Rangeland Weeds

By James A. Young and Charlie D.

Clements, Range Scientists

USDA, Agricultural Research Service

A weed is a plant that grows in an 

environmental setting where it is 

not wanted. The determination of 

the value of a particular plant is 

subjective—one person’s weed may 

be another person’s prized plant. 

Natural resource manager need 

to be familiar with established 

definitions for types of weeds when 

dealing with weed problems.

Classification of Life 

Forms

Traditionally, the life form classifications of weeds on rangelands 

have been annuals, herbaceous 

perennials, and woody perennials. 

The annuals are primarily exotic 

species, many of which have spread 

over vast areas of rangelands until 

they characterize entire landscapes. 

Cheatgrass (Bromus tectorum L.; 

Figure 1) in the Intermountain 

Area or Russian thistle (Salsola 

tragus L.; Figure 2) throughout 

much of the West are examples of 

annual range weed species. Herbaceous perennial weeds may also be 

exotic species. They often reproduce 

both by seed and by creeping roots 

or rhizomes. Herbaceous perennial 

weeds may be landscape characterizing species, such as leafy spurge 

(Euphorbia esula L.), or may be 

restricted to specific environments, 

such as hoary cress (Cardaria draba 

[L.] Desv.). In both instances, 

these herbaceous perennial weeds 

are usually extremely difficult to 

control. Few exotic woody perennials are weeds on rangelands; however, saltcedar (Tamarix ramosissima 

Ledeb.) is an important exception. 

Most of the weeds in this classification are native species that increase 

in density or invade new habitats 

in response to an environmental 

stimulus. Many species of rabbitbrush (Chrysothamnus spp.) or 

singleleaf pinyon (Pinus monophylla

Torr. & Frem.) are native species 

that can become invasive perennial 

woody weeds.

 

Classification by Life Cycle

The basic life cycles of weeds are 

annual, biennial, and perennial. 

Annuals complete their life cycles 

from germination through seed 

production in 1 year. Many species are classified as winter annuals 

because they germinate in the fall 

of 1 year and complete their life 

cycles in the spring of the next year. 

Cheatgrass is an example, although 

it can germinate in spring and still 

complete its life cycle within the 

same year. Halogeton (Halogeton 

glomeratus [Beib.] C.A. Mey.) and 

Russian thistle are often referred 

to as summer annuals because they 

germinate in late spring and complete their life cycles in fall. Biennials require 2 years to complete their 

life cycles. These are broadleaf herbaceous species that usually form a 

rosette of leaves the first year and 

send up a flowering stalk the second 

year. Dyer’s woad (Isatis tinctoria L.) 

is a good example. Perennials are 

often subdivided into short- and 

long-lived perennials. Long-lived 

species are vegetatively propagated 

species that are usually extremely 

difficult to control. Russian knapweed (Acroptilon repens [L.] D.C.) is 

a good example. Short-lived perennials are often biennial species that 

will occasionally persist as perennials. Scotch thistle (Onopordum 

acanthium L.) and Wilson weed 

(Brassica elongata ssp. integrifolia

[Boiss.] Breitr.) are good examples. 

Both these species can be annuals, 

biennials, or short-lived perennials.

Classification by 

Colonizing Ability

Presently, the popular title for 

rangeland weeds is invasive. When 

we introduced this classification 

of weeds we used self-invasive and 

defined the term as weeds that 

colonize a new habitat without 

the conscious efforts of humans. 

In common usage, “self” has been 

dropped.

Classification by Origin

For a given range site, weeds can 

be native, introduced, or exotic. 

In the past, native has been used 

in a broad sense, such as native 

to North America versus native to 

Asia. Gumweed (Grindelia squarrosa (Pursh) Dunal) is an invasive 

species native to the central Rocky 

Mountain States. In California, 

it is considered an invasive exotic 

species. Introduced weeds are 

non-natives that were intentionally 

introduced and escaped cultivation 

versus accidental introductions. 

Saltcedar is a good example. Exotic 

species result from accidental introduction, often as impurities or 

mimics of seeds of crop species.

Classification 

in Legal Terms

Various States and the Federal government have enacted laws 

and regulations defining noxious 

weeds. These laws and regulations 

set limits or outright bans on lots 

NO. 69 DATE 08/12/04

Range

Range

Resource Note 69

Page 1

Figure 1. Cheatgrass (Bromus tectorum L.), an 

annual weed, grows in the Intermountain Area.
of seed entering commerce contaminated with seeds of noxious 

weed species. In addition, these 

regulations may require attempts 

at eradicating infestations of these 

species. In some States, the failure 

of landowners (including governmental agencies) to take such 

action can result in the assessment 

of fines or bills for the cost of 

eradication. The weed regulatory 

agency issues so-called abatement 

notices to the landowner of record 

to control noxious weeds. Many 

States have various levels of noxious weeds. These usually reflect 

the chances of eradication rather 

than the seriousness of the weed 

species. For example, an “A” level 

noxious weed might be a very 

serious pest that, in the judgment 

of the regulatory agency, can still 

be eradicated. A “B” level noxious 

weed might be a species with even 

worse weedy characteristics, but it is 

judged to be so well established that 

eradication is nearly impossible and 

only suppression is economically 

or ecologically feasible. Noxious 

weed classifications vary among 

States. Federal land management 

personnel that have individual 

administrative units in more than 

one State may find themselves dealing with a conflicting list of noxious 

weeds.

Classification 

by Common Name

This is the classification that 

natural resource managers will most 

frequently use, but it is fraught 

with peril. Cheatgrass, six-weeks 

grass, bronco grass, military grass, 

nodding brome, and downy brome 

are all common names for Bromus 

tectorum, which is most frequently 

referred to in the Intermountain 

Area as cheatgrass. The Weed Science Society of America (WSSA) 

maintains a list of accepted common 

names for weeds. Unfortunately, 

this list does not always conform 

to local or even regional common 

usage. For example, WSSA uses 

the common name downy brome 

instead of cheatgrass.

Classification by 

Scientific Names

The use of genus and species 

names for a weed in the Latin 

alphabet is the universal way of 

communicating the name of a 

weed, provided it also includes the 

authority and the flora (taxonomic 

listing) where the description of the 

plant was published. For example, 

it usually does not matter if you 

call a plant cheatgrass or downy 

brome if, the first time you use 

the common name in a technical 

document, you include the complete scientific name, such as (Bromus tectorum L.) as described (for 

example) in Hickman, J. C., editor. 

1993. The Jepson Manuel. University of California Press, Berkeley. A 

natural resource manager who prepares a communication concerning 

noxious weeds must use the correct 

scientific name for the weed. It is 

important to include the authority 

for the name and the flora where 

the name was obtained because 

scientific names can change over 

time. For example, many range 

management students learned 

the scientific name of bluebunch 

wheatgrass as Agropyron spicatum

Pursh. However, on the basis of 

genomic classification, the name 

is now accepted as Pseudoroegneria 

spicata (Pursh) A. Löve.

Conclusion

If you understand and properly 

use the plant classification terminology, you reduce the chances of 

misunderstandings in communications concerning weeds and natural 

resource management.

Contact

James A. Young and Charlie D. 

Clements, Range Scientists

USDA, Agricultural Research 

Service

920 Valley Road

Reno, NV 89512

Phone: 775-784-6057, ext. 229

Fax: 775-784-1712

Email: jayoung@scs.unr.edu and 

charlie@scs.unr.edu 

Range

Resource Note 69

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Figure 2. Russian thistle (Salsola tragus L.) is 

found throughout much of the West.