United States Department of Agriculture 

NATURAL RESOURCES CONSERVATION SERVICE 

Invasive Species Technical Note No. MT-1 

June 2006 

Ecology and Management of Spotted knapweed (Centaurea maculosa Lam.) 

Abstract 

Populations of spotted knapweed have been recorded in every county of Montana making this 

noxious weed one of the more widespread invasive plant species in the state (see Figure 1). In 

addition, it is one of the more economically destructive non-native invaders in western North 

America where it infests about ten million acres. Large-scale, monotypic infestations can reduce 

livestock and wildlife forage, are associated with reduced plant diversity, and can increase 

surface water run-off, soil erosion, and stream sedimentation. 

Spotted knapweed is a tap-rooted, short-lived perennial. The average age of plants in the 

Bitterroot Valley, Montana, was estimated at three to four years and the oldest plants were nine 

years old. Plants grow each year from perennial buds on the root crown at the soil surface, do 

not produce rhizomes or stolons, and therefore the persistence and spread of populations is 

dependent on seeds which can remain viable and dormant in the soil for eight or more years. 

Auxin-type herbicides including 2,4-D, aminopyralid, clopyralid, dicamba, picloram, and 

Figure 1. Spotted knapweed infestations (magenta) reported in Montana in 2001. Each magenta 

dot on the map represents one section reporting spotted knapweed. Permission to use this map was 

granted from Diana Cooksey, Adjunct Instructor, MSU-LRES and Coordinator of Montana 

Agricultural Potential System (MAPS). Additional information about section based maps can be 

found at: http://www.montana.edu/places/mtweeds/databasedev.html . 

NRCS−Montana−Technical Note−Invasive Species−MT-1 1
triclopyr are effective at killing spotted knapweed plants when applied at recommended rates and 

timings. Sulfonylurea herbicides such as metsulfuron and trisulfuron, and imazipic do not kill 

spotted knapweed at label recommended rates. Currently, there are eight flower-head insects and 

five root-boring insects that have been approved and released for biological control of spotted 

knapweed. In addition, sheep and goat grazing have been used to reduce seed production and 

population densities. Cultural control methods including hand pulling, tilling, irrigation, and revegetation are effective when combined with chemical, biological, or grazing control. 

Cultivating a desirable, competitive plant community is the key to sustainable management of 

spotted knapweed regardless of the population control methods used. 

PLANT BIOLOGY 

Taxonomy 

Centaurea maculosa (Lam.) is the most widely used taxonomic name of the species commonly 

referred to as spotted knapweed in North America. It is in the Asteraceae (sunflower) family. In 

its native range of western, central, and eastern Europe, two sub-species have been identified; 

Centaurea stoebe spp. stoebe is biennial, and subspecies C. stoebe spp. micranthos is perennial. 

The perennial subspecies is considered more invasive in Europe than the biennial subspecies. 

The invasiveness of the North American taxon, C. stoebe spp. micranthos, has been ascribed to it 

being perennial because it can tolerate dense vegetation once it has become established, whereas 

the biennial is more dependent on disturbance. Spotted knapweed has also been referred to as 

Centaurea bieberstenii. Centaurea is appropriately derived from the Greek word for Centaurs, 

kentaruion, which were the mythical creatures with human head, arms, and chest, and the rest of 

the body like that of a horse. The unruly Centaurs that lived in herds around Mount Pelion in 

Thessaly, Greece, were a plague to people around them. 

Life history and Identification 

There are four relatively distinct and measurable life history stages of spotted knapweed; seeds, 

seedlings, rosettes, and flowering plants. Spotted knapweed populations maintain and spread by 

the sexual reproduction of seeds. Seed production of spotted knapweed in western Montana 

ranged from 1,000 to 7,800/m2

 on an Idaho fescue habitat-type, and has been reported as high as 

40,000/m2 in Washington. In Montana, viable seeds were recovered from soil that had no seed 

inputs for eight years. Seeds are 1/8-inch (3mm) long, oval, brown to black, and bear a short, 

persistent pappus of simple bristles. Seedlings are first year emergents and are difficult to 

distinguish from seedlings of other forb species. Spotted knapweed can persist for an entire 

growing season in the seedling stage. Rosettes can develop from seedlings within one growing 

season, and can be identified, in most cases, by the distinctive pinnatifid, oblong-lobed leaves 

(see Figure 2). In addition, plants that flower in one year may persist in the following year as 

rosettes. Flowering plants are distinguished by the production of an upright (except when 

heavily grazed or repeatedly mowed) paniculate inflorescence with few to many branches and 

reaching heights of eight inches to four feet depending on the environmental conditions (see 

Figure 3). Development of flowering plants from seedlings in one growing season is common. 

Flower heads develop on branch ends and are distinguished by the comb-like fringed, blacktipped involucre bracts and the pink to light purple flowers (see Figure 4). Each flower head can 

have as many as 30 flowers each producing one seed. Sensitivity analysis of life history stages 

and calculated transitions has identified early summer rosette survival, the transition from the 

rosette stage to the flowering plant stage, flowering plant survival, and seeds produced per 

NRCS−Montana−Technical Note−Invasive Species−MT-1 2
flowering plant as life history stages and transitions critical to spotted knapweed population 

fitness. 

Figure 2. Spotted knapweed rosette showing the pinnatifid, oblong lobes on the 

leaves. 

Figure 3. Spotted knapweed inflorescence showing the paniculate branching. The 

characteristic shape of the inflorescence can be used to locate spotted knapweed 

infestations in the winter. 

NRCS−Montana−Technical Note−Invasive Species−MT-1 3
 

Figure 4. Spotted knapweed flower heads showing the black-tipped 

involucre bracts and the light purple flowers. 

Spread 

Spotted knapweed spreads by seed, however, entire plants can be transported in soil and on the 

root balls of horticultural trees and shrubs. Radial expansion of populations through peripheral 

enlargement of existing stands can be as much as a few meters per year. Seeds can be 

transported long distances when they become embedded in the fur of animals, by rodents that 

cache flower heads for food, and by birds, livestock and wildlife that consume the seed heads 

and pass the seed through their digestive tract. Flower heads attached to the undercarriages of 

vehicles and equipment, seeds in mud attached to equipment, and seeds that drop into shoes of 

hikers can be transported long distances. Spotted knapweed seeds spread in the currents of rivers 

and streams, and through contaminated crop seed and hay. 

Allelopathy 

Two allelochemicals are produced and exuded by the roots of spotted knapweed, nicin and 

catechin. These chemicals have been found to suppress the growth of some plants and may give 

spotted knapweed a competitive advantage when growing with them. The role of 

allelochemicals in spotted knapweed invasiveness has not been determined because it is difficult 

to separate competition for water and nutrient from allelochemical suppression. Both nicine and 

catechin are believed to be short lived in the soil and break down soon after spotted knapweed 

has been removed. 

NRCS−Montana−Technical Note−Invasive Species−MT-1 4
MANAGEMENT

Herbicidal Control 1/

Short-term control of spotted knapweed is effective using herbicides. Table 1 lists herbicides, 

rates, soil half life, and ecotoxicity for chemicals commonly used to control spotted knapweed. 

The length of control (i.e. the time the population regenerates from seeds in the soil) will depend 

on the soil residual activity of the herbicide, soil texture, precipitation, and the competitiveness 

of the plant community. Picloram applied at one pint product per acre (0.25 pounds active 

ingredient/acre) is the standard recommendation for herbicidal control of spotted knapweed and 

can provide 90 percent population reduction for three or more years on loamy soils with a wellmaintained grassland community. However, picloram is a restricted-use herbicide and cannot be 

applied near surface water or where there is a high water table. It is water soluble, mobile, and 

will leach quickly from the rooting zone in sandy soils. Picloram also breaks down in sunlight 

which reduces its residual activity. Because of the residual activity, timing of picloram 

application is not as critical as with other herbicides with less residual activity (indicated by half 

life in Table 1). Spring, early summer, and fall applications result in the greatest control. 

Application during the hot and dry part of the summer should be avoided because uptake into the 

plant is limited when plants are dormant and the active ingredient breaks down rapidly in the 

sun. 

An economical alternative to picloram is 2,4-D a broadleaf selective herbicide, which can be 

applied to sensitive areas or where the use of picloram is prohibited. The timing of 2,4-D 

application is important to maximize control because this herbicide has brief residual soil activity 

(see Table 1). To have the greatest reduction of spotted knapweed populations, 2,4-D should be 

applied after most of the seeds have germinated and before plants flower, generally at the late 

bud stage but before flowers appear. This timing will target early summer rosettes, prevent the 

transition from rosette to flowering plant, and eliminate seed production. As mentioned above, 

these are life history stages and transitions that are most important to spotted knapweed 

population fitness. Repeated annual applications of 2,4-D may be necessary to maintain control 

of plants that regenerate from the soil seed bank. However, reapplication will depend on the 

degree of seedling suppression by competitive plants. 

Other herbicides available for control of spotted knapweed in sensitive areas include products 

that contain dicamba, clopyralid, or triclopyr. These herbicides are more expensive than 2,4-D 

but clopyralid and triclopyr do not injure as many non-target forb species as 2,4-D, which makes 

them an option where preserving the forb community is important to management objectives. 

These chemicals have residual activity that is greater than 2,4-D but less that picloram (see Table 

1). Timing of application for these herbicides for optimum control of spotted knapweed is 

similar to the timing recommended for 2,4-D. 

Aminopyralid is a recently developed chemical that is effective for controlling spotted 

knapweed. Its chemistry is very similar to that of picloram, clopyralid, and triclopyr. The 

mobility of aminopyralid in the soil is very high, however because of low application rates and 

1/

Any mention of products in this publication does not constitute a recommendation by the NRCS. It is a 

 violation of Federal law to use herbicides in a manner inconsistent with their labeling. 

NRCS−Montana−Technical Note−Invasive Species−MT-1 5
short soil half life (see Table 1), most of the chemical remains within the upper 12 inches of the 

soil profile. Aminopyralid can be used to treat ditch banks and other channels that do not carry 

water used for irrigation or drinking. There are few studies of the effectiveness of aminopyralid 

on spotted knapweed control. Trials in plant communities with competitive, well-maintained 

grasses indicate that control of spotted knapweed using aminopyralid is similar to control using 

picloram. 

There are risks associated with herbicidal control of spotted knapweed that should be balanced 

against the resulting benefits, and the risks of alternative management actions, including the no 

action alternative, when making weed management decisions. Potential risks include human and 

environmental health, and economic costs. 

The risks to human and environmental health can be minimized by strict adherence to application 

requirements defined on the herbicide label. The relative risks of chemicals can be compared by 

their ecotoxicity evaluations (see Table 1). Generally, the chemicals listed pose a low risk to 

human and environmental health. However, the degree of risk is dependent on dose and 

frequency of exposure. Frequency of exposure can be minimized by integrating non-herbicidal 

population regulation to reduce the number of herbicide reapplications. This will also reduce 

economic risk. 

Introduced grasses inter-seeded into herbicide controlled spotted knapweed stands can provide 

competition and long-term suppression of recurring infestations if properly managed. 

Maintaining competitive grasses can reduce the number of herbicide applications needed to 

suppress spotted knapweed and therefore reduce environmental risk. 

The greatest environmental risk of herbicidal control of spotted knapweed is injury to plants 

other than spotted knapweed, which is limited in most cases to forbs. While the functional role 

of forbs in ecosystems is not completely understood, we can assume the risk is significant where 

forbs, other than spotted knapweed, make up most of the plant community. Where grasses or 

spotted knapweed dominate the plant community, this risk will be minor. 

Table 1. Chemical and product name, recommended application rate, soil residual half life, 

and ecotoxicity of herbicides commonly used to control spotted knapweed. 

Chemical name Product name Rate 

per Acre 

Half life 

(days) 

Ecotoxicity (LC50/EC50) 

2,4-D Many names 1 to 2 qts. 7 1-10 mg/L 

Aminopyralid Milestone 5 to 7 oz. 30 >100 mg/L 

Clopyralid Stinger 

Transline 

1/3 to 

1-1/3 pts. 

40 >100 mg/L 

Dicamba Banvel 

Clarity 

1 to 3 pts. 10 >100 mg/L 

Picloram Tordon 1 pt. 90 10-100 mg/L 

Clopyralid+2,4-D Curtail 2 to 3 qts. 40 0.1-1 mg/L 

Clopyralid+triclopyr Redeem 1 to 2 qts. 40 0.1-1 mg/L 

NRCS−Montana−Technical Note−Invasive Species−MT-1 6
Biological Control 

Pathogenetic control of spotted knapweed using fungi, bacteria, or viruses has had limited 

success in field applications. Sclerotinia sclerotiorum is a common native soil fungus that is 

very effective in killing spotted knapweed in laboratory experiments. In field situations, 

mortality of spotted knapweed from Sclerotinia is most often associated with damage caused by 

root boring insects. Sclerotinia is pathogenetic to a wide range of plant species including many 

crops. 

Currently there are eight flower-head insects and five root-boring insects that have been 

approved and released for biological control of spotted knapweed in the United States (see Table 

2 below). Most of these insects are available commercially or through state, federal, or private 

programs. Once insects are established they can be collected on site and redistributed. Biocontrol insects may reduce spotted knapweed populations where competitive plants are available, 

but they will not eradicate the population. 

 

Table 2. Biological control insects for management of spotted knapweed, the site of attack on 

the plant, insect life stage and time of collection, and the collection method for redistribution. 

Insect Type Site of 

attack 

Collection Collection method 

Agapeta zoegana Moth Root Larvae/rosette Hand pick 

Bangasternus fausti Weevil Flower 

head 

Adults/bud Sweep net 

Chaetorellia 

acrolophi 

Fly Flower 

head 

Larvae-pupae/seed 

heads(fall-winter) 

Whole plant bouquets 

Cyphocleonus 

achates 

Weevil Root Adults/flowering Sweep net, hand pick 

Larinus minutus Weevil Flower 

head 

Adults/flowering Sweep net 

Larinus obtusa Weevil Flower 

head 

Adults/flowering Sweep net 

Metzneria 

paucipunctella 

Moth Flower 

head 

Larvae-pupae/seed heads 

(fall-winter) 

Whole plant bouquets 

Pelochrista 

medullanda 

Moth Root Larvae/late winter or 

adults/flowering 

Hand pick or sweep 

net 

Pterolonche 

inspersa 

Moth Root Larvae/late winter or 

adults/flowering 

Hand pick or sweep 

net 

Spenoptera 

jugoslavica 

Beetle Root Adults/flowering Sweep net 

Terellia virens Fly Flower 

head 

Larvae-pupae/seed heads 

(fall-winter) 

Whole plant bouquets 

Urophora affinis Fly Flower 

head 

Larvae-pupae/seed heads 

(fall-winter) 

Whole plant bouquets 

Urophora 

quadrifasciata 

Fly Flower 

head 

Larvae-pupae/seed heads 

(fall-winter) 

Whole plant bouquets 

The Urophora seed-head flies were released over 20 years ago in Montana and they are well 

established throughout most of the spotted knapweed-infested areas in the western United States. 

NRCS−Montana−Technical Note−Invasive Species−MT-1 7
These species have been observed to reduce seed production by 50 percent or more. Other 

flower-head feeding insects are not as well distributed, but may be as effective as the Urophora

fly. The Larinus flower-head weevils and Metzneria seed-head moth are established in Montana 

and believed to be effective in reducing seed production. Larinus species prefer hot dry sites and 

Metzneria does best on sites with winter snow cover. The Bangasternus seed-head weevil feeds 

primarily on diffuse and squarrose knapweed and is not reported in Montana. However, it is 

reported to consume up to 100 percent of the seeds per flower head where it is established. 

While the number of seeds produced per flowering plant has been identified as important to 

spotted knapweed population fitness, seed-reducing insects applied alone are usually not 

effective in reducing populations because of the perennial nature of the plants. Spotted 

knapweed seed reduction is greatest where more than one insect is established, however, some 

species will attack and destroy larvae of other flower-head feeding species. Poor establishment 

of the Chaetorellia and Terellia flies is believed to be the result of competition with other 

flower-head insect species. 

Of the root feeding insects, the Agapeta root moth, Cyphocleonus root weevil, and Sphenoptera

root borer are well established in parts of Montana. These species prefer hot, dry, open sites. 

The Pelochrista root moth first released in Montana in 1984 has been slow to establish for 

unknown reasons. The Pterolonche root moth was released and established in Oregon in 1986 

but has not been recovered since 2000 presumably because of the dramatic control of diffuse 

knapweed (another host for this species) by the seed-head weevils. 

 

Reductions in spotted knapweed biomass and density have been noted ten years after the release 

of root-boring weevils. Observations suggest that the Cyphocleonus root weevil reduces the 

longevity of spotted knapweed plants making their duration more biennial than perennial, and 

thus less competitive with perennial grasses. This observation is supported by taxonomic 

information about spotted knapweed plants in Europe. 

Dramatic reductions in spotted knapweed densities have been observed in southwestern Montana 

after the release of flower-head and root-feeding biological control insects. In most cases, two or 

more insect species attack the spotted knapweed population. Ten years after the release and 

establishment of Cyphocleonus achates (see Figure 5) and Larinus spp. in large scale and dense 

populations of spotted knapweed, three different plant communities were observed depending on 

management treatments. Where the biological insects were released and no other management 

was used, the plant community remained dominated by spotted knapweed. Where picloram was 

applied and the biological control insects established on spotted knapweed regenerating from the 

seed bank, cheatgrass was the dominant plant in the plant community. However, where picloram 

was used to establish perennial grasses and the insects established, the perennial grasses 

dominated the plant community. These observations illustrate how plant community 

composition was important in influencing the outcome of management actions. 

 

NRCS−Montana−Technical Note−Invasive Species−MT-1 8
Figure 5. A spotted knapweed root with a Cyphocleonus root weevil gall. This root 

supported three weevil larvae. The gall is the enlarged portion of the root below the 

base of the plant. One newly emerged adult can be seen on the lower side of the root 

gall above the center of the clippers. One larva can be seen on the paper bag below the 

root and above the soil residue. 

As with herbicidal management, there are risks associated with releasing biological control 

insects that include potential human and environmental health, and economic risks. The risk of 

non-target attack is controlled, but not eliminated, by host specificity testing. Many of the 

insects released will attack more than one species of Centaurea, however there have been no 

reported occurrences of insects released for spotted knapweed management attacking individuals 

of native plant species populations. Considering the abundance of host plants where insects have 

been released, there is currently little pressure for insects to adapt to alternative hosts. Other than 

hand pulling, biological control insects are the most target-specific management available for 

spotted knapweed. 

 

Increases in mouse populations have been documented in spotted knapweed infestations where 

the larvae of flower head insects have established and provide a new energy source for mice. 

Increased mouse populations may increase risk of mouse-born diseases to humans; however 

there has been no increase in human disease associated with spotted knapweed or biological 

control insects. 

Establishment, Collection, and Redistribution of Bio-Control Insects 

The minimum number of insects needed to establish a sustainable population is 200 insects, but 

more is better. The spotted knapweed infestation should be at lease two acres in area and 

moderately dense to very dense (three or more knapweed plants per square yard). Sites should 

be open and not forested. Insects collected for redistribution, should be released on the new site 

as soon as possible after they are collected, and kept cool (but not frozen) during collection, 

transport and storage. Collected insects should be kept in breathable containers that allow air 

NRCS−Montana−Technical Note−Invasive Species−MT-1 9
flow and where condensation does not form. Non-wax ice cream containers work well, paper 

bags will work if they are not crushed or saturated when iced, and pillow cases are convenient 

for collecting and transport. Do not use plastic bags. Including some fresh knapweed in the 

container gives the insect something to congregate and feed on. 

 

Release sites may be marked with a steel post to help relocate the site for monitoring. Whole 

plant bouquets can be tied directly to the post or to fence posts. Weevils, moths and flies should 

be released on the ground within three feet of the marking post. The best time to release insects 

is during the cool times of the day in the morning or evening. The post can be used as a photo 

point to monitor any change in the weed population and plant community after release. 

It may take two to three years for an insect population to establish. High release numbers will 

reduce the time of establishment. 

Grazing Control 

Sheep and goats have been used to reduce the biomass and density of spotted knapweed, and 

other forb-eating livestock may also utilize this weed. An advantage of this management is that 

it results in a commodity while providing control of the weed. The degree of spotted knapweed 

population regulation will depend on the timing, intensity, and frequency of spotted knapweed 

utilization by these animals. 

The nutritional content and palatability of spotted knapweed will vary during the growing season 

and affect the way grazing animals use spotted knapweed. Spring and early summer rosettes, 

and bolting plants are most palatable and nutritious to livestock. These life history stages were 

identified in sensitivity analysis as important stages to target in management to reduce spotted 

knapweed population fitness. When plants dry in the summer, nutrition and palatability are 

reduced and livestock select stem leaves, flower heads of spotted knapweed, and other more 

palatable forage plants. 

The diet of sheep is generally divided equally between forb and grass species, or in proportion to 

their availability. When spotted knapweed is more abundant than grasses, sheep will tend to 

consume a greater proportion of it than grasses if palatability is equal. One strategy that has been 

applied is to graze an area first with cattle to utilize the grass followed by sheep grazing to clean 

up the knapweed. The resulting grazing pressure on desirable grasses and the weed is equal 

under this management. 

The intensity of sheep grazing on spotted knapweed can be increased by fencing sheep on 

infestations (see Figure 6). Portable electric fencing has been used successfully to target specific 

problematic infestations. Webbed electric fencing also helps reduce losses to small predators 

including fox, coyotes, and domestic dogs. 

NRCS−Montana−Technical Note−Invasive Species−MT-1 10
Figure 6. A fence line comparison of sheep utilization of spotted knapweed (right) 

and no control of spotted knapweed (left). On the right side, 20 ewes and one ram 

were held on 0.1 acre plot for one to four days twice annually for five years. 

A single grazing during bloom may reduce spotted knapweed seed production, however multiple, 

repeated grazing over a number of years is needed for sheep or goats to reduce spotted knapweed 

biomass and density. In a five year study, annual intensive grazing of spotted knapweed when 

plants bolted in late May, and repeating the treatment after one month rest when plants reinitiated bolting, was effective in reducing spotted knapweed density. Grazing spotted knapweed 

one time with sheep resulted in an increase in spotted knapweed density. 

Sheep grazing may be combined with other management to improve control of spotted 

knapweed. Irrigation followed by repeated sheep grazing has been used to boost the production 

of perennial grasses and reduce spotted knapweed production. Applying this treatment for three 

to five years followed by a single application of 2,4-D resulted in long-term suppression of 

spotted knapweed. 

 

Spotted knapweed seed can pass through the digestive system of grazing animals and remain 

viable. Sheep, goats, and other animals that have grazed in spotted knapweed infestations and 

may have consumed seeds should be held for at least five days before being moved to weed free 

areas to prevent spreading the weed via feces. Grazing weeds with sheep should be timed to 

avoid contact with, and disease spread to, wild sheep. 

Cultural Control 

Cultural methods including hand pulling, mowing, tilling, irrigation, and re-vegetation are 

effective in managing spotted knapweed in certain situations and when combined with other 

control actions. 

NRCS−Montana−Technical Note−Invasive Species−MT-1 11
Hand Pulling. Hand pulling that extracts the root crown is an effective method to temporally 

reduce spotted knapweed on small-scale infestations. Pulling or grubbing the root crown is most 

easily accomplished when the soil is moist and a shovel is used to pry-up the tap root. When the 

soil is dry the plant tends to break-off above the root crown enabling it to regenerate. If 

flowering plants have been pulled, they should be sealed in plastic bags and disposed of in the 

trash to prevent seed spread. Wearing gloves while pulling spotted knapweed will protect 

against potential skin irritation from chemicals produced by knapweed. 

 

Mowing. Spotted knapweed seed production can be reduced by mowing. Mowing when plants 

have bolted to produce a flower, but before flowering, will reduce energy reserves and seeds 

produced. Repeated mowing may be needed if plants reinitiate bolting after the first mowing. 

After repeated mowing, spotted knapweed has been observed to produce flower heads below the 

height of the mower blade. Mowing has not been shown to reduce population densities but may 

prevent population spread by reducing seed production. Consecutive years of mowing followed 

by an herbicide application may improve long-term control of spotted knapweed because of 

reduced seeds to regenerate the population. Washing mowing equipment after mowing spotted 

knapweed will help prevent spread of this weed if the equipment is subsequently used on weed 

free areas. 

Tilling. Shallow tilling that severs the tap root below the root crown has been effective in 

reducing spotted knapweed populations. This treatment may, however, increase regeneration of 

spotted knapweed from the seed bank by exposing buried seeds. Multiple tilling or tilling 

followed by application of herbicide to target establishing seedlings can be used to exhaust the 

supply of viable seeds in the soil. Tilling is only recommended in combination with seeding 

perennial competitive plants. As with any mechanical treatment, equipment should be washed 

after working spotted knapweed infestations and before it is used on weed-free areas to prevent 

the spread of spotted knapweed. 

Irrigation. Irrigation can be used as part of a spotted knapweed management program. 

Irrigation can stimulate the germination of spotted knapweed seeds thus reducing viable seeds in 

the soil. When followed by a control treatment such as herbicide application or sheep grazing, 

populations can be reduced. Irrigation can be used to increase the production of many pasture 

plant species and increase their competitiveness with spotted knapweed. If mature, competitive 

spotted knapweed plants are controlled using herbicides, sheep grazing, or biological control 

insects, the competitive pasture plants may suppress young and establishing spotted knapweed 

plants. 

Prescribed Burning. In Montana, prescribed fire generally results in increased spotted 

knapweed populations and is not used as a control treatment. The effect of fire on plants is 

strongly influenced by the timing of burning. Most fires are prescribed in the spring or fall in 

Montana. In Michigan, annual prescribed fire in the summer reduced spotted knapweed on 

restored warm-season grass prairies. Burning spotted knapweed infested pastures or rangeland 

in the summer in Montana poses a high risk of wildfire and is generally not recommended. 

Fertilization. Fertilization can be used to boost the competitiveness of grasses with spotted 

knapweed when applied in combination with herbicidal or other suppression of spotted 

knapweed. Non-native pasture grasses respond to fertilization with greater biomass production 

than native grasses. Fertilization can also be used to stimulate the growth of re-vegetation plants. 

Fertilization by itself stimulates spotted knapweed growth and reproduction. 

NRCS−Montana−Technical Note−Invasive Species−MT-1 12
Re-vegetation. Sustainable suppression of spotted knapweed populations is less likely without 

desirable perennial plants to compete with spotted knapweed for water, nutrients, and light. On 

disturbed sites, and sites where the competitive vegetation has been lost, re-vegetation may be 

needed. Successful establishment of desirable vegetation can be improved by selecting a site 

adapted seed mix, preparing a firm weed-free seedbed, controlling weeds, and seeding at a time 

that optimizes moisture availability to establishing seedlings. 

 

Species selected for re-vegetating disturbed sites and spotted knapweed infestations should be 

appropriate for management objectives, adapted to site conditions, and competitive with spotted 

knapweed. Management objectives will determine if forage species or native species are seeded 

and the species mixture. The environmental conditions of the site including precipitation, soil 

texture and depth, slope and aspect, will affect species establishment. A diversity of perennial 

grass and forb species that occupy many niches over time and space will most fully utilize 

resources and compete with spotted knapweed. Refer to Montana Plant Materials Technical 

Note 46, ‘Seeding Rates and Recommended Cultivars,’ and Extension Bulletin EB19 ‘Dryland 

Pasture Species for Montana and Wyoming’ for seeding rate guidance and re-vegetation species 

selection. State and area resource specialists can help determine the most appropriate, sitespecific species mix and timing of seeding. 

An ideal seedbed for re-vegetation is one that has pulverized soil on the surface with moderate 

amounts of mulch or plant residue, firm soil below seeding depth, and free of residual 

competitive plants. This may be practically prepared on agricultural land, however, on rangeland 

a no-till drill may be the best method to seed into spotted knapweed infestations that have been 

suppressed. On sites inaccessible to machinery, broadcast seeding with little or no seedbed 

preparation is often the only option. Increasing seeding rate can increase the establishment of 

seeded species that are broadcast. Weed infested sites burned by wildfire have been successfully 

re-vegetated using aerial seeding. 

 

In most cases, herbicidal suppression of spotted knapweed is needed for re-vegetation of infested 

lands. The herbicides listed in Table 1 will control spotted knapweed and reduce competition 

during the establishment period with little or no injury to emerging grass seedlings. Picloram 

provides the longest spotted knapweed suppression of the herbicides listed in Table 1, and 

therefore provides the greatest window of competition-free establishment. This is especially 

important for species that are slow to establish like many of the native grasses. Picloram applied 

at the time of a fall-dormant drill-seeding in spotted knapweed/cheatgrass (Bromus tectorum) 

infestations has resulted in good stands of native and non-native perennial grasses (see Figure 7). 

NRCS−Montana−Technical Note−Invasive Species−MT-1 13
Figure 7. A comparison of a spotted knapweed infestation re-vegetated with bluebunch 

wheatgrass (left) and no re-vegetation (right) on adjacent sites. The grass was drill-seeded 

after a fall picloram application ten years prior to the time of the photograph. The 

Cyphocleonus root weevil and the Larinus flower head weevil are established on both sites. 

Integrated Pest Management 

Integrated Pest Management (IPM) is the application of multiple management actions that are 

mutually supportive, and should include actions that cultivate competitive desirable plants and 

actions that suppress weed populations. The goals of IPM beyond weed control include reducing 

the cost of weed management and reducing herbicide inputs. Keys to successful IPM include 

understanding the biology of the weed, understanding the ecology of the system, and careful 

planning. The biology of spotted knapweed important to IPM has been outlined above and can 

be used to time control actions to reduce early summer rosette, the transition from the rosette 

stage to the flowering plant stage, flowering plant survival, and seeds produced per flowering 

plant. Knowledge of the ecology of the system including climate, soil, and biotic community 

factors is important in designing biological, grazing, and cultural management. 

 

Careful planning is based on three things that very often are not considered in weed 

management: overall management goals, measured threats or risk to management goals, and 

measured progress toward reaching management goals. Specific weed management goals will fit 

into one of the following categories: prevention, early detection and small-scale eradication, 

containment, and large-scale population reduction. Prevention is guided by how spotted 

knapweed spreads and its requirements for establishment. Early detection and small-scale 

eradication is achieved through persistent survey and herbicide application. Spotted knapweed 

populations are contained by herbicidal control of population boarders and satellite populations, 

control actions that reduced seed production, and cultivation of competitive plants. Large-scale 

population reduction is achieved over the long-term by applying management actions that reduce 

the spotted knapweed population fitness and increase the fitness of desirable plant populations. 

NRCS−Montana−Technical Note−Invasive Species−MT-1 14
References 

Coombs, E. M., Coombs, J. K. Clark, G. L. Piper, and A. F. Cofrancesco, Jr. (eds) Biological 

control of invasive plants in the United States. Oregon State Univ. Press, Corvallis. 467 pp. 

Callan, N. W., M. P. Westcott, M Knox, and S. Wall-MacLane. 2004. Grasses for dryland forage 

and weed suppression in western Montana. Western Agricultural Research Center, Montana 

State University, Corvallis MT. 5 pp. 

Davis, E. S., P. K. Fay, T. K. Chincoine, and C. A. Lacey. 1993. Persistence of spotted 

knapweed (Centaurea maculosa) seed in soil. Weed Science 41:57-61. 

Jacobs, J. S. and R. L. Sheley. 1998. Observation: spotted knapweed life history. Journal of 

Range Management 51:665-673. 

Lacey, J. R., C. B. Marlow, and J. R. Lane. 1989. Influence of spotted knapweed (Centaurea 

maculosa) on surface water runoff and sediment yield. Weed Technology 3:627-631. 

Sheley, R. L., J. S. Jacobs, and D. E. Lucas. 2001. Re-vegetating spotted knapweed infested 

rangeland in a single entry. Journal of Range Management 54:144-151. 

Rinella, M. L., R. L. Sheley, J. S. Jacobs, and J. J. Borkowski. 2001. Spotted knapweed 

response to season and frequency effects of mowing. Journal of Range Management 54:52-56. 

Sheley, R. L., J. S. Jacobs, and J. M. Martin. 2004. Integrating 2,4-D and sheep grazing to 

rehabilitate spotted knapweed infested rangeland. Journal of Range Management 51:371-375. 

Sheley R. L., Duncan, C. A., M. B. Halstvedt, and J. S. Jacobs. 2000. Spotted knapweed and 

grass response to three herbicide treatments applied at various growth stages. Journal of Range 

Management 53:176-182. 

Sheley, R. L., J. S. Jacobs, and M. F. Carpinelli. 1999. Spotted knapweed. In: Biology and 

Management of Noxious Rangeland Weeds. Oregon State Press. Pages 350-361. 

Sheley, R. L., J. S. Jacobs, and R. P. Valagala. 1999. High seeding rates enhance intermediate 

wheatgrass establishment in spotted knapweed infested rangeland. Journal of Range 

Management 52:67-73. 

NRCS−Montana−Technical Note−Invasive Species−MT-1 15