United States Department of Agriculture

Field Guide for Managing

Malta Starthistle in the Southwest

Forest 

Service

Southwestern

Region TP-R3-16-01 Revised June 2017
Cover Photos

Top photos: Tonto National Forest

Bottom photo: Forest and Kim Starr, Starr Environmental, bugwood.org

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Malta starthistle (Centaurea melitensis L.)

Sunflower family (Asteraceae)

Malta starthistle is listed as a noxious weed in New 

Mexico. Thisfield guide serves as the U.S. Forest Service’s

recommendations for management of Malta starthistle in 

forests, woodlands, rangelands, and deserts associated with 

the Service’s Southwestern Region. The Southwestern 

Region covers Arizona and New Mexico, which together 

have 11 national forests. The Region also administers 4 

national grasslands located in northeastern New Mexico, 

western Oklahoma and theTexas panhandle.

Description

Malta starthistle (synonyms: Napa starthistle, tocalote)is 

an annual invasive weed with foliage and winged stems

that are grayish to green in color. Its thistle-like appearance 

is similar to yellow starthistle (C. solstitialis), butMalta

starthistle has smaller yellow flowers and longer seedpods 

that are armed with relatively shortspines (less than 1/2 

inch). Strategies for managing both species are similar.

Growth Characteristics

• Winter annual and occasional biennial; grows erect 

to 1 to 2 feet tall.

• Deep, simple taproot.

• Thick leaves held in a basal rosette through winter

and early spring until flower stems bolt; narrow 

leaves smooth-edged near the tip and lobed at the

base; covered with thick, stiff “prickly” hairs and 

dot-like resinous glands that may be overlaid by fine

white “cottony” hairs.

• Produces from 1 to over 100 solitary, spiny, yellow 

flower heads from April through September;flowers

about 1/3 to 1/2-inch long; purple to brown-tinged 

flower base has fine hairs and a branched spine 

surrounding a central spine; insect pollinated.

• Reproduces by seed; 1 to over 60 seeds per flower

head; seeds about 1/10 of an inch long with gray to 

tan stripes.

Ecology 

Impacts/Threats

Malta starthistle is highly competitive and often develops 

dense, impenetrable stands that displace desirable

vegetation. The threat of injury from spines on the seed

heads diminishes recreational opportunities, livestock

grazing, and other resource values. Malta starthistle has 

also been implicated in producing an incurable

neurological disorder in horses known as “chewing 

disease.” Aneurotoxic sesquiterpene lactone in starthistles 

called repin is believed to be the underlying cause of the 

disorder. However, animals in general typically avoid the 

weed because of the sharp spines and hairs.

Site/Distribution

The invasive weed occurs on open disturbed sites such as 

grasslands, rangelands, open woodlands, fields, pastures, 

roadsides, waste places, and cultivated fields. However, it 

is uncommon in desert regions. Malta starthistle isfound 

throughout most western States and some States in the

central, eastern, and southern parts of the United States as 

well. It ranges up to 7,200 feet in elevation.

Spread

Seeds adhere to surfaces and, thus, can be carried for long 

distances on undercarriages of vehicles and road 

maintenance equipment and for shorter distances on

animals and humans. Birds can also transport seeds after 

eating them.

Invasive Features

Malta starthistle is a prolific seed producer. The starthistle 

grows rapidly and is highly adaptable to environmental

variation. It typically out-competes native plant species for 

sunlight, space, water, and nutrients. Other features that

facilitate invasiveness include a deep taproot that accesses 

available soil moisture, winged stems that dissipate heat,

and formidable spines at maturity that deter grazing by

livestock or wildlife.
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Management

Malta starthistle grows rapidly as an invasive plant, and 

seeds may remain dormant in the soil up to 10 years. An 

integrated weed management approach with different 

control methods is usually necessary for long-term 

success. The following actions should be considered when 

planning a management approach to controlstarthistle:

• Maintain healthy plant communities to suppress or 

limit the impact of a starthistle infestation.

• Incorporate sound grazing management with any 

control strategy.

• Detect and eradicate infestations soon after 

discovery. 

• Map known large infestations and keep annual 

records of reported infestations.

• Combine mechanical, cultural, biological, and

chemical methods to control Malta starthistle

populations whenever possible.

• Include monitoring and a follow-up treatment plan 

for missed plants and seedlings.

Table 1 summarizes some management options for

controlling Malta starthistle under various situations. 

Further details on these management options follow the 

table. Choice of individual control method(s) for 

starthistle depends on many local factors including extent

of infestation, current land use, and site conditions such as

Table 1. Management options*

Site Physical Control Cultural Control Biological Control Chemical Control

Roadsides Use machinery such as mowers 

or graders for mechanical

clearing.

Use seed, mulch, and fill 

materials certified to be weedfree.

Implement reporting of 

infestations along roads and

sanitary requirements for 

operation of vehicles.

Control with biocontrol

agents is little researched.

Use truck spraying

equipment. Wash

underneath vehicles to

prevent spread.

Rangeland Use tillage or prescribed fire if 

feasible. May need to use hand 

tools in difficult terrain.

Use seed and forage hay certified 

to be weed-free; use pellets for 

horses in backcountry areas.

When moving livestock or 

vehicles through infested areas, 

inspect and remove any seeds 

from animals, clothing, and

vehicles before entering uninfested areas.

Manage grazing closely to

prevent overuse or 

toxicity to horses.

Consider prescribed 

grazing during spring

with an intense, shortduration approach in 

combination with other 

control methods.

Use ground or aerial

broadcast spraying; 

however, backpack

spraying may be more

practical in areas with

difficult access.

Wilderness 

and other

natural

areas

Hand methods may be needed to

protect certain resources.

Use seed and forage hay certified

to be weed-free; use pellets for

horses in backcountry areas.

Post signs warning visitors to 

remove seeds.

When moving livestock or 

vehicles through infested areas, 

inspect and remove seeds from

animals, clothing, and vehicles

before entering un-infested areas.

Manage grazing closely to

prevent overuse or 

toxicity to horses.

Consider prescribed 

grazing during spring

with an intense, shortduration approach in 

combination with other 

control methods.

Use backpack sprayers.

Broadcast spraying by 

aerial or ground methods

may be used on thicker

stands if allowed.

* Choice of a particular management option must be in compliance with existing regulations for the land resource.
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terrain, accessibility for treatment, microclimate, non-target 

flora and fauna present, etc. Other important considerations 

include treatment effectiveness, overall cost, and the number 

of years needed to achieve control. Typically, more than one 

control method may be needed for a particular site.

Physical Control

Physical methods to control Malta starthistle shouldfocus 

on removal of seed heads and the root system. These

methods usually have to be repeated and must be timed

properly to be most effective.

Manual Methods

Hand pulling and hoeing are effective for small

infestations of Malta starthistle, but this must be done 

repeatedly. Plants should be removed in early bolt before 

flowers have opened and gone to seed. The taproot should 

be removed as much as possible.

Mechanical Methods

If using machinery to manage Malta starthistle, the 

equipmentshould be cleaned immediately after field 

operations to prevent movement of seeds into un-infested

areas.

Tillage – When feasible, frequent tillage with a plow or 

disc will control Malta starthistle. Tillage should be done 

whenthe surface soil is dry since fragmented plant 

segments can regrow in moist soil. Shallow cultivation 

(five or six times a year, 2 weeks apart) should be repeated 

while leaves are present but before plants have flowered. 

Regular cultivation for 2 or more years must be maintained 

for long-term effectiveness.

Mowing – Mowing is commonly used to reduce starthistle 

seed production; however, mowing during early plant 

growth can cause greater production of flowers and seed. 

Some vegetation management experts do not recommend 

mowing at all since mown plants often produce side 

branches with more flowers, even with repeated mowing 

and proper timing. When appropriate, mowing should take 

place only when plants are in late bud or early bloom 

stage. Mowing should occur regularly (e.g.,weekly or 

biweekly) at a level that will remove the lowest branches. 

Leaves should not be left below the level of the cut.

Prescribed Fire

Burning conducted from January to April can eliminate

Malta starthistle during the rosette stage provided there is

a source of fine fuels sufficient to carry an intense, uniform

fire. Malta starthistle may also be burned in early to midsummer (late June to early July) during the early flower

stage. However, prescribed fire operations during this

period may not be feasible in some areas due to the hazard

of causing an uncontrolled fire. Burning at other times

may increase seed production and enhance survival of

established plants. Research currently underway is 

investigating the combination of fire with follow-up

herbicide treatments for improved control, but results are 

unknown at this time.

Cultural Control

Early detection and plant removal are critical for

preventing establishment of Malta starthistle. The local 

public should be educated to help prevent Malta starthistle 

from becoming established. Vehicles, humans, and 

livestock should be discouraged from traveling through 

infested areas; and a program to check and remove seeds 

from vehicles and livestock after going through infested 

areas should be implemented to help stop dispersal. Seed 

and materials used for mulch, forage, or fill should be 

certified to be weed-free; pellets may be used for horses in 

backcountry areas.

Biological Control

Grazing

Sheep, goats, and cattle may graze Malta starthistle in

early spring when plants have developed flowering stems

but before they have spiny heads. Excessive grazing favors 

growth of Malta starthistle over grass species.Although

grazing can reduce the presence of starthistle, owners of 

horses and other livestock should ensure thatsuitable

alternative forage is available. Owners should also look for 

signs of toxicity or so-called “chewing disease” in 

starthistle stands that have flowering heads.

As compared to yellow starthistle, less is known about 

managing Malta starthistle with livestock grazing. Infested 

land may need to rest for 11⁄2 years after treatment

(herbicide, reseeding, disking, etc.) before grazing can
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Table 2. Classical biocontrol agents approved for Malta starthistle

Species Type of

Agent Site of Attack Impact onHost Use/Considerations for Release

Bangasternus orientalis weevil Eats flowers and developing

seed

Limited Little researched

Puccinia juncea var.

solstitialis

rust fungus Undetermined Unknown Released in California

occur. Grazing methods successfully used in controlling 

yellow starthistle will likely also control Malta starthistle 

to some degree. Therefore, prescribed grazing applied in 

an intensive, short-duration approach may be effective for 

managing Malta starthistle, especially when combined 

with other control methods.

Classical Biological Control

Biocontrol agents for Malta starthistle have not been 

researched as well as for yellow starthistle, although

some biocontrol agents may affect both species. The 

two approved biocontrol agents known to affect Malta 

starthistle are shown in table2.

Organisms (insects, pathogens, etc.) used as biocontrol 

agents in southwestern States should be adaptable to 

arid environments and local conditions. Public, tribal, 

and private land managers may obtain biocontrol agents 

for release directly from local offices of the USDA 

Animal and Plant Health Inspection Service (APHIS) 

when these agents are available. Other sources for 

biocontrol agents include locally developed insectaries 

or private companies. 

A permit must be obtained from APHIS before 

biocontrol agents can be transported across State lines. 

Regulations and permit applications (PPQ 526 permit 

forms) pertaining to interstate shipment of biocontrol 

agents can be found at https://www.aphis.usda.gov/ 

aphis/ourfocus/planthealth/import-information/permits/ 

regulated-organism-and-soil-permits/sa_apply/ 

ct_plantpest_howtoapply. Although biocontrol agents 

may be collected and released internally in a given State 

without an APHIS permit, the State’s department of 

agriculture or agricultural extension service should be 

consulted for any regulations relating to movement of 

these agents within the State.

Chemical Control

Malta starthistle is best controlled with post-emergent 

broadleaf herbicides since these chemicals generally 

have little or no effect on grass species. The main entry

for herbicide into the plant is through the leaves with 

only minor entry through the roots. All herbicides listed 

in table 3 will effectively control Malta starthistle when 

properly applied. However, these herbicides will also 

impact other broad-leaved species that have emerged. 

This includes woody species that may also be impacted. 

Each herbicide product has specific requirements and 

restrictions; therefore, it is important to read the label 

carefully and follow all instructions when mixing and 

spraying. Aquatically approved herbicide formulations 

and surfactants must be used in or near water.

Herbicide Application

The most effective period to spray Malta starthistle is 

from December through April during the seedling to 

early rosette stage since lower rates of herbicide can be 

applied. When in the late rosette or bolting stage, higher 

rates should be used. Herbicides should be applied 

before flowering when good growing conditions exist 

and there is 4 to 6 inches of growth. Since Malta 

starthistle is typically an annual, application of 

herbicide during or after flowering is ineffective.

Herbicides to control Malta starthistle may be applied

by backpack or hand-held sprayers, ATV or UTV 

sprayers, or conventional boom sprayers that are pulled 

or attached to a tractor or truck. Any equipment used to 

spray herbicide should be calibrated. Precautionary 

measures should be taken to protect desirable plants. If 

Malta starthistle infestations are large enough, aerial 

application with herbicide on targeted areas may be 

warranted.
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Table 3. Herbicide recommendations

Common

Chemical

Name (active 

ingredient)

Product

Example1

Broadcast 

Treatment 

(rate per

acre)

Spot 

Treatment 

(spray 

solution)2

Time of

Application Remarks

Clopyralid Reclaim 2/3–1 pint 1–3% Early rosette stage; 

use higher rate at 

bolting to bud 

stage.

Wet foliage thoroughly. Do not 

spray when plants are defoliated by 

late freeze, hail, insects, or other

unfavorable conditions. Effects are

shown within 2 to 4 weeks.

Clopyralid + 2,4-D3 Curtail 0.25–1 pint Same as above. After most rosettes

have emerged, but

before buds form.

Same as above.

Aminocyclopyrachlor

+ chlorsulfuron

Perspective 3–4.5 ounces Consult label

for spot

applications.

Lower rate for 

rosette; higher rate 

at bolting. Fall or 

spring.

Selective herbicide used on noncrop sites; may cause temporary 

injury to some grassspecies.

Aminocyclopyrachlor

+ metsulfuron methyl

Streamline 4.75–9.5 ounces Same as 

above.

Same as above. Same as above.

Aminopyralid + 

2,4-D3

GrazonNext 1.5–2 pints 1–3% Same as above. Same as above.

Picloram4 Tordon 22K 1–3 pints Same as above. Same as above. Same as above.

Picloram4 + 2,4-D3 Grazon P+D 1–2 quarts Same as above. Same as above. Same as above.

Dicamba + 

diflufenzopy

Overdrive 4–8 ounces Same as above. Same as above. Same as above.

Dicamba + 2,4-D3 Weedmaster 1 pint to 1 quart 3–5% Same as above. Same as above.

2,4-D3 several

manufacturers

1–2 quarts 5–10% Same as above. Same as above.

Metsulfuron Escort 1 ounce NA Same as above. May take 2 to 3 months to show 

effects.

Metsulfuron + 2,4-D3

+ dicamba

Cimarron Max Rate III:

1 ounce (Part A) 

4 pint (Part B)

NA Same as above. May take 1 to 3 months to show 

effects.

Imazapyr Arsenal 1 pint 1% All stages Spray to have total plant control

(e.g., along roadsides). May take 2 to 

3 months to show effects.

In addition to spray drift, nontarget plants may also be killed or 

injured by imazapyr through 

runoff, residue movement in soil, 

or root exudates from treated 

plants.

1 Trade names for products are provided for example purposes only, and other products with the same active ingredient(s) may be available. Individual 

product labels should be examined for specific information and appropriate use with Malta starthistle.

2 Spray solution is the herbicide/water ratio in a spray mix that may be used for spot treatment with backpack or hand-held sprayers. The amount of product 

applied during an annual growing season must not exceed the maximum application rate per acre as specified by the product label – refer to the product label 

for the site type and application.

3 2,4-D is a restricted use pesticide in New Mexico only. A certified applicator’s license is required for purchase and use.

4 Restricted-use pesticide - A certified applicator’s license is required for purchase and use.
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Integrated Control Methods

An option for controlling Malta starthistle is to use a 

treatment combination of herbicide-prescribed fire. In 

this sequence, clopyralid herbicide is applied during the 

first year of treatment. The clopyralid will substantially 

reduce starthistle while allowing grasses to become 

established. Herbicide treatment can then be followed by 

burning in the following year (or possibly in 2 years). The

treatment combination of herbicide-prescribed fire greatly 

reduces starthistle infestations to insignificant or very low 

levels. The combination also benefits range plant 

communities by increasing species diversity and enhancing 

the quality and quantity of forage.

Management Strategies

Initial treatment of Malta starthistle should attempt to 

eliminate as much of the weed population as possible. 

Secondary treatmentshould include monitoring and 

additional control measures such as spot spraying with 

backpack sprayers or prescribed fire. More than one 

control method may be necessary over years of treatment.

Herbicide application may be necessary for restoration of 

areas infested with Malta starthistle. Infested areas can be

treated initially by intensive spraying to control the 

starthistle. Deep-rooted native perennial grasses are then 

seeded to establish erosion control. Native broadleaf forbs

such as lupines may be seeded at a later time to restore a

more balanced mix of plants into the system. This 

reseeding strategy is potentially useful for roadways and

may be adapted to other treatment situations depending on 

local circumstances.

In nearly all cases involving Malta starthistle management, 

a long-term commitment of greater than 3 years is usually 

necessary to deplete the seed bank. Since it is ordinarily 

useless to treat an area only one time without retreatment, 

sufficient resources must be allocated for the area where 

control is attempted. After initial treatment, it is especially 

important that resources are also available to respray or 

retreat the treated area as necessary. Previously treated 

areas should be monitored continuously to control 

recovering starthistle.

References and

Further Information

DiTomaso, J.M., W.T. Lanini, C.D. Thomsen, T.S. 

Prather, C.E. Turner, M.J. Smith, C.L. Elmore, 

M.P. Vayssieres, and W.A. Williams. 2007. Pest 

notes: yellow starthistle. Publ. No. 7402. Univ. 

Calif. Agric. Nat. Resources. Davis, CA.

DiTomaso, J. Yellow starthistle information. UC Weed 

Research Inform. Center. Available at 

http://wric.ucdavis.edu/yst/index.html.

DiTomaso, J., G.B. Kyser, and M.S. Hastings. 1999. 

Prescribed burning for control of yellow starthistle 

(Centaurea solstitialis) and enhanced native plant 

diversity. Weed Sci. 47:233-242.

Donaldson, S., and D. Rafferty. Identification and 

management of Malta Starthistle (Centaurea 

melitensis L.). Univ. of Nev. Coop. Exten. Serv. 

Fact Sheet-02-86. Reno, NV 89520. Available at 

http://www.unce.unr.edu/publications/files/nr/2002/

FS0286.pdf

Turner, J.L., K. Duncan, and J. LeFevre. 2011. Russian 

knapweed and yellow starthistle poisoning of 

horses. Guide B-170. Coop. Ext. Service, College 

of Agriculture, Consumer, and Environmental 

Sciences. New Mexico State Univ. Las Cruces, 

NM. Available at http://aces.nmsu.edu/pubs/_b/b710.pdf.

Suggested Web Sites

For information on invasive species:

http://www.invasivespeciesinfo.gov/

http://www.invasive.org/weedus/index.html

For information about calibrating spray equipment: NMSU 

Cooperative Extension Service Guide A-613 

Sprayer Calibration. Available at 

http://aces.nmsu.edu/pubs/_a/A-613.pdf

Herbicide labels online:

http://www.cdms.net/LabelsMsds/LMDefault.aspx
For more information or

other field guides, contact:

USDA Forest Service 

Southwestern Region

Forest Health

333 Broadway Blvd., SE 

Albuquerque, NM 87102

Or visit the Southwestern Region’s 

website for invasive species:

http://www.fs.usda.gov/goto/r3/invasivespecies

The use of trade or firm names in this publication is for reader 

information only and does not imply endorsement of any 

product or service by the U.S. Department of Agriculture. 

Recommendations made here for 

pesticide use are not obligatory, 

nor do they imply that discussed 

uses have been registered. All 

uses of pesticides must be 

registered by appropriate State 

and/or Federal agencies before 

they can be applied. 

CAUTION: Pesticides can be 

injurious to humans, domestic 

animals, desirable plants, and fish or other wildlife—if they 

are not handled or applied properly. Use all pesticides 

carefully and lawfully. Follow recommended practices for the 

disposal of surplus pesticides and pesticide containers.