any southwestern state.

NOTES: Frequently escapes cultivation. Though 

not listed as noxious in any southwestern state, 

its potential ecological impact and 

distribution are both high.

Evergreen ash: c) bark, d) leaf and leaflets (John M. Randall, The Nature Conservancy, www.

bugwood.org, e) fruits (c,e Forest & Kim Starr)

Look-alike: velvet ash (Patrick 

J. Alexander, USDA-NRCS 

PLANTS Database)

c d e
Iberian & Purple starthistle

Centaurea iberica Trevir. ex Spreng. & C. calcitrapa L.

36 New Invaders of the Southwest PINKISH PURPLE FLOWERS

Iberian starthistle: a) plant with flower heads (Eitan f), b) leaf, c) seeds (b,c ©2001 CDFA)

Purple starthistle: d) plant with flower heads (Xemenendura), e) leaf and stem (Solanum), f) seeds 

(D. Walters and C. Southwick, USDA, www.bugwood.org) 

c

b

a

f

e

d
Family Asteraceae

New Invaders of the Southwest 37

SYNONYMS: Iberian starthistle (IS): Iberian knapweed; Purple starthistle (PS):

red starthistle

ORIGIN: southern Europe, Middle East (both species)

GROWTH TRAITS: Both species are herbaceous annuals to short-lived 

perennials typically growing 2-4’ tall (0.6-1.2 m) from a stout taproot. Plants 

often grow as rosettes with spiny centers the first year and then flower the second 

year. Stems and leaves are covered with fine hairs. Lower and basal leaves are 

divided or deeply lobed while upper leaves are not. All leaves often have a light 

green or gray-green appearance. Flowering occurs from July to October when 

numerous lavender to purple flower heads (3⁄4-1” or 2-21⁄2 cm in diameter) 

develop at stem tips. Floral bracts are tipped with 1” long (21⁄2 cm) straw-colored 

spines. The two species can be differentiated by their seeds. IS produces plumed 

seeds while seeds of PS are plumeless.

REPRODUCTION: Both species spread only by seed. Most seed germinates 

the first year, but buried seeds of both species can remain viable in the soil for 

approximately three years.

 

HABITAT: Both species are found in dry and/or disturbed areas, especially along 

roadways and in dry, grazed rangeland. 

LOOK-ALIKES: Both species resemble other members of the Centaurea genus. 

Starthistles can be distinguished by the very long spines extending from bract 

tips. Yellow starthistle differs from these two species with its yellow flower color 

and winged stems. 

NOXIOUS WEED LISTINGS: IS: CA, AZ (Prohibited), NV (A); PS: CA, AZ 

(Prohibited), NV & NM (A)

NOTES: Unpalatable to 

livestock; may cause 

the fatal “chewing 

disease” in horses.

Iberian starthistle Purple starthistle
Three-lobed morning glory

Ipomoea triloba L.

38 New Invaders of the Southwest PINKISH PURPLE FLOWERS SYNONYMS: littlebell, Aiea morning glory

ORIGIN: tropical Americas (southern Mexico, Caribbean, Central America)

GROWTH TRAITS: Annual vine with stems growing up to 10’ long (3 m) from 

an extensive root system. Vines are highly twining so appear much shorter than 

they are, and they do not climb much higher than grasses and low shrubs. Leaves 

are alternate, 3⁄4-43⁄4” long (2-12 cm), and vary in shape from deeply 3-5 lobed 

to heart-shaped. Cut stems and leaf petioles exude a milky sap. Flowers appear 

in leaf axils throughout summer and into fall. Flowers are funnel-shaped with 

fused petals, 1⁄2-1” across (11⁄4-21⁄2 cm), and vary in color from white to pink to 

pale purple with darker centers. Fruits are round, brown capsules containing 2-4 

dark brown seeds.

REPRODUCTION: By seed only. It is unknown how long seeds remain viable 

in the soil.

 

Three-lobed morning glory: a) plant (Neha Vindhya), b) patch (J.M.Garg)

a b
Family Convolvulaceae

New Invaders of the Southwest 39

HABITAT: Tolerates a wide variety of conditions 

in warm climates and is found in cultivated 

fields, grassy swamp margins, in hedgerows, along 

roadsides, and in waste places in sandy ground.

LOOK-ALIKES: The vining habit combined 

with white to purple funnel-shaped flowers help 

differentiate this species from unrelated lookalikes. Several native and exotic morning glory 

species occur in North America and can be 

distinguished by their differences in flower color, 

leaf shape, and root structures. The weedy field 

bindweed (Convolvulus arvensis) is a perennial 

with a rhizomatous root system and smaller, arrowhead-shaped leaves. Hedge 

bindweed (Calystegia sepium) is a similar native but typically weedy species that 

climbs much higher on vegetation or structures (61⁄2-13’ or 2-4 m) compared to 

three-lobed morning glory. Hedge bindweed leaves are also narrower and much 

less lobed, appearing nearly triangular. Sweet potato (Ipomoea batatas) has larger 

flowers (11⁄4-23⁄4” or 3-7 cm across) and sweet potato tuber roots. 

NOXIOUS WEED LISTINGS: AZ (Prohibited)

NOTES: Seed germination increased following 

scarification.

Three-lobed morning glory: c) leaf (J.M.Garg), d) flower (USDA APHIS PPQ Archive, USDA 

APHIS PPQ, www.bugwood.org), e) seed capsules (Forest & Kim Starr)

Look-alike: sweet potato (Forest 

& Kim Starr)

c d e
Highway iceplant, Sea fig & hybrids

Carpobrotus edulis (L.) N. E. Br. & C. chilensis (Molina) N. E. Br.

40 New Invaders of the Southwest PINKISH PURPLE FLOWERS

Highway iceplant: a) plant closeup (Carsten Niehaus), b) infestation (Hans Hillewaert), 

c) yellow flower (Carsten Niehaus), d) cross-section of fruit (©2014 Zoya Akulova)

Sea fig: e) plant (Joseph M. DiTomaso, University of California - Davis), f) flower (Joaquim Alves 

Gaspar), g) cross-section of fruit (©2014 Zoya Akulova) 

g

f

e

d

c

a

b
Family Aizoaceae

New Invaders of the Southwest 41

SYNONYMS: Highway iceplant (HI): hottentot fig; Sea fig (SF): iceplant

ORIGIN: South Africa (both species)

GROWTH TRAITS: Both species are perennial, ground-hugging succulents 

that send out fibrous roots at every node. Each plant forms large mats up to 

165’ (50 m) in diameter. The fleshy leaf segments are 3-sided, 11⁄2-51⁄2” long 

(4-14 cm), waxy, and green but lined red with age. Flowering occurs almost 

year-round, peaking in spring. Flowers have numerous bright, narrow petals and 

numerous yellow stamens. The fruit is fleshy and contains several glossy brown 

seeds. HI leaf segments are widest below the middle. Flowers are yellow, pink, 

or magenta and 21⁄2-6” in diameter (6-15 cm). SF leaf segments are widest above 

the middle. Flowers are magenta and 11⁄2-21⁄2” in diameter (4-6 cm). Flowers of 

hybrids are magenta and intermediate in size between the two species.

REPRODUCTION: Both species spread by seed and vegetatively. Every plant 

node can produce roots and shoots, so even small plant segments readily grow.

 

HABITAT: Both species capitalize on disturbance and can be found in coastal 

scrub, sand dunes, salt marshes, and along cliffs, roadways, and railway lines 

further inland. Both species are drought and salt tolerant, prefer full sun, and 

are sensitive to frost.

LOOK-ALIKES: The fleshy, triangular leaf segments, sprawling form, and 

magenta (sometimes yellow) daisy-like flowers differentiate these species from 

unrelated plants. See above for traits differentiating highway iceplant from sea fig.

NOXIOUS WEED LISTINGS: Neither species noxious in any SW state.

NOTES: Though not listed as noxious, their potential ecological impact and 

distribution are both high. While considered distinct species in California, there is 

reportedly much hybridization between Carpobrotus edulis and C. chilensis, making

identification difficult. The 

origins of C. chilensis are 

somewhat obscure. It 

may itself be a hybrid 

species. Further study 

is required. 

Highway iceplant Sea fig
Camelthorn

Alhagi maurorum Medik.

42 New Invaders of the Southwest PINKISH PURPLE FLOWERS SYNONYMS: Caspian manna, Persian manna, Alhagi camelorum Fisch., Alhagi 

pseudalhagi (M. Bieb.) Desv. ex B. Keller & Shap.

ORIGIN: Mediterranean, Eurasia

GROWTH TRAITS: Perennial shrub growing 2-3’ tall (0.6 to 0.9 m) from a 

deep and extensive rhizomatous root system. Stems and leaves are gray-green. 

Leaves are small, elliptic with smooth margins, and occur alternately up the 

stem. Sharp, yellow-tipped spines 1-2” long (21⁄2-5 cm) are produced in leaf axils. 

Small, pea-like flowers (having a banner, wing and keel, typical of the pea family) 

are produced from June to August. Flowers range from pink to maroon and are 

borne on short, spine-tipped branches. Reddish-brown seed pods are constricted 

between the individual seeds and are tipped with a small beak. Seeds are dark 

brown and kidney-shaped. Plant growth varies with environmental conditions; 

thorns are smaller and fewer and leaves are larger and more numerous in moister 

climates.

Camelthorn: a) plant section (John M. Randall, The Nature Conservancy, www.bugwood.org), 

b) large plant, from above (Eitan f)

a b
Family Fabaceae

New Invaders of the Southwest 43

REPRODUCTION: By seed and the rhizomatous 

root system. Seeds may remain viable in the soil for 

many years.

 

HABITAT: Found in moist to dry areas in disturbed 

soil.

LOOK-ALIKES: Though individual characteristics 

of camelthorn may resemble other plants, the 

combination of small, pink, and pea-like flowers, 

long spines, short height, and undivided, smoothmargined leaves help differentiate this species. 

Russian salt tree (Halimodendron halodendron) 

is also a spiny shrub with pink pea-like flowers. 

However, Russian salt tree grows larger (up to 10’ or 3 m tall), has compound 

leaves, has wider and lighter-colored flowers, and has seed pods that are not 

constricted.

NOXIOUS WEED LISTINGS: CA, AZ (Prohibited), NV & NM (A)

NOTES: Seeds require scarification and do not 

germinate while exposed to light. Germination 

is highest following flash floods or after 

seeds pass through the digestive system 

of animals.

Camelthorn: c) leaves and thorns (Steve Dewey, Utah State University), d) flowers (Eitan f), 

e) seed pods from spine (John M. Randall, The Nature Conservancy) (c,e www.bugwood.org)

Look-alike: Russian salt tree 

(Sten Porse)

c d e
Russian salt tree

Halimodendron halodendron (Pall.) Voss

44 New Invaders of the Southwest PINKISH PURPLE FLOWERS SYNONYMS: Robinia halodendron Pall.

ORIGIN: Eurasia

GROWTH TRAITS: Multi-branched perennial shrub growing up to 10’ tall 

(3 m) from a deep and rhizomatous root system. Leaves are divided into 2-6 

(usually 4) leaflets clustered on short spurs. Leaflets are egg-shaped, narrower 

at their base, and have tiny tips atop their broad ends. Spines arise from leaf 

bases and from branch tips. Flowers are pea-like (having a banner, wing and 

keel, typical of the pea family) and range from reddish-purple to pinkish-white. 

Flowers are 0.6-0.7” long (15-18 mm) and occur in clusters of 2-4 on short 

spurs in summer. Flowers have 10 stamens with 9 of the filaments fused into a 

tube. Seed pods are inflated, 2-parted, oblong, 0.6-1.2” long (11⁄2-3 cm), and 

turn woody and black at maturity. 

REPRODUCTION: By seed and vegetatively through rhizomes. Seed viability 

is unknown, though seeds of related species can remain viable for several years.

Russian salt tree: a) plant (Sten Porse), b) infestation (Pavel Buršík)

a b
Family Fabaceae

New Invaders of the Southwest 45

HABITAT: Typically found in dry, disturbed sites 

including roadsides, field edges, and nearby 

hillsides. Tolerates saline soil. 

LOOK-ALIKES: Though individual characteristics 

of Russian salt tree may resemble other plants, 

the combination of pink pea-like flowers, long 

spines, and compound leaves help differentiate this 

species. Camelthorn (Alhagi maurorum) is also a 

spiny shrub with pink pea-like flowers. However, 

camelthorn grows smaller (up to 3’ or 0.9 m tall), 

has undivided leaves, has narrower and darkercolored flowers, and has reddish seed pods that are 

constricted between the seeds.

NOXIOUS WEED LISTINGS: CA

NOTES: Introduced as an ornamental shrub. 

Seed pods have been considered good forage 

for livestock, but some seeds can survive 

ingestion, spreading the plant in grazed 

areas.

Russian salt tree: c) flowers (Sten Porse), d) leaflets and spine (Lazaregagnidze), e) maturing fruits 

(Yuriy Danilevsky)

Look-alike: camelthorn (Eitan f)

c d e
Red sesbania

Sesbania punicea (Cav.) Benth.

46 New Invaders of the Southwest RED FLOWERS SYNONYMS: scarlet wisteria, rattlebox, bladderpod, Piscidia punicea Cav., 

Daubentonia tripetii Poit, Daubentonia punicea (Cav.) DC.

ORIGIN: South America

GROWTH TRAITS: Deciduous shrub or small tree typically growing up to 

13’ tall (4 m). The root system is not very extensive, especially in waterlogged 

conditions. The many slender, thornless branches have smooth green bark 

when young, turning red then brown at maturity. Leaves are alternate, 4-8” 

long (10-20 cm), and divided into 5-20 pairs of opposite leaflets. Each leaflet is 

oval-shaped and often ends in a tiny pointed tip. Flowering occurs throughout 

summer. Flowers occur in drooping clusters 3-10” long (8-25 cm). Each flower 

is approximately 1” long (21⁄2 cm), red or orange-red, and pea-like (having a 

banner, wing and keel, typical of the pea family). Fruits are seed pods 2.4-3.1” 

long (6-8 cm) with pointed tips and 4 longitudinal wings. Pods turn dark brown 

and open when mature to release 4-10 small, light brown seeds. 

Red sesbania: a) young plant (Chris Evans, Illinois Wildlife Action Plan), b) branches of mature 

plant (John D. Byrd, Mississippi State University) (a,b www.bugwood.org)

a b
Family Fabaceae

New Invaders of the Southwest 47

REPRODUCTION: By seed only. Seeds germinate 

rapidly and remain viable in the soil for no more 

than two years.

 

HABITAT: Predominantly found in moist soils 

including riverbanks, drainage ditches, and 

depressions where moisture accumulates, but 

can expand into pastures and dry grasslands after 

prolonged periods of rainfall.

LOOK-ALIKES: Many species in the pea family 

(Fabaceae) have similar compound leaves, growth 

form, and preferred habitat. The combination of 

large red flower clusters, thornless branches, shrubby or small tree size, and 

presence in moist soils help differentiate red sesbania. The native hemp sesbania 

(Sesbania exaltata) can occur in similar habitats and has similar leaf shape and 

size. Hemp sesbania has yellow flowers and is typically smaller overall.

NOXIOUS WEED LISTINGS: CA

NOTES: Widely used as an ornamental planting 

because of its attractive flowers and foliage, but 

has escaped cultivation. It is especially 

problematic in South Africa where an 

extensive biological control program has 

been enacted to combat this weed. Foliage, 

flowers, and seeds are toxic to livestock and 

humans.

Red sesbania: c) leaf and leaflets (Rebekah D. Wallace, University of Georgia), d) flowers (Chris 

Evans, Illinois Wildlife Action Plan), e) fruits (Barry Rice, sarracenia.com) (c-e www.bugwood.org)

Look-alike: hemp sesbania 

(Barry Rice, sarracenia.com, 

www.bugwood.org)

c d e
Canary Island knapweed

Volutaria canariensis Wagenitz

48 New Invaders of the Southwest WHITE FLOWERS SYNONYMS: Canary Island starthistle

ORIGIN: Canary Islands

GROWTH TRAITS: Herbaceous annual typically growing to 24” tall (60 cm) 

from a taproot in its native habitat, though much larger individuals have been 

collected in North America. The plant is multi-branched; all branches and stems 

have narrow wings. Leaves are deeply lobed with individual lobes secondarily 

lobed or toothed. Leaves become smaller and less lobed further up the stem. 

Leaves and stems have small hairs. Flowering occurs from spring to summer. 

Flower heads are 0.4-0.6” in diameter (1-11⁄2 cm) and appear singly or in just a 

few numbers at the ends of stems. Floral bracts are tipped with broadly triangular 

and tan-colored spines covered in tiny, tangled hairs. Some bracts are elongated, 

extending beyond florets. Outer petal-like florets are sterile while inner florets 

each produce a single seed. All florets are short; inner florets are hairy at their 

Canary Island knapweed: a) plant (Kate Harper, http://tchester.org/bd/species/asteraceae/volutaria_

canariensis.html), b) multiple smaller plants and habitat (Krzysztof Ziarnek, Kenraiz)

a b
Family Asteraceae

New Invaders of the Southwest 49

base and are often tinged in purple. Seeds are 

elliptic, tan, and have stiff pappus as long as seeds.

REPRODUCTION: By seed. Seeds remain viable 

in the soil for multiple years.

 

HABITAT: Found in hot, arid conditions. Seeds 

sprout following infrequent rain events.

LOOK-ALIKES: Several related Centaurea species 

are invasive in the Southwest. Most can be 

differentiated by having yellow florets and/or very 

long, spiny bracts. Spotted, squarrose, and diffuse 

knapweed (Centaurea stoebe, C. squarrosa, and 

C. diffusa, respectively) have a similar growth form and bract shape, however 

spotted and squarrose knapweed have pinkish florets. Only diffuse knapweed 

can have white florets. All three have longer florets, non-winged stems, and are 

densely hairy, giving the foliage a gray-green appearance.

NOXIOUS WEED LISTINGS: Not listed as noxious in any SW state.

NOTES: Though not listed as noxious in any 

southwestern state, its potential ecological 

impact and distribution are both high. 

Discovered in southern California in 

2010, a handful of plants spread to nearly 

1,000 in just a few years.

Canary Island knapweed: c) leaves (Kate Harper), d) flower head with purple-tinged florets 

(Krzysztof Ziarnek, Kenraiz), e) top of flower head with white florets, f) single inner floret (e,f Tom 

Chester)(c,e,f http://tchester.org/bd/species/asteraceae/volutaria_canariensis.html)

Look-alike: diffuse knapweed 

(Jennifer Andreas, Washington 

State University Extension)

c d e f
Carolina horsenettle

Solanum carolinense L.

50 New Invaders of the Southwest WHITE FLOWERS SYNONYMS: bull nettle, apple of Sodom

ORIGIN: North America, including regions as far west as (moving south) Ontario 

(Canada), South Dakota, Nebraska, New Mexico (US), and Sonora (Mexico).

GROWTH TRAITS: Herbaceous perennial with spiny stems growing up to 

3.2’ tall (1 m) from a deep taproot with an extensive rhizomatous root system. 

Leaves are triangular to oval with deep, irregular lobes. Both surfaces are covered 

with fine hairs and can have spines protruding from the midvein. Flowering 

occurs throughout the summer from April to October. Flowers have 5 whitish 

petals with 5 bright yellow stamens. Petals are fused at their bases. Fruits are 

clusters of round berries that resemble small tomatoes with light green stripes, 

turning yellow as they mature.

REPRODUCTION: By seed and creeping rhizomes. Seed may remain viable in 

the soil for up to 10 years.

 

Carolina horsenettle: a) plant (Ohio State Weed Lab Archive, The Ohio State University, www.

bugwood.org), b) infestation (©2014 Keir Morse)

a b
Family Solanaceae

New Invaders of the Southwest 51

HABITAT: Found in arid to semi-arid disturbed 

areas such as pastures, roadsides, and railroad 

margins with sandy or loam soils.

LOOK-ALIKES: The 5-petaled flowers with 

yellow stamens and berry fruits help differentiate 

this species from unrelated look-alikes. Several 

similar-looking species of native and exotic 

Solanum occur in North America. Most of these 

can be differentiated by having non-lobed leaves, 

different colored fruits, non-spiny stems/leaves, 

or different colored flowers with non-fused petals. 

Sticky nightshade (Solanum sisymbriifolium) has 

similar flowers but produces red berries, and its 

lobed leaves appear almost fern-like. Buffalobur 

(Solanum rostratum) has more deeply lobed leaves and produces yellow flowers. 

NOXIOUS WEED LISTINGS: AZ (Prohibited), CA, NV (B)

NOTES: Though native to portions of North 

America, it is regularly invasive inside and 

outside of its native range. All parts 

are poisonous. Spines can break off 

painfully after puncturing human skin. 

Carolina horsenettle: c) leaf with spines (Rebekah D. Wallace, University of Georgia), d) flower 

(Fritz Flohr Reynolds.), e) fruits and stem spines (James H. Miller & Ted Bodner, Southern Weed 

Science Society) (c,e www.bugwood.org)

Look-alike: buffalobur 

nightshade (Jennifer Andreas, 

Washington State University 

Extension)

c d e
Crystalline iceplant

Mesembryanthemum crystallinum L.

52 New Invaders of the Southwest WHITE FLOWERS SYNONYMS: common iceplant, Cryophytum crystallinum (L.) N. E. Br.

ORIGIN: Africa, Mediterranean

GROWTH TRAITS: Succulent, herbaceous annual (most typical) to short-lived 

perennial with trailing and branched stems growing up to 3.2’ wide (100 cm) 

and 4-8” high (10-20 cm) from a shallow and very short root system. Leaves are 

succulent, 0.8-4” long (2-10 cm), oval or triangular in outline with very wavy 

margins, and light green to red. Stems, leaves, and flower receptacles are covered 

in tiny, clear, blister-like projections resembling frozen dew. Flowering largely 

occurs in spring and early summer, but flowers can be found nearly year-round. 

Flowers are 0.3-0.6” in diameter (7-15 mm), occur on fleshy red receptacles, have 

numerous narrow, white petals aging to pink, and have multiple yellow-white 

stamens and styles. Fruits are 5-celled capsules that burst to release numerous 

brown seeds when conditions are moist.

REPRODUCTION: By seed. Seed viability is unknown.

 

Crystalline iceplant: a) plant (Yummifruitbat), b) small patch (Winfried Bruenken)

a b
Family Aizoaceae

New Invaders of the Southwest 53

HABITAT: Found primarily in saline soils near the 

coast in sage scrub, coastal bluffs and cliffs, and 

other open, disturbed areas without frost.

LOOK-ALIKES: The white petals and clear, tiny, 

blister-like projections covering the plant help 

differentiate this species from unrelated species as 

well as many other iceplants. The exotic slender 

iceplant (Mesembryanthemum nodiflorum) has a 

similar form and habitat, is also covered with the 

clear blisters, and varies from light green to red. 

Slender iceplant is smaller with stems growing 

to 8” long (20 cm), and is lined with knob-like, 

cylindrical, fleshy leaves up to 0.8” long (2 cm). Slender iceplant flowers are 

white but less than 0.4” in diameter (1 cm). 

NOXIOUS WEED LISTINGS: Not listed as noxious in any southwestern state.

NOTES: Though not listed as noxious in any 

southwestern state, its potential ecological 

impact and distribution are both high. 

The stems, leaves, and fruits are edible, 

though the plant accumulates salt. 

After plant death, the salt leaches from 

the decaying plant into the surrounding 

soil, inhibiting the growth of less salt-tolerant 

species.

Crystalline iceplant: c) leaves (Frank Vincentz), d) flower, e) blisters on flower receptacle 

(d,e Yummifruitbat)

Look-alike: slender iceplant 

(Frank Vincentz)

c d e
Greater swinecress

Lepidium coronopus (L.) Al-Shehbaz

54 New Invaders of the Southwest WHITE FLOWERS SYNONYMS: creeping watercress, creeping wart cress, Coronopus squamatus 

(Forssk.) Asch.

ORIGIN: Mediterranean

GROWTH TRAITS: Typically an annual (but occasionally a biennial) growing 

prostrate on the ground from a taproot. Flowering stems are up to 1’ long (30 

cm) but also grow prostrate on the ground. Leaves are alternate, 2-12” long 

(5-30 cm), and are deeply divided into lobes that are toothed or lobed again. 

Flowering occurs from May to October. Flowers are grouped in compact clusters 

in leaf axils. Each flower has 4 tiny white petals and is 1/8” across (3 mm). Fruits 

are small, very wrinkled, flattened, and round with 2 chambers. Each chamber 

contains 1 yellow-brown seed.

REPRODUCTION: By seed only. It is unknown how long seeds may remain 

viable in the soil.

 

Greater swinecress: a) plant (Joseph M. DiTomaso, University of California - Davis, www.bugwood.org), 

b) illustration (Jacob Sturm & Johann Georg Sturm, Deutschlands Flora in Abbildungen, © expired) 

a b
Family Brassicaceae

New Invaders of the Southwest 55

HABITAT: Capitalizes on disturbance and can 

be found in agricultural fields, orchards, turf, 

roadsides, ditchbanks, and other disturbed areas. 

Tolerates saline soils.

LOOK-ALIKES: Though many prostrate weeds 

have similar leaves and growth form, greater 

swinecress can be differentiated by having its 

flowers in compact clusters from leaf axils and by 

its flowers having 4 white petals. Lesser swinecress 

(Lepidium didymum) closely resembles greater 

swinecress but has longer stems, and the foliage of 

lesser swinecress gives off a skunk-like odor when 

crushed or broken.

NOXIOUS WEED LISTINGS: AZ (Prohibited), CA

NOTES: While the foliage of a closely related 

and very similar species (lesser swinecress) has a 

skunk-like odor, the leaves and stems of 

greater swinecress do not.

Greater swinecress: c) leaves and flowers (Joseph M. DiTomaso, University of California - Davis, 

www.bugwood.org), d) flowers (©2001 CDFA), e) fruits (Roger Culos)

Look-alike: lesser swinecress 

(Rasbak)

c d e
Onionweed

Asphodelus fistulosus L.

56 New Invaders of the Southwest WHITE FLOWERS SYNONYMS: asphodel, hollow-stem asphodel

ORIGIN: Mediterranean

GROWTH TRAITS: Herbaceous annual to short-lived perennial growing up to 

30” tall (76 cm) from a fibrous root system. The roots have a series of tuber-like 

parts at stem bases. There are multiple hollow, rigid flowering stems per plant. 

All leaves are basal, giving the plant a bunchgrass appearance. Leaves grow up 

to 20” long (50 cm) and are hollow, though not perfectly cylindrical. Flowering 

occurs in late summer. Flowers are approximately 1⁄2” across (13 mm) and have 

3 true petals and 3 sepals resembling petals. Petals and sepals are elliptic, white 

(sometimes slightly pink), and have a conspicuous reddish-brown midvein. 

Flowers have 6 stamens with white filaments and reddish-brown tips. Fruits are 

wrinkled, round capsules that contain 6 triangular, rough, brown seeds.

REPRODUCTION: By seed. Seeds remain viable in the soil for many years.

 

Onionweed: a) plant (USDA APHIS PPQ Archive, www.bugwood.org), b) infestation (Fan of 

Zhirkov)

a b
Family Liliaceae

New Invaders of the Southwest 57

HABITAT: Found in disturbed places including 

pastures, field edges, roadsides, and dunes. 

Recently becoming problematic in grasslands and 

other wildland sites post burning. 

LOOK-ALIKES: Though onionweed superficially 

resembles Allium species (true onion and garlic), 

onionweed lacks an onion or garlic odor. Allium

spp. have similar leaves, and their flowers have 

6 petals/sepals just like onionweed. However, 

Allium spp. flowers all arise from a single point 

in a spherical inflorescence rather than alternately 

up the stem. The combination of symmetrical 

flowers and clumped, basal, and hollow leaves help 

differentiate onionweed from other unrelated lookalikes.

NOXIOUS WEED LISTINGS: Not listed as 

noxious in any southwestern state.

NOTES: Though not listed as noxious in any 

southwestern state, it is federally listed, and 

its potential ecological impact and 

distribution are both high.

Onionweed: c) leaves (USDA APHIS PPQ Archive), d) flowers (Donald Hobern, Copenhagen, 

Denmark), e) seed (Steve Hurst, USDA-NRCS PLANTS Databae)(c,e www.bugwood.org)

Look-alike: Allium spp. 

(R.A. Howard, Smithsonian 

Institution, USDA-NRCS 

PLANTS Database)

c d e
African rue

Peganum harmala L.

58 New Invaders of the Southwest WHITE FLOWERS SYNONYMS: Syrian rue, wild rue, Esfand, harmel

ORIGIN: northern Africa, Asia, Europe

GROWTH TRAITS: Perennial shrub with a low-growing, bushy habit. The 

plant reaches 2-3’ in height (0.6-0.9 m) and grows from a stout taproot with 

short, creeping lateral roots. Green stems are stiff, fleshy, highly branched, and 

hairless. Leaves are alternate, fleshy, bright green, 0.8-2” long (2-5 cm), and 

irregularly divided into linear segments. Flowering occurs from spring through 

fall. The white, 5-petaled flowers are 1” in diameter (21⁄2 cm) and can be found 

on stalks from leaf axils. Each flower has 10-15 yellow stamens. The fruit is a 2- 

to 4-celled leathery capsule containing 45-60 dark brown, angular seeds. Seeds 

have a distinctive smell, especially when crushed. Aboveground plant parts die 

back in winter.

REPRODUCTION: Primarily by seed, though severed roots can produce new 

shoots. Seeds remain viable in the soil for many years.

 

African rue: a) plant (Yuriy Danilevsky), b) small infestation (Oregon Department of Agriculture) 

a b
Family Zygophyllaceae

New Invaders of the Southwest 59

HABITAT: Found in disturbed environments 

including roadsides and fields in desert to semidesert areas. Often found in soils with high salinity.

LOOK-ALIKES: Vaguely resembles shrubby species 

of sandwort (Eremogone spp.) which also have dark 

green and long, linear leaves and white, 5-petaled 

flowers. Shrubby sandworts usually grow no taller 

than 2⁄3’ (20 cm). Flowers of African rue have 10 to 

15 yellow-tipped stamens while those of sandworts 

have 10 white stamens tipped with brown.

NOXIOUS WEED LISTINGS: AZ (Prohibited), 

CA, NV (A), NM (B)

NOTES: The plant is toxic to both livestock and 

people; the fruit and seeds are most poisonous. 

De-hulled seeds yield edible oil similar 

to cottonseed oil. Plants have been used 

medicinally and for producing red dye in 

the Middle East.

African rue: c) leaves, d) flower (c,d Yuriy Danilevsky), e) fruit (e Douglas Barbe, California 

Department of Food and Agriculture, www.bugwood.org)

Look-alike: sandwort (Steve 

Dewey, Utah State University, 

www.bugwood.org)

c d e
Bridal veil broom

Retama monosperma (L.) Boiss.

60 New Invaders of the Southwest WHITE FLOWERS SYNONYMS: Genista monosperma (L.) Lam., Spartium monospermum L., Lygos

monosperma (L.) Heywood

ORIGIN: Mediterranean

GROWTH TRAITS: Perennial shrub growing up to 10’ tall (3 m) and 20’ across 

(6 m) from a deep taproot. Young plants are dominated by a leading stem; plants 

become more branched and broaden with age. Stems are slender, green, and 

drooping. Leaves are small, linear, and usually simple (not divided into leaflets). 

Leaves are quickly deciduous, making stems leafless much of the year. Flowers 

are small, have purple sepals, and have petals that are white and pea-like (having 

a banner, wing and keel, typical of the pea family). Flowers appear in short 

clusters of 2-20 from the stems in early spring, though some plants have been 

observed flowering all year in warm climates. The fruits are pods that are nearly 

round, typically 0.4” long (1 cm), and contain 1-2 seeds each. 

Bridal veil broom: a) plant (Javier Martin), b) infestation prior to blooming (Jean-Paul Peltier) 

a b
Family Fabaceae

New Invaders of the Southwest 61

REPRODUCTION: By seed. Seeds can remain 

viable in the soil for several years.

 

HABITAT: Does well in dry, rocky, infertile soils 

such as sage scrub, chaparral, and grasslands.

LOOK-ALIKES: Multiple exotic broom species are 

established in similar habitats in North America, 

such as Scotch broom (Cytisus scoparius). These 

other brooms resemble bridal veil broom when 

not in bloom or not fruiting. Their larger yellow 

flowers and long seed pods help differentiate them 

from bridal veil broom, which has small white 

flowers and nearly round pods.

NOXIOUS WEED LISTINGS: CA

NOTES: Likely an escaped ornamental. In 

southern California, an infestation grew from 

10 acres to over 2,000 acres in 6 years.

Bridal veil broom: c) stems and leaves (James Gaither), d) flowers (Javier Martin), e) fruits (Fouad 

Msanda)

Look-alike: Scotch broom 

(Steve Dewey, Utah State 

University, www.bugwood.org)

c d e
Croftonweed

Ageratina adenophora (Spreng.) R. M. King & H. Rob.

62 New Invaders of the Southwest WHITE FLOWERS SYNONYMS: eupatory, sticky snakeroot, Eupatorium adenophorum Spreng., 

Eupatorium glandulosum Michx.

ORIGIN: southern Mexico

GROWTH TRAITS: Small, shrubby perennial growing up to 3-61⁄2’ tall (1-2 

m) from a short, thick taproot with numerous side branches. Multiple stems are 

purplish and shortly branched at their tips. Stems are glandular hairy at first, 

becoming woody with age. Stems can root when they touch the ground, leading 

to a sprawling growth form. Leaves are opposite, triangular, have deep veins, 

have numerous rounded teeth on margins, and grow 1-3.2” long (5-8 cm). Buds 

appear in late winter, and flowering occurs in early spring. Inflorescences are 4” 

in diameter (10 cm) and consist of multiple flower heads 1⁄4” across (6 mm). 

Each flower head is comprised of 10-60 tubular, white or pale pink florets. Seeds 

are dark brown, narrow, and topped with a pappus of long, sparse hairs.

REPRODUCTION: Primarily by seed and by producing roots when stems 

touch the ground. Seeds typically remain viable for up to three years.

 

Croftonweed: a) plant (Forest & Kim Starr), b) infestation (Frank Vincentz) 

a b
Family Asteraceae

New Invaders of the Southwest 63

HABITAT: Capitalizes on disturbance along 

roadsides, pastures, open woodlands, forest 

margins/clearings, and riparian zones in subtropical 

and warmer temperate regions. 

LOOK-ALIKES: The combination of opposite 

leaves, purplish stems, and clusters of white flowers 

help differentiate this species from other shrubby 

look-alikes. Redosier dogwood (Cornus sericea) also 

has colored stems, opposite leaves, and clustered 

white flowers. Redosier dogwood stems are red, and 

its inflorescences consist of individual 4-petal flowers 

rather than clustered flower heads. Several Ageratina

are native to the USA, though only seven occur west of Texas, and three of those 

have alternate leaves. Of the remaining, A. herbacea has yellow-green and smaller 

leaves, A. lemmonii leaves are typically stalkless, A. paupercula has much narrower 

leaves, and A. rochrockii grows shorter (up to 27” or 70 cm) from slender rhizomes.

NOXIOUS WEED LISTINGS: Not listed as noxious in any southwestern state.

NOTES: Though not listed as noxious in any 

southwestern state, it is federally listed, and 

its potential ecological impact and 

distribution are both high. Fatally toxic 

to livestock, especieally horses. The root 

gives off a carrot-like odor when damaged.

Croftonweed: c) leaves (John Tann), d) flowers (Christian Hummert), e) seeds (Julia Scher, USDA 

APHIS PPQ, www.bugwood.org)

Look-alike: redosier dogwood (Albert 

Herring, Superior National Forest)

c d e
Syrian beancaper

Zygophyllum fabago L.

64 New Invaders of the Southwest WHITE FLOWERS SYNONYMS: Syrian bean-caper

ORIGIN: Eurasia

GROWTH TRAITS: Typically a shrubby perennial growing up to 3’ tall (0.9 

m) from a deep, woody taproot with creeping rhizomes. In cold climates, the 

plant may behave as an annual. Stems are multi-branched and hairless. Leaves 

are opposite and each composed of 2 leathery, oval-shaped leaflets. Leaflets have 

smooth margins and are up to 1” long (21⁄2 cm). Flowers appear on stalks at leaf 

axils from spring through early summer. Flowers have 5 green sepals and 5 white 

or cream petals with salmon-colored markings. The 10 stamens are orangish and 

extend beyond the petals. Fruits are 5-celled, drooping capsules up to 11⁄2” long 

(4 cm) that open at maturity to release the small, dark seeds.

REPRODUCTION: By seed, rhizomes, and root fragments. It is unknown how 

long seeds may remain viable in the soil.

 

Syrian beancaper: a) plant (Philmarin), b) infestation (Javier Martin) 

a b
Family Zygophyllaceae

New Invaders of the Southwest 65

HABITAT: Found in dry, open habitats with welldrained soil including rangeland, roadsides, and 

desert areas.

LOOK-ALIKES: The opposite compound leaves, 

white flowers with exerted orange stamens, hanging 

capsule fruits, and shrubby growth form are not 

found in combination on any possible look-alike. 

Most species resembling Syrian beancaper have two 

or less of those traits. Various species in the Fabaceae 

may resemble young and small Syrian beancaper, 

including alfalfa, sweetclover, and medick. These 

can be distinguished by having 3 leaflets on 

alternate leaves and pea-like flowers. Common snowberry (Symphoricarpos albus) 

is a shrub with opposite leaves and whitish flowers; however its leaves are not 

compound, its flowers occur in small drooping pairs, its fruits are white berries, 

and it typically grows in more moist habitats than Syrian beancaper. 

NOXIOUS WEED LISTINGS: CA, NV (A)

NOTES: The flowers have a taste and scent 

similar to caper.

Syrian beancaper: c) leaflets, flower, and immature fruit (Dell O. Clark, California Department of 

Food and Agriculture, www.bugwood.org), d) flower, e) split fruits and seeds (d,e Philmarin)

Look-alike: common snowberry 

(Frank Vincentz)

c d e
Brazilian peppertree

Schinus terebinthifolius Raddi

66 New Invaders of the Southwest WHITE FLOWERS SYNONYMS: Brazilian pepper, Florida holly, Christmasberry

ORIGIN: South America

GROWTH TRAITS: Perennial sprawling shrub or small tree growing 10-32’ 

tall (3-10 m) from a shallow root system. Plants are typically multi-stemmed, 

each with smooth bark that becomes furrowed and scaly with age. Stems can 

grow upright, reclining, or nearly vine-like. Leaves are alternate and divided into 

2-14 opposite leaflets and a single terminal leaflet. Leaflets are dark green, lanceshaped but rounded, 1-4” long (21⁄2-10 cm), and have prominent midveins and 

finely toothed margins. Plants are either male or female; both sexes produce 

large clusters of small, white, 5-petaled flowers from leaf axils in late fall. Some 

individuals may flower again in spring. Fruits appear on female plants in large 

clusters and are tiny, berry-like, and red when mature.

Brazilian peppertree: a) tree (James Steakley), b) infestation (Randy Westbrooks, Invasive Species 

Prevention Specialist, www.bugwood.org) 

a b
Family Anacardiaceae

New Invaders of the Southwest 67

REPRODUCTION: By seed and suckering from 

roots/stems damaged during control efforts. Seeds 

remain viable in the soil for less than six months.

 

HABITAT: Found along abandoned fields, roadsides, 

rights-of-way, levees, marshes, shorelines, and 

undisturbed habitats in surrounding hillsides. 

It tolerates a wide range of conditions at low 

elevations.

LOOK-ALIKES: Many species may look similar 

from a distance (e.g. Ilex or holly species typically 

found in eastern states), but Brazilian peppertree can 

be differentiated by its combination of compound leaves and profuse red fruits. 

The related Chinese pistache (Pistacia chinensis) is an exotic ornamental with an 

even number of leaflets, reddish foliage in fall, and blue fruits (when ripe).

NOXIOUS WEED LISTINGS: Not listed as noxious in any southwestern state.

NOTES: Though not listed as noxious in any 

southwestern state, its potential ecological 

impact and distribution are both high. 

This species escaped cultivation as an 

ornamental and is especially problematic 

in Florida, and is becoming a concern in 

California and beyond. Plants give off a strong 

odor of turpentine or peppers when crushed.

Brazilian peppertree: c) leaf with leaflets (James H. Miller, USDA Forest Service), d) flowers 

(Rebekah D. Wallace, University of Georgia), e) fruits (Stephanie Sanchez) (c-e www.bugwood.org)

Look-alike: Chinese pistache 

(Magnus Manske)

c d e
Mexican fan palm

Washingtonia robusta H. Wendl.

68 New Invaders of the Southwest WHITE FLOWERS SYNONYMS: Mexican palm, Washington fan palm, skyduster

ORIGIN: Mexico

GROWTH TRAITS: Tree typically growing 40-50’ tall (12-15 m) from a fibrous 

root system, though some individuals may reach 80’ tall or more (24.3 m). The 

single trunk is ringed by old leaf bases and tapers from 2’ (60 cm) in diameter 

at the base to 8” (20 cm) near the top. The tree is topped by 20-25 erect leaves. 

Leaves are 3-6’ long (0.9-1.8 m), compound, and split into numerous linear 

leaflets in the arrangement of a fan. Leaf petioles are brownish at their bases 

and edged with thick, sharp teeth. Leaflets frequently have threads along their 

margins. Old leaves remain attached to the trunk, forming a long brown skirt 

beneath healthy leaves. During spring and summer, small white flowers appear 

in clusters on branched inflorescences that are 8-10’ long (2.4-3 m) and hang 

beyond the leaf canopy. These are followed by black berry-like fruits ~1⁄2” in 

diameter (1.2 cm). 

Mexican fan palm: a) tree with fronds attached (Patti Anderson, Division of Plant Industry, www.

bugwood.org), b) tree with fronds dropped, c) infestation (b,c Forest & Kim Starr)

a b c
Family Arecaceae

New Invaders of the Southwest 69

REPRODUCTION: By seed only. It is unknown 

how many years seeds remain viable in the soil, 

though seeds of most palm species lose viability 

quickly.

HABITAT: Found in well-drained soil with full to 

partial sun and most often with moderate moisture, 

including urban areas, riparian zones, canyons, 

roadsides, and orchards. Can tolerate drought.

LOOK-ALIKES: The fan-like leaves with leaflets 

arising from one point help differentiate Mexican 

fan palm from other palm species whose leaflets are 

arranged linearly (feather-like). The native California fan palm (Washingtonia 

filifera) has very similar growth and fan-like leaves. California fan palm grows 

shorter with a thicker trunk, has leaf petioles more green in color and without 

obvious thorns, has more threads along leaflets, and is more cold and drought 

tolerant. Both fan palms hybridize.

NOXIOUS WEED LISTINGS: Not listed as noxious in any southwestern state.

NOTES: Though not listed as noxious in any 

southwestern state, its potential ecological 

impact and distribution are both high. 

It has long been used as an ornamental, 

but frequently escapes to become 

problematic in rural and riparian areas.

Mexican fan palm: d) leaves and thorny leaf petioles (Forest & Kim Starr), e) inflorescences 

(Ginabovara), f) fruit (Mmcknight4)

Look-alike: California fan palm 

(Forest & Kim Starr)

d e f
Austrian & Creeping yellow cress

Rorippa austriaca (Crantz) Besser & R. sylvestris (L.) Besser

70 New Invaders of the Southwest YELLOW FLOWERS

Austrian yellow cress: a) basal leaves, b) flowers, c) fruits (a-c Elizabeth Bella, USDA, bugwood.org)

Creeping yellow cress: d) plant (Joseph DiTomaso, University of California - Davis, www.bugwood.

org), e) flowers and leaves (SB Johnny), f) fruits (Robert Mohlenbrock, USDA SCS, USDA 

PLANTS Database) 

f

e

d

c

b

a
Family Brassicaceae

New Invaders of the Southwest 71

SYNONYMS: Austrian yellow cress (AYC): Austrian fieldcress, Nasturtium 

austriacum Crantz; Creeping yellow cress (CYC): creeping fieldcress

ORIGIN: Eurasia (both species)

GROWTH TRAITS: Both species are herbaceous perennials with rhizomes. 

Stems are 1-3’ tall (0.3-0.9 m). Flowers have 4 petals and 6 stamens, are 

arranged alternately at branch tips, and appear late spring through summer. 

AYC: Stem leaves are alternate, 1-2” long (21⁄2-5 cm), oblong, hairless, and can 

have slightly or very toothed margins. Basal leaves are twice as long and deeply 

lobed, sometimes appearing dissected. Stems are slightly hairy, and upper leaves 

clasp the stem. Flowers are yellow and 1⁄4” (1⁄3 cm) in diameter. Fruits are small 

and spherical but are rarely produced. CYC: Plants often form a dense, creeping 

mat. Leaves are more finely divided than AYC and grow up to 8” long (20 cm). 

Flowers are yellow and less than 1⁄8” (0.6 cm) in diameter. Fruits are long (0.4” 

or 1 cm), thin, and somewhat constricted. 

REPRODUCTION: Both species spread primarily by rhizomes.

 

HABITAT: Both species frequent moist and disturbed areas along roadsides, 

fields, and pastures. CYC almost always occurs near water.

LOOK-ALIKES: Many Brassicaceae in the Southwest have similar flowers and 

leaves. The exact combination of leaf shape, short stature, moist habitat, and 

small flower size differentiate these species from others. The rounded fruit of 

AYC differentiates it from the long, slender-fruited CYC. 

NOXIOUS WEED LISTINGS: AYC: AZ (Prohibited), CA, NV (A); CYC: CA

NOTES: A hybrid of 

these species is more 

invasive in Europe 

than either alone.

Austrian yellow cress Creeping yellow cress
Sahara mustard

Brassica tournefortii Gouan

72 New Invaders of the Southwest YELLOW FLOWERS SYNONYMS: Asian mustard, African mustard, wild turnip

ORIGIN: northern Africa, Mediterranean, Middle East

GROWTH TRAITS: Herbaceous annual growing 4-40” tall (10-100 cm) from 

a stout taproot. Stems are multi-branched near the top of the plant. Lower stems 

have small, stiff, white hairs. Basal leaves are 3-20” long (8-50 cm), deeply lobed 

but not divided, have toothed margins, and have a rough texture with small stiff 

hairs. Stem leaves are small and oblong, appearing bract-like further up the stem. 

Flowering typically occurs from February to April; flower stalks spread away 

from the stem. Flowers are dull yellow, inconspicuous, consist of 4 petals and 6 

stamens, and are 0.6” in diameter (11⁄2 cm). Fruits are 11⁄2-21⁄2” long (3-6 cm) 

and spread outward from stems at 45 degree angles. Each fruit has an obvious 

beak at the tip that is 0.4-0.6” long (10-15 mm) and has 1 seed inside.

REPRODUCTION: By seed. Seeds can remain viable for more than three years.

 

Sahara mustard: a) plant, b) infestation (a,b James M. André)

a b
Family Brassicaceae

New Invaders of the Southwest 73

HABITAT: Primarily invades disturbed, arid habitats 

and is typically found along roadsides and dry creek 

beds. Tolerates high salt concentrations.

LOOK-ALIKES: Many Brassicaceae species in the 

West have similar flowers and fruits. The undivided 

leaves of this plant help differentiate it. Most lookalikes have leaves that are either more deeply lobed 

(at times appearing divided), or leaves that clasp 

the stem. The exotic wild mustard (Sinapis arvensis) 

looks similar with its large basal leaves and more 

naked upper branches. However, wild mustard 

leaves are not as deeply lobed and its flowers are 

larger and a brighter yellow.

NOXIOUS WEED LISTINGS:

NV (B), NM (Watch List)

NOTES: The weak stems enable this plant to 

break and tumble in the wind, spreading seeds 

far and wide. When the seed coats are 

moistened, they become very sticky and 

readily adhere to people, animals, and 

objects.

Sahara mustard: c) leaves (Stan Shebs), d) flowers, e) seed pods (d,e Patrick Alexander, USDA 

NRCS PLANTS Database)

Look-alike: wild mustard 

(Olivier Pichard)

c d e
Shortpod mustard

Hirschfeldia incana (L.) Lagr.-Foss.

74 New Invaders of the Southwest YELLOW FLOWERS SYNONYMS: buchanweed, hoary mustard and Mediterranean mustard, Sinapis 

incana L., Brassica adpressa Boiss., Brassica geniculata (Desf.) J. Ball

ORIGIN: Mediterranean

GROWTH TRAITS: Herbaceous, upright annual, biennial, or perennial 

growing 1-3.2’ tall (30-100 cm) from a deep and slender taproot with multiple 

side branches. The upper half of the plant splits into multiple spreading branches. 

Stems and leaves are covered in hairs. Basal leaves are up to 14” long (35 cm), 

have toothed margins and indented veins, and are deeply lobed but not divided; 

the terminal lobe is the largest. Stem leaves become smaller and less lobed further 

up the stem. Flowering typically occurs from May to October; flower stalks 

spread away from the main stem. Flowers are yellow, consist of 4 petals and 6 

stamens, and are 0.6” in diameter (11⁄2 cm). Leaves are typically retained during 

flowering. Seed pods are 1⁄2” long (13 mm), have an obvious beak at the tip, and 

are held closely against the stem.

Shortpod mustard: a) small plant, b) large plant (a,b Harry Rose)

a b
Family Brassicaceae

New Invaders of the Southwest 75

REPRODUCTION: Primarily by seed, but may 

regenerate from damaged roots/root crowns. It is 

unknown how long seeds remain viable in the soil.

HABITAT: Capitalizes on disturbance, occurring 

along roadsides, in waste places, fields, grasslands 

and creek bottoms.

LOOK-ALIKES: Many Brassicaceae species in the 

West have similar flowers and fruits. The undivided 

leaves of this plant help differentiate it. Most lookalikes have leaves that are either more deeply lobed 

(at times appearing divided), or leaves that clasp 

the stem. The exotic black mustard (Brassica nigra) looks most similar. However, 

black mustard typically grows taller, has smaller flowers, and always grows as an 

annual.

NOXIOUS WEED LISTINGS: Not listed as noxious in any southwestern state.

NOTES: Though not listed as noxious in any 

southwestern state, its potential ecological 

impact and distribution are both high. 

The plant is becoming an increasing 

problem in California. Young leaves and 

flowers are edible.

Shortpod mustard: c) basal leaves, d) flowers (Pato Novoa), e) fruits (c,e Harry Rose)

Look-alike: black mustard 

(Magnus Manske)

c d e
Wall rocket

Diplotaxis tenuifolia (L.) DC.

76 New Invaders of the Southwest YELLOW FLOWERS SYNONYMS: perennial wall rocket, Lincoln weed, sand mustard, Sisymbrium 

tenuifolium L.

ORIGIN: Eurasia

GROWTH TRAITS: Herbaceous perennial typically growing 8-28” tall (20-

70 cm) from a deep taproot. Basal leaves are up to 7” long (18 cm), have a 

prominent and light-colored midvein, and are deeply lobed but not divided. 

Lobes are asymmetrical, and lobe tips are rounded. Most leaves are basal; leaves 

grow alternate, smaller and less lobed further up the stem. Flowering occurs 

throughout summer. Flowers are 0.3-0.6” in diameter (8-11⁄2 cm) and have 4 

yellow petals and 6 yellow stamens. Fruits are narrow pods 3⁄4-2” long (2-5 cm) 

with an obvious beak at their tip. Seeds are small, yellow-brown, and round. All 

parts of the plant have a peppery flavor when consumed.

REPRODUCTION: By seed. It is unknown how long seeds remain viable in 

the soil.

 

Wall rocket: a) plant (AnRo0002), b) infestation (Rasbak)

a b
Family Brassicaceae

New Invaders of the Southwest 77

HABITAT: Capitalizes on disturbance and is 

found along roadsides, waste places, streambanks, 

pastures and railway lines. Tolerates a wide variety 

of soils, but is most often encountered in sandy, 

well-drained soil with high calcium content.

LOOK-ALIKES: Many Brassicaceae species in the 

West have similar yellow flowers and/or long and 

narrow fruits. Most look-alikes do not have the 

peppery flavor of wall rocket, and they have leaves 

that either have larger lobes or leaves that clasp 

the stem. The widely cultivated arugula (rocket, 

Eruca vesicaria subsp. sativa) has very similar leaves 

and long beaked fruits as well as a peppery flavor. 

However, arugula is an annual, has white flowers 

with purplish veins, and has shorter fruits.

NOXIOUS WEED LISTINGS: NM (Watch List)

NOTES: Consumption by grazing livestock can 

taint meat and dairy.

Wall rocket: c) leaves (Leo Michels), d) flower, e) fruits (d,e Rasbak)

Look-alike: arugula (Javier 

Martin)

c d e
Capeweed

Arctotheca calendula (L.) Levyns

78 New Invaders of the Southwest YELLOW FLOWERS SYNONYMS: South African capeweed, cape dandelion, cape gold, Arctotis 

calendula L, Cryptostemma calendula (L.) Druce

ORIGIN: South Africa

GROWTH TRAITS: Low-growing, herbaceous, winter annual to perennial with 

a stoloniferous root system. Stems are hairy, ribbed, and creep on top of or just 

below the surface, rooting at nodes and forming new rosettes. Stems can reach 

up to 9’ long (23⁄4 m) in a single growing season and form a dense groundcover. 

Leaves are 2-10” long (5-25 cm) and deeply and irregularly lobed. Leaves are 

bright green and lightly hairy above; they are densely hairy and white beneath. 

Flowering occurs nearly year-round but peaks in spring. Flower heads are ~2” 

in diameter (5 cm) and occur singly on hairy stalks 6-8” tall (15-20 cm). Outer 

florets (<20) are petal-like, sterile, and yellow with purple-tinged bases. Inner 

florets are numerous, tiny, and dark purple or yellow. Inner florets produce oval, 

brown achenes, each surrounded by a dense tuft of light, woolly hairs. Seeds are 

sterile in the infertile form of the plant frequently used in landscaping.

Capeweed: a) plant (Xemenendura), b) infestation (Forest & Kim Starr)

a b
Family Asteraceae

New Invaders of the Southwest 79

REPRODUCTION: Infertile form spreads only via 

stolons. Fertile form spreads with seeds and stolons. 

Seed viability is unknown.

 

HABITAT: Found in disturbed areas with welldrained soil and full sun including coastal sites, 

vacant urban areas, riparian zones, and roadsides. 

Tolerates wet or dry soils, but is limited by frost.

LOOK-ALIKES: The large flower heads with yellow 

outer florets, and tiny, usually darker, inner florets 

help set this species apart from many others in the 

Asteraceae family. It can be differentiated from 

most remaining look-alikes by being very low-growing and having deeply lobed 

leaves. Cape marigold (Dimorphotheca sinuata) is a winter annual that has also 

escaped cultivation. Cape marigold sometimes has similar yellowish flowers with 

purple-tinged florets and can form dense infestations. Cape marigold does not 

have stolons, or hairy leaf undersides, or woolly hairs around seeds.

NOXIOUS WEED LISTINGS: CA

NOTES: An infertile form widely used as an 

ornamental has frequently escaped cultivation. 

A fertile form that was more recently 

introduced has become problematic as 

well. These are considered by some to be 

separate species.

Capeweed: c) leaves (Forest & Kim Starr), d) flower head (Ghislain Chenais), e) fruit (Deena 

Walters, USDA APHIS PPQ CPHST ITP, www.bugwood.org)

Look-alike: cape marigold 

(Malcolm Manners)

c d e
Crown daisy

Glebionis coronaria (L.) Cass. ex Spach

80 New Invaders of the Southwest YELLOW FLOWERS SYNONYMS: garland chrysanthemum, chop suey green, Chrysanthemum 

coronarium L.

ORIGIN: Mediterranean, southwestern Asia

GROWTH TRAITS: Herbaceous, upright annual growing up to 3.2’ tall (1 m) 

from a taproot. The main stem spreads into numerous branches with smooth 

surfaces. Leaves are alternate, up to 3” long (8 cm) and deeply lobed. Lobes are 

themselves secondarily lobed into tiny, rounded-tooth projections. Inflorescences 

appear at stem tips from spring through early summer. Flower heads are 0.8-2.4” 

in diameter (2-6 cm) with numerous tiny, yellow florets in the center and 13-

21 petal-like florets on the periphery. Petal-like florets are either solid yellow or 

white with yellow bases. Flower heads produce numerous small brown seeds 

(achenes) that have no pappus.

REPRODUCTION: By seed only. Seeds remain viable in the soil for 3-4 years.

 

Crown daisy: a) plant (MathKnight and Zachi Evenor), b) infestation (MathKnight)

a b
Family Asteraceae

New Invaders of the Southwest 81

HABITAT: Found under a variety of conditions 

in riparian areas, coastal dunes, prairies, and 

scrubland.

LOOK-ALIKES: The daisy-like inflorescence 

helps differentiate this species from unrelated 

look-alikes. Several chamomile species (spanning 

multiple genera) have similar flower heads and 

similar, deeply-lobed leaves. Many look-alikes can 

be differentiated by having solid white petal-like 

florets (rather than yellow or white with yellow). 

Other look-alikes with all yellow florets have leaves 

that are less lobed or divided. The exotic golden 

chamomile (Cota tinctoria) has similar leaves and flower heads, however its 

stems are typically less branched and have fewer leaves below the flower head. 

Golden chamomile leaves are also typically more jaggedly lobed and symmetrical 

compared to crown daisy.

NOXIOUS WEED LISTINGS: Not listed as noxious in any southwestern state.

NOTES: Though not listed as noxious in any 

southwestern state, its potential ecological 

impact and distribution are both high. 

Frequently escapes cultivation to invade 

disturbed areas.

Crown daisy: c) foliage (Dalgial), d) yellow inflorescence (Laitche), e) white and yellow inflorescence 

(Joaquim Alves Gaspar)

Look-alike: golden chamomile 

(Oskar Gran)

c d e
Globe chamomile

Oncosiphon piluliferum (L. f.) Källersjö

82 New Invaders of the Southwest YELLOW FLOWERS SYNONYMS: stinknet, Cotula pilulifera L. f., Matricaria pilulifera Druce

ORIGIN: South Africa

GROWTH TRAITS: Herbaceous annual typically growing 1-2’ tall (15-60 

cm) from a taproot. Stems are multi-branched, have longitudinal grooves, and 

are often covered in very fine hair. Leaves are alternate, covered in small hairs, 

and finely divided (appearing somewhat fern-like). Leaves are up to 2” long (5 

cm), becoming progressively smaller and less divided up the stem. Flower heads 

appear singly at branch tips in spring. Flower heads are up to 1⁄2” in diameter 

(11⁄4 cm) and resemble round, yellow globes. Each is comprised of numerous 

tiny, tubular, yellow florets. Achenes are up to 0.08” long (2 mm) with a short 

crown of white pappus scales. All parts of the plant have a very pungent and 

usually disagreeable odor. 

REPRODUCTION: By seed. Seed viability is unknown. 

Globe chamomile: a) plant (©2014 Phillip Ruttenbur, plantworlds.org), b) infestation (Liz 

Makings, swbiodiversity.org)

a b
Family Asteraceae

New Invaders of the Southwest 83

HABITAT: Capitalizes on disturbance; frequently 

found in urban waste places, crops, pastures, 

coastal scrub, and along roadsides.

LOOK-ALIKES: Globe chamomile’s inflorescences 

lack petal-like florets and consist of numerous 

tiny, tubular florets. This helps differentiate it 

from all unrelated look-alikes. Within the family, 

many other native and exotic species have similar 

flower heads, including the weedy pineappleweed 

(Matricaria discoidea), which very closely resembles 

globe chamomile. Pineappleweed is typically lower 

growing (3-12” or 7-30 cm), its flower heads are 

more cone-shaped, and both the leaves and flower heads give off a distinctive 

pineapple odor when crushed.

NOXIOUS WEED LISTINGS: Not listed as noxious in any southwestern state.

NOTES: Though not listed as noxious in any 

southwestern state, its potential ecological 

impact and distribution are both high. 

This ornamental is escaping cultivation 

to become problematic in Arizona and 

California.

Globe chamomile: c) leaf and stem, d) flower heads (Max Licher, swbiodiversity.org), e) dried flower 

head with achenes (c,e ©2014 Thomas Palmer)

Look-alike: pineappleweed 

(Walter Siegmund)

c d e
Rush skeletonweed

Chondrilla juncea L.

84 New Invaders of the Southwest YELLOW FLOWERS SYNONYMS: skeleton weed, hogbite, nakedweed, gum succory

ORIGIN: Eurasia, Mediterranean

GROWTH TRAITS: Herbaceous biennial or perennial growing 1-4’ tall (1⁄3-

1.2 m) from a rhizomatous root system. Plants produce multiple stems that are 

wiry and rigid, giving the plant an overall skeleton appearance. Bottom portions 

of stems are covered with stiff, golden-reddish, and downward pointing hairs; 

upper stems are nearly or fully hairless. Rosettes have deeply lobed leaves; stem 

leaves are alternate and very narrow. Flower heads appear in later summer singly 

or in small clusters at leaf axils or stem tips. Each flower head is 1⁄2” across (1 

cm) and consists of ~9-12 yellow florets that produce seeds without fertilization. 

Seeds are small, brown, and topped by tufts of pappus. All parts of the plant 

exude a milky latex sap when damaged.

REPRODUCTION: By seed and vegetatively from the rhizomes. Seeds may 

remain viable in the soil for up to four years.

Rush skeletonweed: a) plant (Rachel Winston, MIA Consulting), b) infestation (Laura Parsons, 

University of Idaho)

a b
Family Asteraceae

New Invaders of the Southwest 85

HABITAT: Found on rangeland, abandoned 

cropland, roadsides, and other disturbed places 

often in semiarid conditions with cool, moist 

winters and hot summers. 

LOOK-ALIKES: Many weedy species are similar to 

rush skeletonweed with their lobed rosette leaves, 

wiry stems with stiff hairs, pappus-tufted seeds, 

and milky latex sap from damaged tissue, but they 

differ in a few other features. Chicory (Cichorium 

intybus) produces large blue flower heads. Prickly 

lettuce (Lactuca serriola) has roughly lobed leaves 

continuing up the stem, each with a line of spines 

on the undersides of the midvein.

NOXIOUS WEED LISTINGS:

AZ (Prohibited), CA, NV (A)

NOTES: This weed is well established and very 

problematic in northwestern North America, 

where biological control is actively 

utilized. Multiple approved agents 

are available. The weed is becoming an 

increasing concern in the Southwest.

Rush skeletonweed: c) rosette leaves and stem spines (Richard Old, XID Services, Inc, www.

xidservices.com, www.bugwood.org) d) flower head (Rachel Winston, MIA Consulting,), e) seeds 

(Leo Michels)

Look-alike: prickly lettuce (Mary 

Ellen Harte, www.bugwood.org)

c d e
Sicilian starthistle

Centaurea sulphurea Willd.

86 New Invaders of the Southwest YELLOW FLOWERS SYNONYMS: sulphur knapweed

ORIGIN: Mediterranean

GROWTH TRAITS: Upright, herbaceous winter annual typically growing 1-3’ 

tall (1⁄3-1 m) from a taproot. Large plants are branched; small plants have a single 

stem. Stems have wings ~1⁄4” wide (5-6 mm) at their largest. Leaves are alternate, 

may be lightly toothed or lobed, and become smaller up the plant. Leaves and 

wings are yellow-green and sparsely covered with stiff hairs. Flowering occurs 

from May to July when 1” (21⁄2 cm) flower heads develop singly at the ends of 

stems. Florets are yellow and numerous. Floral bracts are tipped with 3-5 pairs of 

small spines and a central main spine up to 1” long (21⁄2 cm). Spines are strawcolored at their tips and dark brown at their bases. Seeds are 1⁄4-1⁄3” long (5-8 

mm), glossy, brownish-tan, and with dark brown pappus as long as seeds.

Sicilian starthistle: a) plant with immature capitula, b) senescing infestation (a,b Joseph DiTomaso, 

University of California - Davis, www.bugwood.org)

a b
Family Asteraceae

New Invaders of the Southwest 87

REPRODUCTION: By seed. It is unknown how 

long seeds remain viable in the soil, but seeds of 

related species are viable for up to three years. 

 

HABITAT: Open, disturbed sites including 

grasslands, fields, pastures, roadsides, and waste 

places.

LOOK-ALIKES: The spiny capitula help 

differentiate this species from non-starthistle 

species in the Northwest. Yellow florets separate 

this species from purple-colored starthistles. Malta 

and yellow starthistle (Centaurea melitensis and 

C. solstitialis, both exotic) have yellow florets, but 

both have gray-green foliage due to numerous white cottony hairs. Flower heads 

of Sicilian starthistle are larger than Malta and yellow starthistle, and Sicilian 

starthistle seeds are twice as large and darker than seeds of the other two. 

NOXIOUS WEED LISTINGS: AZ (Prohibited), CA

NOTES: Though not yet documented in the 

Southwest, Sicilian starthistle is established in 

neighboring northern areas and is suited 

to the climate of the Southwest. Where 

the species co-occur, Sicilian starthistle 

often out-competes yellow starthistle.

Sicilian starthistle: c) leaves and winged stem (Joseph DiTomaso, University of California - Davis, 

www.bugwood.org), d) flowering capitulum, e) senescing capitulum with seeds, from above 

(d,e Daniel Montesinos)

Look-alike: yellow starthistle 

(Jennifer Andreas, Washington 

State University Extension)

c d e
Smooth & Woolly distaff thistle

Carthamus creticus L. & C. lanatus L.

88 New Invaders of the Southwest YELLOW FLOWERS

Smooth distaff thistle: a) plants (©2001 CDFA), b) capitula, c) seeds (b,c Zoya Akulova)

Woolly distaff thistle: d) plant (Javier Martin), e) capitulum, f) seeds (e,f Philmarin) 

f

e

d

c

b

a
Family Asteraceae

New Invaders of the Southwest 89

SYNONYMS: Smooth distaff thistle (SDT): Carthamus baeticus (Boiss. & Reut.) 

Pérez Lara, C. lanatus L. ssp. baeticus (Boiss. & Reut.) Nyman, C. lanatus L. ssp. 

creticus (L.) Holmboe; Woolly distaff thistle (WDT): downy safflower, saffron 

thistle, C. lanatus L. ssp. lanatus

ORIGIN: SDT: Mediterranean, WDT: Mediterranean, Eurasia

GROWTH TRAITS: Both species are herbaceous winter annuals growing upright 

to 3.2’ tall (1 m) from slender, fibrous taproots. Stems are straw-colored. Stems 

and leaves are covered in tiny glandular hairs. Stem leaves are alternate, stiff, 

weakly clasp the stem, and often spread backwards. Leaves have lobes tipped with 

sharp spines. Basal leaves are larger than stem leaves and are typically absent at 

flowering. Flower heads appear solitary on stem tips in summer. Each capitulum 

has several rows of bracts and yellow florets. Outer bracts are stiff, leaf-like, and 

spread backward; inner bracts are smaller, spiny, and less lobed. Seeds are brown 

achenes. Outer seeds are rough and lack pappus; inner seeds are smoother with 

stiff, persistent pappus. SDT: Hairs on stems, leaves, and flower heads are only 

slightly woolly. Seed pappus is 0.3-0.4” long (8-10 mm). WDT: Hairs on stems, 

leaves, and flower heads are very woolly. Seed pappus is 0.4-0.5” long (10-13 mm). 

REPRODUCTION: Both species spread by seed only. Most seeds germinate 

within three years, though some may remain viable in the soil for eight.

 

HABITAT: Both species are found in disturbed, open sites including agricultural 

fields (especially grain), grasslands, and pastures.

LOOK-ALIKES: Their spiny nature differentiates these species from nonthistle look-alikes. Most other thistles do not have yellow florets or stiff, thick, 

backward-spreading leaves. Golden thistle (Scolymus hispanicus) is most similar 

but has milky sap and winged stems. 

NOXIOUS WEED LISTINGS: Both species: CA

NOTES: Both weeds are 

sometimes classified 

of subspecies of 

Carthamus lanatus.

Their gene numbers 

differ, however, and the 

species do not hybridize. Smooth distaff thistle Woolly distaff thistle
Stinkwort

Dittrichia graveolens (L.) Greuter

90 New Invaders of the Southwest YELLOW FLOWERS SYNONYMS: stinkweed, camphor inula, cape khakiweed, Erigeron graveolens L.,

Inula graveolens (L.) Desf.

ORIGIN: Mediterranean

GROWTH TRAITS: Herbaceous annual typically growing 8-30” tall (20-75 

cm) from a taproot with extensive, smaller side roots. Stems are multi-branched. 

The entire plant is sticky to the touch, being covered in small, white, and slightly 

glandular hairs with a strong camphor-like odor. Stems and branches are covered 

in alternate, linear leaves 0.4-11⁄2” long (1-4 cm). Flower heads are produced in 

leaf axils and stem tips throughout fall. Each capitulum is 0.2-0.3” in diameter 

(5-7 mm). Outer florets are yellow and larger than inner florets which are yellow 

to red. Seeds are brown, oval achenes with a hairy pappus. 

REPRODUCTION: By seed. Seeds remain viable in the soil for up to three years.

 

Stinkwort: a) plant, b) infestation (a,b Javier Martin)

a b
Family Asteraceae

New Invaders of the Southwest 91

HABITAT: Primarily invades disturbed, open 

habitats, such as cultivated land, abandoned fields, 

overgrazed pastures, roadsides, and tidal margins.

LOOK-ALIKES: With its narrow leaves and bushy 

appearance, stinkwort resembles young Russian 

thistle (Salsola tragus) and kochia (Bassia scoparia), 

though its yellow flower heads and sticky, odorous 

foliage help differentiate it from these look-alikes. 

Many tarweeds (Centromadia, Hemizonia and 

Holocarpha spp.) are sticky, odorous, and have 

yellow flower heads. However their foliage often 

appears more spiny than stinkwort, and their flower 

heads are typically larger. The native mountain 

tarweed (Madia glomerata) has small yellow flower heads, linear leaves, and is 

sticky and odorous. However, mountain tarweed grows much smaller and less 

branched, and its flower heads are typically clumped.

NOXIOUS WEED LISTINGS: Not listed as noxious in any southwestern state.

NOTES: Though not listed as noxious in any 

southwestern state, its potential ecological 

impact and distribution are both 

high. Touching the foliage can cause 

dermatitis, itchy skin, or blistering. 

When grazed, this plant can cause milk or 

meat to be tainted.

Stinkwort: c) leaf, bud and stem (Javier Martin), d) flower head, e) mature achenes, still attached 

(d,e ©2014 Keir Morse)

Look-alike: mountain tarweed 

(Matt Lavin, Bozeman, MT, 

USA)

c d e
Bermuda buttercup

Oxalis pes-caprae L.

92 New Invaders of the Southwest YELLOW FLOWERS SYNONYMS: buttercup oxalis, African wood-sorrel, Oxalis cernua Thunb., 

Bolboxalis cernua (Thunb.)

ORIGIN: southern Africa

GROWTH TRAITS: Perennial herb growing from a bulb with a single, short, 

vertical stem that is largely underground. Leaves form a loose basal rosette. Leaf 

petioles are less than 4.7” long (12 cm). Each leaf has 3 heart-shaped leaflets 

arising from the same point, with each leaflet up to 1.4” in diameter (3.5 cm). 

Leaflets often have tiny purple spots. Flowering occurs from late fall through 

early spring. Flowering stems are leafless, typically less than 12” long (30 cm), 

and are topped by clusters of 3-20 flowers that arise from the same point. Each 

flower is 1-11⁄2” in diameter (21⁄2-4 cm), has 5 bright yellow petals fused at their 

base, and has 10 stamens. Fruits and seeds have not been observed in North 

America; the plant instead spreads vegetatively. Small, whitish “bulbils” develop 

on the stem at the base of the rosette leaves. Bulbs are larger (0.3-1.2” long or 

8-30 mm), brown, and produced underground on roots. 

Bermuda buttercup: a) plant (MathKnight and Zachi Evenor), b) infestation (Harry Rose)

a b
Family Oxalidaceae

New Invaders of the Southwest 93

REPRODUCTION: Vegetatively through bulbs 

and bulbils. Does not produce seed.

 

HABITAT: Found in coastal dunes, scrub, oak 

woodlands, gardens, turf and lawns, urban areas, 

orchards, agricultural fields, and open or semishaded hillsides.

LOOK-ALIKES: Bermuda buttercup leaves 

somewhat resemble those of clovers, however clover 

flowers are very different with their distinctive 

pea-like appearance (having a banner, wing and 

keel). Creeping woodsorrel (Oxalis corniculata) is 

a similar exotic species. Creeping woodsorrel has creeping, above-ground stems 

and does not produce underground bulbs. Creeping woodsorrel also has smaller 

leaflets and smaller flowers (up to 0.6” in diameter or 11⁄2 cm) borne in smaller 

clusters on short stalks.

NOXIOUS WEED LISTINGS: Not listed as noxious in any southwestern state.

NOTES: Though not listed as noxious in any 

southwestern state, its potential ecological 

impact and distribution are both 

high. Introduced as an ornamental, 

but has escaped cultivation to become 

problematic in a variety of habitats. 

Toxic to livestock when consumed in large 

quantities.

Bermuda buttercup: c) leaf, d) flower (MathKnight), e) roots with bulbs (c,e Daniel Feliciano)

Look-alike: creeping woodsorrel 

(J.M.Garg)

c d e
Fennel

Foeniculum vulgare Mill.

94 New Invaders of the Southwest YELLOW FLOWERS SYNONYMS: sweet fennel, common fennel

ORIGIN: Europe, Mediterranean

GROWTH TRAITS: Herbaceous, upright perennial growing 3.2-10’ tall (1-3 

m) from a deep taproot. Stems (typically 10-20) re-sprout from the root crown 

each year. Stems are hollow, longitudinally grooved, and branched at nodes. 

Leaves are alternate, expanded at the base, and clasp the stem. Leaves grow up 

to 16” long (40 cm) and are finely divided into thread-like leaflets. Flowering 

typically occurs from August to October. Flowers appear on compound, flattopped umbels typically 4” in diameter (10 cm). Individual flowers are yellow 

with 5 petals often curling inwards. Fruits are brown, long and oval-shaped, and 

have obvious ridges.

REPRODUCTION: By seed, regeneration from the root crown, and root 

division. Seeds may remain viable in the soil for many years.

HABITAT: Frequently escapes cultivation to invade open waste areas with dry, 

Fennel: a) plant (©2010 Steven Thorsted), b) infestation (H. Zell)

a b
Family Apiaceae

New Invaders of the Southwest 95

sandy soils including roadsides, vacant lots, 

abandoned fields, and riverbanks.

LOOK-ALIKES: Numerous Apiaceae species are 

present in North America and resemble fennel 

with their yellow umbel flowers and hollow stems. 

Most look-alikes don’t grow as tall or upright as 

fennel, or have thicker and less-dissected leaves. 

Dill (Anethum graveolens) also escaped cultivation 

and is similar with its yellow umbel inflorescences, 

thread-like leaves, upright growth, and aromatic 

foliage. However, dill is an annual and grows only 

16-24” tall (40-60 cm). Three varieties of fennel are present in the United States. 

Two are very similar in appearance (var. vulgare and var. dulce) and have been 

observed escaping cultivation to become weedy. The third (var. azoricum) is 

cutivated as a vegetable and not invasive. It can be differentiated by its swollen 

bases which give that part of the plant a bulbous appearance.

NOXIOUS WEED LISTINGS: Not listed as noxious in any southwestern state.

NOTES: Though not listed as noxious in any 

southwestern state, its potential ecological 

impact and distribution are both high. It 

is already widely established in California, 

but is also becoming problematic elsewhere 

in the Southwest. The plant has been used 

for medicinal and culinary purposes since 

Roman times and two varieties have escaped 

cultivation in the USA. All parts of the plant have a strong anise-like odor.

Fennel: c) leaf and stem (©2010 Steven Thorsted), d) inflorescence (H. Zell), e) fruits (Philmarin)

Look-alike: dill (ekenitr)

c d e
Dudaim melon

Cucumis melo L. var. dudaim (L.) Naudin

96 New Invaders of the Southwest YELLOW FLOWERS SYNONYMS: Queen Anne’s melon, smell melon, Cucumis dudaim L.

ORIGIN: Africa, southern Asia

GROWTH TRAITS: Vining annual with prostrate stems growing up to 33’ long 

(10 m) from a taproot. Some vines root from leaf nodes that touch the ground, 

helping the plant form large, extensive mats. Vines have several branches near the 

base. Each vine is slender, angled in cross-section, and has short stiff hairs that 

give it a rough feel. Leaves are alternate, covered in short hairs, shallowly lobed 

(3-7), and approximately 3” long (8 cm). A single, unbranched tendril arises 

from each leaf node. Flowers appear in leaf nodes from March to November. 

Flowers are typically 1” in diameter (21⁄2 cm) and have 5 yellow petals fused at 

their bases to form a shallow cup. Fruits (melons) are oblong to round, 1.2-2.4” 

wide (3-6 cm), and green when young but turn either mottled/striped yellowishorange or solid yellowish-orange at maturity. They are produced until the first 

hard frost. Seeds within are sticky, teardrop-shaped, and 0.2” long (1⁄2 cm). 

Dudaim melon: a) plant (John D. Byrd, Mississippi State University, www.bugwood.org), 

b) infestation (©2001 CDFA)

a b
Family Cucurbitaceae

New Invaders of the Southwest 97

REPRODUCTION: By seed. Seeds remain viable 

in the soil for up to 20 years.

 

HABITAT: Found in disturbed areas with plenty of 

moisture, including agricultural fields and margins, 

roadsides, and ditchbanks.

LOOK-ALIKES: Dudaim melon can be 

differentiated from related cultivars of Cucumis 

melo by its leaves that are only shallowly lobed, its 

smaller flowers, and its distinctive fruits which are 

much smaller, smoother, and colored differently 

than related species. Wild cucumber (Echinocystis 

lobata) is an annual vine with small but spiny fruits 

and long clusters of white flowers.

NOXIOUS WEED LISTINGS:

AZ (Prohibited), CA

NOTES: Several subspecies and varieties of 

muskmelon (Cucumis melo) are cultivated, 

including honeydew, crenshaw, casaba, 

cantaloupe, Persian melon, Christmas 

melon, and Armenian cucumber. 

Dudaim melon can readily hybridize 

with cultivated varieties, making this 

weed especially problematic in muskmelon 

agricultural areas.

Dudaim melon: c) leaves and flower, d) flowers (Howard F. Schwartz, Colorado State University, 

www.bugwood.org), e) seed pods (c,e ©2001 CDFA)

Look-alike: wild cucumber 

(Barbara Tokarska-Guzik, 

University of Silesia, www.

bugwood.org)

c d e
Canary Island St. Johnswort

Hypericum canariense L.

98 New Invaders of the Southwest YELLOW FLOWERS SYNONYMS: Canary Island hypericum, Hypericum canariense var. floribundum

(Aiton) Bornm., Hypericum floribundum Aiton

ORIGIN: Canary Islands

GROWTH TRAITS: Perennial, multi-stemmed shrub growing up to 10’ tall (3 

m) from a rhizomatous root system. Leaves are opposite, waxy, lance-shaped, 

typically 2-23⁄4” long (5-7 cm), and have a prominent midvein. Leaves are 

green throughout winter, turning orange in the summer before dropping to the 

ground. In spring and early summer, clusters of yellow-orange flowers appear at 

branch tips. Flowers are ~0.6” in diameter (11⁄2 cm) and have 5 petals, 5 sepals, 

3 styles, and numerous stamens. Petals are much larger than sepals, and the 

sepals have tiny hairs along their margins. Fruits are oval-shaped capsules. When 

mature and dried, fruits shed hundreds of tiny seeds that are brown and 0.04” 

long (1 mm).

Canary Island St. Johnswort: a) shrub, b) infestation (a,b Forest & Kim Starr)

a b
Family Hypericaceae

New Invaders of the Southwest 99

REPRODUCTION: By seed and vegetatively 

through the rhizomes. It is unknown how long 

seeds remain viable in the soil.

 

HABITAT: Found in disturbed areas in grasslands, 

coastal scrub, moist hillsides, and along roadsides, 

but is expanding inland. Tolerates high elevations.

LOOK-ALIKES: The combination of five large 

yellow petals, numerous stamens, and opposite 

leaves help differentiate this weed from unrelated 

look-alikes. Within the genus, several native and 

exotic species are established in North America. 

Most Hypericum occurring in the Southwest are forbs rather than shrubs. 

Canary Island St. Johnswort’s long, lance-shaped leaves and its small sepals with 

hairy margins help differentiate young plants from those of non-shrub species. 

Hooker’s St. Johnswort (Hypericum hookerianum) is an exotic shrub that is 

cultivated and also escaped in CA. Its petals and leaves are more rounded, and it 

grows shorter and bushier than Canary Island St. Johnswort.

NOXIOUS WEED LISTINGS: CA

NOTES: Often grown as an ornamental, but 

has escaped cultivation in North America 

and other parts of the world.

Canary Island St. Johnswort: c) leaves, d) flower (Eric in SF), e) fruits (c,e Forest & Kim Starr)

Look-alike: Hooker’s St. 

Johnswort (H. Zell)

c d e
Woman’s tongue tree

Albizia lebbeck (L.) Benth.

100 New Invaders of the Southwest YELLOW FLOWERS SYNONYMS: lebbeck, Mimosa lebbeck L., Acacia lebbeck (L.) Willd.

ORIGIN: Indomalaya, New Guinea, northern Australia

GROWTH TRAITS: Deciduous tree typically growing 49-65’ tall (15-20 m) 

from a shallow but extensive root system. The tree is multi-stemmed when 

growing in the open, but capable of having a single trunk (1.6’ or 1⁄2 m in 

diameter) when grown on plantations. The crown is spreading and umbrellashaped. The bark is grayish-brown and lightly fissured. Each leaf is divided into 

2-4 smaller sections which are each divided further into 2-11 pairs of leaflets. 

Leaflets are oval-shaped, 0.6-21⁄2” long (11⁄2-61⁄2 cm), and close at night when 

leaflets are young. Raised glands occur on the undersides of leaf stalks at leaf 

bases and between pairs of leaflets. Fluffy inflorescences appear at branch tips or 

leaf axils in early spring. Inflorescences are yellow-green, 2-3” in diameter (5-71⁄2 

cm), and consist of 15-40 non-showy flowers, each with more than 10 very long 

stamens fused at their base. Seed pods are 6-12” long (15-30 cm), light brown, 

papery-leathery, occur in large numbers, and contain 3-12 flat, brown seeds. 

Woman’s tongue tree: a) tree in fruit (Forest & Kim Starr), bark (Dinesh Valke)

a b
Family Fabaceae

New Invaders of the Southwest 101

REPRODUCTION: By seed. Seeds remain viable 

in the soil for up to 30 years.

 

HABITAT: Tolerates a wide variety of conditions 

and is frequently found on riverbanks, roadsides, 

and dry hillsides, as well as in savannahs, forests, 

and bushy areas.

LOOK-ALIKES: Several species in this family 

(native and exotic) resemble woman’s tongue tree 

with their compound leaves and similar leaflets, 

fluffy inflorescences, and long seed pods. Many 

look-alikes do not have yellow inflorescences. The 

most similar species belong to Acacia or Mimosa.

The stamens of Acacia spp. are not fused at their base. Mimosa spp. have 10 or 

fewer stamens per flower while Acacia spp. and woman’s tongue tree both have 

more than 10.

NOXIOUS WEED LISTINGS: Not listed as noxious in any southwestern state.

NOTES: Though not listed as noxious in any 

southwestern state, its potential ecological 

impact and distribution are both high. 

The wood is valued for timber in its 

native range. Throughout the world, the 

tree is frequently used in erosion control, 

though it quickly escapes into nearby 

habitats.

Woman’s tongue tree: c) leaves and flowers, d) flowers, e) seed pods (c-e Forest & Kim Starr)

Look-alike: Acacia mearnsii 

(Forest & Kim Starr, Starr 

Environmental bugwood.org)

c d e
Glossary

102 New Invaders of the Southwest GLOSSARY achene A small, one-seeded fruit that does not split at 

maturity

alternate Where leaves appear singly at stem nodes, on 

alternate sides of the stem

annual A plant that completes its life cycle in one year and 

then dies

awn A hair- or bristle-like appendage extending from 

florets of many grasses

axil The angle between the upper side of a leaf or stem 

and the stem or branch that supports it.

basal Located at the base of a plant or plant part

biennial A plant that lives two years, typically flowering and 

fruiting its second year

bolting Plant stage at which the flower stalk begins to grow

bract A small, leaf-like structure below a flower

bulbil Tiny secondary bulb that forms in the angle between 

a leaf and stem or in place of flowers on certain plants

capitulum (pl. capitula) Seed head or flower head of a plant in the sunflower 

family

compound leaf A leaf consisting of two or more leaflets borne on the 

same leaf stalk

deciduous Sheds its leaves annually

density Number of individuals per unit area

divided Synonym for compoud leaf

erect Grows upright and vertical as opposed to prostrate 

(spreading on the ground)

exotic Not native 
New Invaders of the Southwest 103

floret One of the small, closely clustered flowers forming 

the head of a composite flower in the sunflower 

family or the flowering unit of a grass spikelet, 

consisting of the flower and its two enveloping bracts

flower head A special type of inflorescence consisting of numerous 

florets that actually look like one flower

forb Herbaceous plant (does not have woody stems)

herbaceous Does not have woody stems

hyperaccumulator A plant capable of growing in soils with very high 

concentrations of metals, absorbing these metals 

through their roots, and concentrating extremely 

high levels of metals in their tissues

inflorescence The flowering part of a plant

involucre A circle of bracts under an inflorescence

lag phase First stage of a typical plant invasion during which 

populations remain at low levels for several years. 

Plants ofen become abundant during the next phase

leaflet A leaf-like part of a compound leaf. Though it 

resembles an entire leaf, a leaflet is not attached to 

the main plant stem or branch as a leaf is, but rather 

on a the leaf stalk

ligule A thin outgrowth at the junction of leaf and leafstalk 

of many grasses and sedges

lobed A leaf with shallow or deep, rounded segments, as in 

a thistle rosette leaf

native A plant that originated in the geographic area of discussion

node Part of the stem of a plant from which a leaf, branch, 

or root grows

nontarget effect When control efforts affect a species other than the 

species they were enacted to control (can be positive 

or negative)
104 New Invaders of the Southwest GLOSSARY opposite Where leaves appear in twos at stem nodes, on 

opposite sides of the stem

ovate Shaped like an egg, with the base wider than the tip

pappus A tuft of hairs, scales, or bristles at the base of an 

achene in flowers of the sunflower family

perennial A plant that lives for more than two years

petiole Leaf stalk that usually attaches it to a plant stem

prostrate Grows flat along the ground as opposed to growing 

erect (upright)

receptacle Part of the stem to which the flower is attached

rhizome A modified stem of a plant that grows horizontally 

underground, often sending out roots and shoots 

from its nodes

rosette A compact, circular, and normally basal cluster of 

leaves

scarification Cutting the seed coat using abrasion, thermal stress, 

or chemicals to encourage germination

seed head Synonym for capitulum of a plant in the sunflower 

family. Consists of a receptacle and florets

senescence Final stage in a plant’s life cycle

sepal A small, (typically) green petal-like structure beneath 

petals that protect petals in bud and support petals 

during open flowering

spadix Inflorescence with several tiny flowers clustered on a 

narrow, fleshy stem

spathe Leaf-like curved bract surrounding a spadix

stamen The pollen-producing reproductive organ of a flower
New Invaders of the Southwest 105

stolon Stem which grows at the soil surface or just below 

ground that forms adventitious roots at the nodes, 

and new plants from the buds (also called runner)

style A stalk structure in female flower parts

succulent Plants having some parts that are more than normally 

thickened and fleshy, usually to retain water in arid 

climates or soil conditions

taxonomy The classification of organisms in an ordered system 

that indicates natural relationships. The science, 

laws, or principles of classification; systematics

tiller Side shoot that arises from the base of the stem in 

grasses

toothed Leaf margin that is regularly incised, such as for a saw

tussock Tuft or clump of growing bunchgrass

umbel An inflorescence which consists of a number of short 

flower stalks which spread from a common point, 

somewhat like umbrella ribs. They can be simple or 

compound (the single flowers are replaced by many 

smaller umbels called umbellets)

variegated Plant foliage having leaves that are edged or patterned 

in a second color, especially white as well as green

whorled Where multiple leaves or flowers radiate outward 

from a single stem node

winter annual A plant that germinates in autumn, lives through the 

winter, and produces seed and dies in the following 

season
Selected References

106 New Invaders of the Southwest REFERENCES Affernia, M. 2012. Euphorbia dendroides L.: A semi-succulent shrub. Euphorbia 

World 8 (2): 24-26.

Aikio, S., R.P. Duncan and P.E. Hulme. 2010. Lag-phases in alien plant invasions: 

separating the facts from the artefacts. Oikos 119:370–378.

Ashigh, J., J. Wanstall and F. Sholedice. 2010. Troublesome weeds of New Mexico. New 

Mexico State University, College of Agricultural, Consumer and Environmental 

Sciences Cooperative Extension Service. Las Cruces, New Mexico. 105 pp.

Batlle, I. and J. Tous. 1997. Carob tree. Ceratonia siliqua L. Promoting the conservation 

and use of underutilized and neglected crops. 17. Institute of Plant Genetics and 

Crop Plant Research, Gatersleben/International Plant Genetic Resources Institute, 

Rome, Italy.

Bell, C.E. 1991. Creeping Wartcress (Coronopus squamatus): A New Weed in 

Southeastern California. Weed Technology 5(3): 635-638.

Bossard, C.C., J.M. Randall, and M.C. Hoshovsky. 2000. Invasive Plants of 

California’s Wildlands. University of California Press. Berkeley, CA. 360 pp.

Brigham, C. 2006. Euphorbia terracina: Be afraid, be very afraid. Presentation by the National 

Park Service, Santa Monica Mountains National Recreation Area. 28 pp. [Online]. 

Available: http://www.cal-ipc.org/symposia/archive/pdf/2006/EDBrigham.pdf.

Briefel, J. 2012. Ravenna grass. Invasive Plant Alert, Ed. and M. Frey and A. DuPrey. 

National Park Service, National Capital Region Exotic Plant Management 

Team, Washington, DC. 

CABI. 2014. Invasive Species Compendium, CABI Publishing, Wallingford, UK.

Calflora: Information on California plants for education, research and conservation, 

with data contributed by public and private institutions and individuals, including 

the Consortium of California Herbaria. [web application]. 2014. Berkeley, 

California: The Calflora Database [a non-profit organization].Available: http://

www.calflora.org/ (Accessed: Jul 27, 2014). 

DiTomaso, J.M. and E.A. Healy. 2007. Weeds of California and Other Western States. 

University of California Agriculture and Natural Resources. Oakland, CA.

DiTomaso, J.M., G.B. Kyser et al. 2013. Weed Control in Natural Areas in the 

Western United States. Weed Research and Information Center, University of 

California. 544 pp.

Encycloweedia. 2014. Integrated Pest Control, California Department of Food and 

Agriculture. University of California, Davis, Cooperative Extension Service, 

Weed Science Program. Available: http://www.cdfa.ca.gov/plant/ipc/weedinfo/

winfo_table-sciname.html. [2014, June 21].
New Invaders of the Southwest 107

Harris, F., K. Harper, and T. Chester. 2014. Volutaria canariensis, Canary Island 

knapweed. [Online]. http://www.tchester.org/bd/species/asteraceae/volutaria_

canariensis.html. [18 August 2014].

Hauser, A. Scott. 2008. Pennisetum ciliare. In: Fire Effects Information System, 

[Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Research Station, Fire Sciences Laboratory (Producer). Available: http://www.

fs.fed.us/database/feis/ [2014, August 17].

Jepson Flora Project (Eds.: Bruce G. Baldwin, David J. Keil, Staci Markos, Brent 

D. Mishler, Robert Patterson, Thomas J. Rosatti, Dieter H. Wilken). 2013. 

Jepson eFlora, http://ucjeps.berkeley.edu/IJM.html, accessed on Jun 11 2014.

Meyer, Rachelle. 2011. Schinus terebinthifolius. In: Fire Effects Information 

System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Research Station, Fire Sciences Laboratory (Producer). Available: 

http://www.fs.fed.us/database/feis/ [ 2014, August 1].

ODA. 2014. Noxious Weed Control Program. [Online]. Oregon Department 

of Agriculture. Available: http://www.oregon.gov/ODA/PLANT/WEEDS/

Pages/statelist2.aspx [2014, June 1].

USDA, ARS, National Genetic Resources Program. 2014. Germplasm Resources 

Information Network - (GRIN) [Online Database]. National Germplasm 

Resources Laboratory, Beltsville, Maryland. Available: http://www.ars-grin.

gov/cgi-bin/npgs/html/taxon.pl?403159 [2014].

WNWCB. 2014. [Online]. Washington State Noxious Weed Control Board. 

Available: http://www.nwcb.wa.gov. [2014, June 1].

Zouhar, Kris. 2005. Fire Effects Information System, [Online]. U.S. Department 

of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences 

Laboratory. Available: http://www.fs.fed.us/database/feis/ [2014, June 20].

References for Plant Identification and Terminology

Elpel, T. J.. 2006. Botany in a day: the patterns method of plant identification. 

HOPS Press, LLC. 221 pp.

Go Botany: https://gobotany.newenglandwild.org/simple/

Harrington, H.D. and L.W. Durrell. 1957. How to Identify Plants. Swallow Press, 

Chicago, IL. 203 pp.

Wildflowers and Weeds: http://www.wildflowers-and-weeds.com/Plant_Families/

Patterns_in_Plants.htm


English to Metric Conversions (Length)

Symbol When You Know Multiply By To Find Symbol

in or ” inches 2.54 centimeters cm

ft or ’ feet 0.305 meters m

yd yards 0.914 meters m