United States Department of Agriculture 

Field Guide for Managing Diffuse, 

Meadow, Spotted, and Squarrose 

Knapweeds in the Southwest

Forest 

Service

Southwestern

Region TP-R3-16-05 Revised June 2017
Cover Photos

Top left: Meadow knapweed – Cindy Roche, Bugwood.org 

Top right: Squarrose knapweed – Steve Dewey, Utah State University, Bugwood.org

Bottom left: Diffuse knapweed – Norman E. Rees, USDA Agricultural Research Service, Bugwood.org 

Bottom right: Spotted knapweed – Joseph M. DiTomaso, University of California, Davis, Bugwood.org

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1 

Diffuse knapweed (Centaurea diffusa Lam.)

Meadow knapweed (C. pratensis Thuill.)

Spotted knapweed (C. stoebe)

Squarrose knapweed (C. virgata Lam. ssp. squarrose) 

Sunflower family (Asteraceae)

Diffuse, meadow, spotted, and squarrose knapweeds are

invasive weeds common to western States. Diffuse and

spotted knapweeds are listed as noxious weeds in Arizona

and New Mexico. Squarrose knapweed has been listed as a

noxious weed in Arizona. Although meadow knapweed is not

currently found in Arizona or New Mexico, it is includedin

this guide to facilitate early identification and control.

This field guide serves as the U.S. Forest Service’s

recommendations for management of diffuse, meadow, 

spotted, and squarrose knapweeds in forests, woodlands,

rangelands, and deserts associated with its Southwestern

Region. The Southwestern Region covers Arizona andNew

Mexico, which together have 11 national forests. The Region

also administers 4 national grasslands located in northeastern

New Mexico, western Oklahoma, and the Texaspanhandle.

Description

Collectively, knapweeds are invasive plants that range from 

annual to biennial to perennial in growth form. At maturity, 

all knapweeds have deep taproots; basalrosettes; and highly

branched flowering stems with white, pink, or purple thistlelike disk flowers. Proper identification should always be 

made before managing knapweeds. Phyllaries(involucral

bracts) and seed appearance are key distinguishing features

by which species can be identified. The pappus (tuft of hairs) 

on each seed is especially important for identification. Table 

1 provides important growth and plant features associated

with Centaurea species addressed in this field guide.

Table 1. Growth characteristics

Species Life Span Growth and

Root Habit

Vegetative/Flower

Appearance

Phyllary 

Characteristics

Reproductive Method

and Seed Appearance

Diffuse 

knapweed

Short-lived

perennial, biennial,

or sometimes an

annual

Prostrate rosette

base; stems 6 to 24

inches tall. Long, 

stout taproot.

Basal leaves 4 inches 

long, stems branched in

the upper half; white or 

pink-purple flowers.

Crab or comb-like; 1/3 

inch long terminal spine 

with 4 to 5 lateral spines.

Seed only. Seeds are dark 

brown, oblong, 1/8 inch 

long with a pappus of 

short, pale bristles.

Meadow 

knapweed

Perennial Prostrate rosette

base; stem 20 to 42

inches tall. Woody

or fleshy taproot.

Deep green leaves;

rose to purple flowers,

sometimes white;

central flowers shorter 

than outer.

Light to dark brown with

a deeply fringed margin;

metallic golden at time

of flowering.

Seed and via root or crown 

fragments. Seeds are pale

tan and plumeless; 1/8 inch 

long.

Spotted

knapweed

Short-lived

perennial or 

biennial

Prostrate rosette

base; stem 24 to 48

inches tall.

Basal leaves, 8 inches 

long, deeply-lobed;

resin-dotted stem leaves

smaller, not lobed;

alternate; pink or purple

flowers.

Stiff, black-tipped with

soft, spine-like fringe

at the tip; shorter center

spine. Black tips make

flower head appear 

spotted.

Seed and via lateral roots. 

Seeds are black or brown 

with pale vertical lines;

0.1 inch long with bristly 

pappus half the length of 

the seed.

Squarrose 

knapweed

True perennial;

long lived; woody 

base.

Prostrate rosette

base; stem 18 to 36

inches tall.

Grey-green, deeply

lobed basal leaves;

upper leaves linear, 

entire. Purple or pink 

flowers.

Similar to diffuse (shortspiny), but central

terminal bract bent

backward; pale-green to

straw colored or may be

purplish.

Seed only. Pale to dark 

brown seed with pale

vertical stripes; short white

pappus; 1/8 inch long.
2 

Ecology

Impacts/Threats

Knapweeds are highly competitive, persistent plants; 

dense, impenetrable stands of knapweed can displace

desirable vegetation. They are often the first plants to 

establish on disturbed sites, roadsides, or areas cleared in 

preparation for development. Spotted and diffuse 

knapweeds are aggressive weeds that rapidly invade

disturbed rangeland, pasture, and fallow cropland. While 

meadow and squarrose knapweeds have a more limited

range, they can quickly out-compete desirable native

plants. Knapweeds have high amounts of phytotoxins, and 

a high knapweed density at a site can make native plants 

appear to be sick and soils seem barren.

Site/Distribution

Diffuse knapweed is wide ranging, although it prefers 

shrub-steppe and dry forest zones. Meadow knapweed 

prefers moister and cooler habitats such as forest openings 

or along rivers and streams. Spotted knapweed has the

widest distribution of the four species and is present in all 

western states. It prefers grasslands and open forests. 

Squarrose knapweed has limited distribution and prefers 

dry, open rangeland with shallow soils. 

Diffuse and spotted knapweeds are found mainly in 

northern parts of New Mexico and Arizona. Meadow 

knapweed is currentlyfound in California and may reach 

other States in the Southwest relatively soon.

Spread

Knapweed seed is easily dispersed by wind and water.

Seed can also be spread in hay that is not certified to be 

weed free. Seed adhering to surfaces and undercarriages of 

vehicles (especially road maintenance equipment) can be

carried for long distances. Seeds may be carried forshorter

distances on animals and humans. Birds transport and 

disperse seed after eating them. Mature stems of diffuse

and squarrose knapweed break off at the base and tumble

over the landscape during winter, thereby dispersing seed.

Invasive Features

Knapweeds readily establish on disturbed sites, especially

along roadsides, railways, waste and cleared areas, and

overgrazed rangeland. Mechanical disturbance can favor 

knapweeds over grass species. However, disturbance is not 

necessary for knapweed invasions to occur. Spotted 

knapweed develops a symbiotic association with a soil 

fungus that can divert carbon from grasses. This reduces 

the ability of grass species to compete and may shift

vegetative composition toward a knapweed monoculture.

Management

Knapweed species are difficult to control, and an integrated 

managementstrategy with a combination of control methods is 

typically necessary to manage infested sites. The following

actions should be considered when planning a

management approach:

• Maintain healthy plant communities to prevent or 

limit knapweed infestations. This may involve

improving grazing management practices to prevent

plant spread.

• Limit disturbance and/or promptly revegetate 

disturbed areas.

• Check purchased hay for presence of weed seeds; 

feed certified weed-free hay or pellets to horses used

in backcountry areas.

• Survey for new populations of knapweed and 

eradicate them as early as possible.

• Map and keep annual records on large infestations.

• Use a combination of mechanical, cultural, 

biological, and chemical methods for effective 

control.

• Include monitoring and follow-up treatment for 

managing missed plants and seedlings.

Table 2 above summarizes some management options for

controlling knapweed under various situations. Further 

details on these management options are explained below. 

Choice of method(s) to use for knapweed control depends 

on the extent and density of an infestation, current land 

use, and site conditions (accessibility, terrain, 

microclimate, other flora and fauna present, etc.). Other 

important considerations include treatment effectiveness, 

overall cost, and number of years needed to achieve

complete control. More than one control method may be
3 

Table 2. Management options*

Site Physical Control Cultural Control Biological Control Chemical Control

Roadsides,

fence lines,

or non-crop 

areas

Mow in the bud to early

flower growth stage and 

repeat as necessary for plant 

suppression. Mowing mature 

plants with seed is not 

recommended. Consider using 

mowing as part of a combined 

approach.

Use seed, mulch, and fill 

materials certified to be weedfree. 

Implement requirements for 

vehicle operations and educate

road maintenance personnel to

identify and report infestations

along roads.

Consider using a biocontrol 

agent (gall-forming flies and

weevils) in combination with

another control method.

Effectiveness of agents may be 

limited due to possible 

disturbances in agent life cycles 

from roadside operations.

Use of grazing animals may aid 

in plantsuppression and support 

of other control methods.

Use truck or tractor

spraying equipment.

Wash underneath 

vehicle after spraying to

prevent spread.

Rangeland,

pasture, or

riparian

corridors

Mowing - Same as above.

On suitable sites, consider 

cultivation in combination

with herbicide spraying and

later reseeding. Include a 

cover crop when reseeding

with desired species. Tillage 

alone is usually not

recommended for knapweed 

control.

Burning does little to control,

but it may be used to prepare

for herbicide application or to 

dispose of hand-pulled debris.

Use seed and forage hay 

certified to be weed-free; use 

pellets for horses in backcountry 

areas.

After passing through infested

areas, inspect and remove any

seed from animals, clothing, and 

vehicles before entering treated 

or un-infested areas.

Use weed screens on irrigation

water intakes.

Consider using a biocontrol 

agent (gall-forming flies and

weevils) in combination with

another control method.

Effectiveness of classical 

biological control agents varies 

by specific location.

Closely manage grazing to

prevent overuse. Avoid grazing

in areas where knapweed seed

has ripened inside seed head.

Use ground or aerial

broadcast spraying. 

Backpack or ATV

spraying may be more

practical in areas

difficult to access. 

Wash underneath 

vehicle after application 

to prevent spread.

Wilderness,

other

natural

areas, and/ 

or small

infestations

Remove by pulling, hoeing, or 

digging 2 to 4 times per year. 

Diffuse and spotted

knapweeds should be severed

at least 2 inches below root 

crown to prevent re-sprouting. 

Squarrose knapweed should 

be severed at least 8 inches 

below soil surface.

Use seed and forage hay 

certified to be weed-free; use 

pellets for horses in backcountry 

areas.

After passing through infested

areas, inspect and remove any

seed from animals and clothing

before entering treated or uninfested areas.

Post signs warning visitorsto

inspect for seeds and remove

them from animals, clothing,

and vehicles when leaving an

infested area.

Same as above. Use backpack or handheld sprayers.

Broadcast spraying by 

aerial or ground 

methods may be used 

on thicker stands, if 

allowed. Wash

underneath vehicle after 

application to prevent 

spread.

* Choice of a particular management option must be in compliance with existing regulations for the land resource.
4 

needed for a particular site.

Physical Control

Physical methods to control knapweeds should focus on 

reducing seed production and preventing germination,

which are mainly accomplished through removal of seed

heads and the root system. These methods usually have to

be repeated and must be timed properly to be effective. 

Physical control methods can be more effective when 

combined with other control methods (see the “Integrated

Control Methods” segment at the end of this section).

Manual Methods

Hand pulling, hoeing, or digging – Hand removal can

be effective for small knapweed infestations, but it

usually must be repeated as many as 2 to 4 times per year 

for multiple years. Plants should be removed in early bolt

before flowers have opened and gone to seed, and the 

taproot should be removed as much as possible. For 

spotted knapweed, it is important to remove the entire 

crown since it can regrow from root fragments. After 

hand removal, knapweed populations may return from 

new or missed plants. It may be helpful to stake areas that 

have been pulled and then monitor closely for new

seedlings.

Proper disposal of debris is essential in preventing

knapweed spread. Mulching or incinerating are 

acceptable disposal methods, especially for plants 

removed before the flowering or seed set stages.

Mechanical Methods

If using machinery to manage knapweed, the equipment 

should be cleaned after use to prevent movement of seed 

into un-infested areas.

Tillage – Tillage with a plow, disc, or other implement

may favor further invasion, especially with diffuse and

spotted knapweeds. This practice is usually not

recommended; however, disc tillage may be used in

certain agronomic situations as part of a combined

control and reseeding strategy.

Mowing – Mowing during the early vegetative and bolt

stages is a commonly used practice to reduce flowering and 

seed production. Mowing mature plants that have already

flowered is not recommended as this facilitates seed dispersal 

and spread. Some vegetation management experts do not 

recommend mowing at all. Mowed knapweed plants often 

produce side branches with greater numbers of flowers, even 

with repeated mowing and proper timing. Consequently, 

mowing should be combined with another control method. 

Prescribed Fire

Burning is likely to result in crown re-sprouts and increased

seed germination, especially in spotted and diffuse 

knapweeds. Therefore, fire by itself is not an effective 

means to control knapweeds; but it can be combined with 

other control methods. 

Cultural Control

Proper identification of knapweed species is important for 

their management. Land managers, the local public, and

road crews should be educated on knapweed identification

and ecology so they can assist with the reporting of

suspected infestations. 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. Vehicles and 

livestock should be checked for knapweed seed after going 

through infested areas. If possible, use weed screens on 

irrigation water intakes within infested areas to prevent 

seed from being transported by irrigation canals. In some 

cases, reseeding with native perennial grass may be useful 

after controlling knapweed.

Biological Control

Grazing

Diffuse, meadow, spotted, and squarrose knapweeds canbe 

grazed by sheep, goats, and cattle, especially in spring

during early growth stages. Intense short-term grazing in

spring or when desirable grasses are dormant can reduce

young knapweed plants. Meadow knapweed is more readily 

grazed than the other three knapweed species and is

intolerant of continuous, heavy grazing. Mature knapweed 

plants are usually avoided by grazing animals; however, 

knapweed seed can be inadvertently eaten and spread in

manure. Therefore, care should be taken when moving 

livestock from infested to un-infested areas.
5 

Table 3. Classical biocontrol agents approved for knapweeds

Species Type of

Agent

Knapweed 

Species 

Attacked

Site of Attack/Impact Use/Considerations for Release

Agapeta 

zoegana 

moth Spotted 

(primarily) 

and diffuse

Larvae feed in roots. There can be multiple larvae 

in the roots. Larvae are mobile and can move a 

short distance to other plants. Larval feeding can 

kill young plants; larger plants often do not 

flower. 

A. zoegana was first released in the 

United States in 1984 and is now 

established in most western states.

Bangastemus 

fausti 

weevil Spotted, 

diffuse, 

squarrose, 

and meadow

Adults feed on foliage and in the flower base, 

destroying the flowers and ovules before they 

produce seeds. Weevils can consume 95 to 100 

percent of the seed. In the fall, attacked seed 

heads have a characteristic emergence hole 

similar to emergence holes of Larinus species.

Prefers hot, dry sites. 

Chaetorellia 

acrolophi 

fly Spotted, 

squarrose, 

and diffuse

This fly feeds on flowers and seed heads; it does 

not cause plants to form galls. Larval feeding can 

significantly reduce seed production; a single 

larva can destroy all of the seeds in a single seed

head. 

C. acrolophi prefers plants in moist 

habitats and is generally associated 

with scattered plants rather than in 

dense stands of spotted knapweed. 

This fly is not widely distributed

Cyphocleonus 

achates 

weevil Spotted 

(primarily), 

diffuse, and 

squarrose 

Larvae feed in roots. Small plants can be killed as 

a direct result of larval feeding. Most damage is 

done when multiple larvae occupy a root or when 

the attacked roots are small. Older larvae cause a 

gall to form in the root, which acts as a metabolic 

sink. Plants are stunted and some survive only 

one season after being infested with C. achates. 

Tunneling in the root also exposes the plant to 

bacterial and fungal infection that can cause 

additional secondary injury.

C. achates is not a strong flyer and 

consequently has been slow to 

establish and spread. In hot weather, 

adults can be seen on the tops of the 

plants. This is probably the best rootboring biocontrol agent available for 

knapweed today. 

Larinus 

minutus 

weevil Diffuse, 

spotted, 

squarrose, 

and meadow

Adult feeding can severely defoliate plants. 

Larval feeding reduces seed production; a single 

larva can destroy the contents of an entire diffuse 

knapweed seed head. Emerging adults make 

characteristic emergence holes in the center of 

affected seed heads similar to the emergence 

holes created by B. fausti and L. obtusus. 

Affects spotted and diffuse 

knapweeds. Can establish on squarrose 

and meadow knapweeds. Very 

effective when combined with other 

root knapweed biocontrol agents. 

Larinus 

obtusus 

weevil Spotted 

(preferred) 

and diffuse

One or two larvae destroys most of the 

developing seeds in the head. Any seeds not eaten 

become part of the pupal chamber. Adult feeding 

on foliage can reduce photosynthesis and plant 

vigor. Emerging adults make characteristic holes 

in the center of affected seed heads, similar to the 

emergence holes created by B. fausti and L. 

minutus.

L. obtusus prefers moist sites in 

contrast to the other seed head weevils 

for knapweed, which prefer and thrive 

in drier sites.
6 

Table 3. Classical biocontrol agents approved for knapweeds (cont.)

Species Type of

Agent

Knapweed 

Species 

Attacked

Site of Attack/Impact Use/Considerations for Release

Metzneria 

paucipunctella 

moth Spotted 

(preferred), 

diffuse, and 

meadow 

Larvae feed on developing seeds. Each larva can 

destroy an average of eight seeds and reduce the 

viability of others. Older larvae web seeds 

together preventing seeds from dispersing over 

long distances.

M. paucipunctella can suffer severe 

mortality during cold winters. Moth 

feeding complements the biological 

control caused by Urophora species. 

M. paucipunctella larvae are 

aggressive and will kill one another or 

other knapweed seed head-infesting 

larvae. White-footed deer mice are 

known to eat many of the larvae 

during the winter months.

Pelochrista 

medullana and 

P. inspersa

moths Diffuse and 

spotted 

Damage to the roots is similar to that caused by 

A. zoegana. Larvae cause considerable root 

damage and as a result, plants attacked by the 

larvae are stunted and produce fewer flowers. 

The infested root becomes spongy and easy to 

pull from the ground. Feeding damage reduces 

root storage.

P. inspersa is established in British 

Columbia and Idaho, P. medullana has 

not yet been found in the United 

States.

Sphenoptera 

jugoslavica 

beetle Diffuse 

(preferred), 

spotted, and 

squarrose 

Larvae mining the roots can cause significant 

impact; adult feeding on the leaves is much less 

damaging. The larvae cause a gall-like swelling 

in the knapweed root near the crown. The 

depletion of root carbohydrates can kill the plant 

or retard rosette growth. Attacked plants are often 

stunted and produce fewer seeds the following 

season.

This beetle prefers hot, dry sites with 

shallow, stony soils typical of those 

infested with diffuse, spotted and 

squarrose knapweed. 

Terellia virens fly Spotted 

(preferred) 

and diffuse

T. virens larvae cause considerable destruction of 

seeds; partial feeding damage on other seeds can 

reduce viability of the remaining seeds by up to 

90 percent.

T. virens prefers plants on southfacing slopes and dry locations.

Urophora 

affinis

gall-forming 

fly

Spotted, 

diffuse, 

squarrose, 

and meadow

Attacks early bud and seed head stages causing 

the formation of a woody gall. Larvae directly 

destroy seeds within the gall. Galls drain 

nutrients from other parts of the plant resulting in 

fewer seed heads and reduced vegetative growth.

U. affinis does not disperse as well as 

U. quadrifasciata and other seed headfeeding agents. On sites with both U. 

affinis and U. quadrifasciata infesting 

knapweed, U. affinis tends to 

dominate.

Urophora 

quadrifasciata

gall-forming 

fly

Spotted, 

diffuse, 

squarrose, 

and meadow

Papery gall forms within seed head. Each larva 

destroys two seeds. Damaged florets are 

destroyed and adjacent florets abort.

There does not appear to be a decrease 

in the number of seed heads on plants 

attacked by U. quadrifasciata. The fly 

spreads rapidly, more so than U. 

affinis.
7 

Classical Biological Control

Thirteen biocontrol agents (moths, flies, weevils, and a 

beetle) have been introduced throughout the U.S. for 

knapweed control (see table 3). Biocontrol agents may 

reduce but will not eradicate knapweed populations. 

However, these biocontrol agents can be highly effective 

when used in combination with other control methods such 

as herbicides, mechanical methods, or cultural practices. 

For further information on classical biocontrol of 

knapweeds, see Winston et al. (2011) in the “Reference 

and Further Information” section of this field guide.

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-soilpermits/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

Knapweeds are best controlled with a selective, postemergent, broadleaf herbicide that has little or no effect on 

associated native grassspecies (table 4). Commonly used 

herbicides for knapweed control include picloram,

dicamba, and clopyralid applied alone or in combination 

with 2,4-D. Two new herbicides, aminopyralid and 

aminocylopyrachlor, are labeled for knapweed control and 

are also effective. Choice of herbicides should be based on

cost, availability, and effectiveness of individual products.

The main herbicide point of entry into the plant for all 

products listed in table 4 is through the leaves with only

minor entry through the roots. These herbicides target 

emerged, broad-leaved plantspecies so caution should be 

taken if non-target species need to be protected. Consult 

the label for specific information.

Herbicide Application

A single herbicide spray application will reduceknapweed 

populations; however, it is important to anticipate the need

for follow-up spot treatments for several years to ensure 

long-term control success. Typically, native grasses

respond favorably after knapweed control and will 

increase in cover within a year of spraying. When native 

grasses are sparse at the time of spraying, it may be 

necessary to reseed with competitive perennial species in 

the fall or spring after herbicide control.

The most effective time to spray spotted or diffuse 

knapweeds is in the fall during the seedling to early rosette

stages since lower rates of herbicide can be applied. In 

spring, higher rates should be used to spray plants during 

the late rosette or bolting stage; or before flowering when 

there are 4 to 6 inches of growth and good growing 

conditions. Effectiveness of herbicide spraying is lower 

when plants are drought-stressed or leaf-damaged; 

therefore, herbicide application is not recommended under 

severe growing conditions. Consider rotating herbicides

from year to year to prevent development of resistance.

Any equipment used to spray herbicide should be 

calibrated. Herbicides may be applied using various types 

of broadcast equipment such as ATVs or UTVs, or 

conventional boom sprayers that are pulled or attached to a 

tractor or truck. For individual plant treatment (IPT), 

knapweed foliage should be wetted thoroughly with a 

backpack or hand-held sprayer that has a single nozzle 

When using picloram or another post-emergent herbicide, 

spray an extra 10 to 15 feet around the infested area to 

ensure control of root sprouts and seedlings. Precautionary 

measures should be taken to protect desirable native plants 

(including woody species) during application. Label 

instructions and guidelines for mixing, application, and 

grazing restrictions following herbicide treatment should 

always be followed. 
8 

Table 4. Herbicide recommendations

Common Chemical

Name (active 

ingredient)

Product

Example1

Broadcast 

Treatment 

(rate per 

acre)

Spot 

Treatment 

(spray 

solution)2

Time of

Application Remarks

Picloram3 Tordon 22K 1–2 pints 1–3% Fall during 

rosette stage or in

spring during bolt 

to bloom stage;

use higher rate at 

bolting to bud 

stage.

Persistent, selective herbicide. May pose 

a risk to groundwater in permeable soils

or in areas where the water table is near

the surface.

Picloram3 + 2,4-D4 Grazon P+D 2–3 quarts 1–3% Same as above. Adding 2,4-D in spring broadens 

spectrum of activity but may damage

desirable forbs and shrubs.

Aminocyclopyrachlor 

+ chlorsulfuron

Perspective 4.75–8 ounces

Use a high 

quality

adjuvant as 

recommended

on the label.

Add 5–9 

grams of dry

flowable

powder to1 

gallon of 

water.

Most effective in 

late fall after frost 

but before soil

freeze.

Labeled for non-crop use includes natural 

areas such as wildlife management areas, 

wildlife habitats, recreation areas,

campgrounds, trailheads, and trails).

May also be used on public, private, and

tribal lands as part of an early detection 

and rapid response (EDRR) in treating 

infestations of invasive weed species.

Persistent; selective; may cause temporary 

injury to some grassspecies.

Aminocyclopyrachlor 

+ metsulfuron methyl

Streamline 4.75–8 ounces Same as 

above.

Same as above. Same as above.

Aminopyralid Milestone 5–7 fluid

ounces

3–5% Fall during 

rosette stage or in

spring during bolt 

to bloom stage; 

use higher rate at 

bolting to bud 

stage.

Labeled for use on sensitive areas, such

as wildlife management areas and natural

areas. May be applied up to water’s edge.

Limited grazing restrictions.

Clopyralid Reclaim 1/3–1 1/3 pints 1–3% Same as above. More selective than 2,4-D or dicamba, 

but may injure legumes such as clover.

Transline 2/3–1 pints 1–3% Persistent in soil; very soluble in water

and mobile in soil; potential to leach into

groundwater.

Clopyralid + 2,4-D4 Curtail 4 pints 1–3% Same as above. Same as above.

Clopyralid + triclopyr Redeem 2 pints 1–3% Same as above. Same as above.

2,4-D4 Several

manufacturers

1–2 quarts 5–10% Early spring;

apply when 

flower stems 

begins to

elongate.

Less expensive, also less effective alone.

Not persistent; need to apply annually to

control new seed germination.
9 

Table 4. Herbicide recommendations (cont.)

Common Chemical

Name (active 

ingredient)

Product

Example1

Broadcast 

Treatment 

(rate per 

acre)

Spot 

Treatment 

(spray 

solution)2

Time of

Application Remarks

Dicamba Banvel,

Vanquish,

Clarity

1–2 pints 1–3% Fall or early 

spring; apply

to rosettes 

before

bolting.

Use higher rate for older or dense stands.

Dicamba + 2,4-D4 Weedmaster 2 pints to 1 

quart

3–5% Same as above. Adding 2,4-D in spring broadens 

spectrum of activity but may damage

desirable forbs and shrubs.

Banvel,

Vanquish, or

Clarity with

2,4-D

Tank mix 1 

pint dicamba

with 2 pints

2,4-D

1–3% 

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 knapweeds.

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

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

.

Integrated Control Methods

The following combinations of control methods should

be considered for managing knapweed infestations that 

cover large areas:

• Mow–herbicide – Mow during late bud to early 

bloom, or graze during bolt in spring. Repeat as 

necessary during the growing season, and spray 

regrowth in fall. Consider reseeding the next spring 

with a variety of desirable native perennial species. 

Periodically monitor for newly emerged seedlings 

and spot treat them. 

• Tillage–herbicide–reseed – When managing a 

large, dense infestation on a tillable area where 

reseeding will be necessary, the area should be 

cultivated during the summer using a disc or plow. 

The knapweed should be allowed to regrow before 

a broadcast spray is applied in fall with a truckmounted or boom sprayer. Reseed in late fall or the 

next spring with a variety of adaptable perennial 

native species. Periodically monitor for newly 

emerged knapweed seedlings and spot treat them. 

Evaluate the need for using a fertilizer, cover crop, 

or animal waste to boost the health of planted 

species. Conduct soil testing to confirm fertilizer 

needs.

• Burn–herbicide – Use prescribed burning in 

summer, allow new shoots and seedlings to 

emerge, and follow with herbicide spraying in the 

fall. Consider reseeding with a variety of desirable 

native perennial species in the next spring. 

Periodically monitor for newly emerged seedlings 

and spot treat them.

• Herbicide–grazing – Treat infestation by 

broadcast spraying with a selective herbicide and 

practice improved grazing management. If native 

grasses do not respond naturally, then consider 

reseeding with perennial forage species. 

Periodically monitor for newly emerged seedlings 

and spot treat them.
10

Management Strategies

To manage knapweeds, the first priority should be to 

prevent knapweeds from establishing in areas where they 

are currently absent. Next, treat small infestations on 

otherwise healthy sites. Satellite and small, isolated 

populations should be eradicated by pulling, grubbing, or 

spot spraying individual plants. Removed plants should be 

piled and destroyed by fire or mulching. Large infested 

areas should be contained and gradually eradicated over 

time if possible, starting around the perimeter. Periodically

monitor for newly emerged seedlings and spot treat them.

Always evaluate the need to reseed treated areas to 

encourage competition from desirable plants, especially 

perennial native grasses. A complete restoration program 

may be required to control large knapweed populations. 

Prior to treating an infested site, the plant community

should be evaluated as to how it will respond once

knapweed is removed. In many instances, native plants (if 

present) will reoccupy the site naturally, thereby

precluding the need for reseeding. In areas where

reseeding is needed, evaluate soil conditions and select 

control methods that can enhance the seeding process. 

Because nitrogen is often limiting, evaluate the need for 

using a fertilizer, cover crop, or animal waste to boost the 

health of planted species. Conduct soil testing for fertilizer 

needs. 

The key to successful knapweed control is long-term 

planning, integrated management, and monitoring of 

treatment areas on an annual basis. Regardless of the 

management approach, knapweeds typically cannot be 

eliminated within a single year or by using only one 

method. In most cases, at least 3 or more consecutive 

years of treatment are necessary to deplete knapweed seed 

in the soil. 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. Always closely follow knapweed 

control efforts with monitoring and be prepared to spot 

treat surviving plants and seedlings until none can be 

found. Failure to perform monitoring and follow-up 

treatments may result in recolonization of knapweed.

References and Further

Information

Beck, K.G. 2003 (rev. 2008). Diffuse and Spotted 

Knapweed. Colorado State University Extension

Natural Resource Series no. 3.110. Available at

http://www.ext.colostate.edu/pubs/natres/03110. pdf

(accessed March 2010).

Carpinelli, M. 2005. Weeds Gone Wild Fact Sheet: Spotted 

Knapweed. USDA-ARS, Burns, OR. Available at

http://www.nps.gov/plants/ ALIEN/fact/cest1.htm

(accessed March 2010). 

DiTomaso, J.M. and E.A. Healy. 2007. Weeds of

California and Other Western States. University of

California Agriculture and Natural Resource 

Publication. 3488. pp. 235–249.

Dow AgroSciences. 2004. Milestone Factsheet. Available 

at http://www.dowagro.com/range/ 

products/milestone.htm (accessed March 2010)

Duncan, K., K. McDaniel, and M. Renz. 2005. Chemical

Weed and Brush Control for New Mexico 

Rangelands. NM State University College of 

Agriculture and Home Economics and Cooperative

Extension Service. Las Cruces, NM. pp. 8–9.

Lym, R.G. 2004. Know Your Knapweeds. NDSU 

Extension Service Publication W-1146 Available at 

http://www.ag.ndsu.edu/pubs/plantsci/weeds/ 

w1146.pdf (accessed March 2010)

Roché, C.T. and D.E. Johnson. 2003. Weeds: Meadow 

Knapweed (Centaurea pratensis Thuill.).

Washington State University Cooperative 

Extension publication PNW0566. Available at

http://cru.cahe.wsu.edu/CEPublications/pnw0566/P

NW0566.pdf (accessed March 2010
11

Roché, C.T. and L.C. Burrill. 2008 (reprint from 1994). 

Weeds: Squarrose Knapweed. Oregon State 

University Extension Service Publication 

PNW422-E. Available at http://extension. 

oregonstate.edu/catalog/pdf/pnw/pnw422-pdf 

(accessed March 2010)

USDA Plants Database. 2010. Available at

http://plants.usda.gov/index.html (accessed 

March 2010)

Whitson, T.D., et al. (ed.) Weeds of the West. Western

Society of Weed Science in cooperation with 

Cooperative Extension Services, University of 

Wyoming. Available at

http://plants.usda.gov/java/invasiveOne?pubID

=WSWS (accessed March 2010).

Winston, R., M. Schwarzlander, C. Bell Randall, and R. 

Reardon. 2011. Biology and Biological Control 

of Knapweeds, FHTET-2011-05. Morgantown, 

WV. Available at

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

technology/pub_programareas.shtml

Suggested Web Sites

Encycloweedia datasheets by California Department of 

Food andAgriculture: Available at

http://www.cdfa.ca.gov/phpps/IPC/weedinfo/

centaurea.htm

For information about calibrating spray equipment: 

NMSU Cooperative Extension Service Guide A613 Sprayer Calibration. Available at 

http://aces.nmsu.edu/pubs/_a/A613

Forestry images: Available atwww.forestryimages.org

Herbicide labels online: Available at

http://www.cdms.net/

Invasive Plant Atlas of the United States: Available at

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