United States Department of Agriculture 

NATURAL RESOURCES CONSERVATION SERVICE 

Invasive Species Technical Note No. MT-20 

May 2008 

Ecology and Management of Diffuse Knapweed (Centaurea diffusa Lam.) 

By 

Jim Jacobs, Invasive Species Specialist, NRCS, Bozeman, Montana 

Sharlene Sing, Assistant Research Professor, Montana State University, Bozeman, Montana

Figure 1. Diffuse knapweed flowerheads.

Abstract 

A close relative of spotted knapweed in the Asteraceae taxonomic family, diffuse knapweed is 

typically biennial, reproducing exclusively by seed. It forms a rosette with a central crown and 

tap root in the juvenile stage and a single upright stem one to three feet (0.3 to 0.9 m) tall with 

numerous spreading branches at maturity. White (occasionally purple) flowers are borne in 

heads with spiny bracts (see Figure 1). It is generally found on more arid sites than spotted 

knapweed. Native to the Mediterranean region, diffuse knapweed was first recorded in Montana 

from Mineral County in 1951 and by 2008 had been reported from 39 of Montana’s 56 counties. 

Its dense, spiny overstory reduces the availability of desirable forage plants to livestock and 

wildlife, and grass production can be reduced by over 90% in heavy infestations. 

Declines in diffuse knapweed populations in western North America have been attributed to the 

lesser knapweed flower weevil (Larinus minutus), one of 14 approved biological control insects 

it is host to. Herbicidal management can best be achieved by application of any of the following 

chemicals at the rosette to bolt stages: aminopyralid, clopyralid, dicamba, picloram, and 2,4-D. 

Grazing management using sheep or goats should be applied during the rosette and bolt stages 

before the formation of spiny flowerheads, and animals should be removed when 50% of grass 

forage has been utilized. Tillage can be used to remove plants and release the seed bank. 

Persistent hand pulling and grubbing may be practical on small scale infestations. Diffuse 

knapweed re-sprouts following fire, including intense wildfire. Cultivating and re-vegetating 

competitive plants will improve the longevity of control applications and reduce re-establishment 

from the seed bank. 

NRCS−Montana−Technical Note−Invasive Species−MT-20 1
Plant Biology 

Identification 

Basal leaves are whorled from the central crown of the elongated taproot to form a low-lying 

rosette. Rosette leaves grow up to eight inches long (20 cm) and two inches (5 cm) wide, have 

short stalks, and are deeply divided into lobes on both sides of the midrib. Stem leaves are 

similarly divided but sessile, alternately arranged on the stem, and become progressively smaller 

and less divided higher up the stem. The uppermost leaves are small and bract-like. Stems grow 

from one to three feet tall (0.3 to 0.9 m), with numerous branches giving the plant a ball-shaped 

form that tumbles in the wind when broken off. The flowerheads are urn-shaped, about 0.2 inch 

(3 to 6 mm) wide and up to 0.63 inch (16 mm) long. The bracts surrounding the heads are 

yellowish-green with buff or brown margins fringed with spines, and ending with a long 

spreading, stiff spine. Heads are solitary or in clusters of two or three at the branch ends. Most 

flowers are white, but pink or purple flowers are not uncommon. The achenes (seeds) are 0.12 

inch (3 mm) long, light-brown to black, with a pappus of bristle-like hairs that are scale-like to 

less than 1 mm long. 

Life History 

Normally a biennial, diffuse knapweed can complete its life cycle in a single year when 

germination occurs in the early spring. Otherwise, this species behaves as a short-lived 

perennial, depending on prevailing environmental conditions. Plants generally die after seed 

production, and re-growth from the root crown after seed production is rare. Seeds germinate in 

the fall or spring and seedling survival is dependent on seasonal precipitation. Because 

germination occurs throughout the growing season, seedling emergence is distributed over time. 

These factors combined with a continuous seed rain and sequential establishment ensures 

population occupancy of available safe sites with individuals of different age classes (seedlings, 

rosettes, and flowering adults). Diffuse knapweed populations effectively draw resources from 

different levels of the soil profile, reducing intra-specific, while increasing inter-specific 

competition. This near-complete utilization of soil resources effectively results in monocultures 

of diffuse knapweed in heavy infestations. Under intense intra-specific competition, plants die 

before flowering or live for several years as rosettes and die after seed production. Maximal root 

elongation occurs as seedlings develop into rosettes. Plants that overwinter as rosettes will bolt 

in early May. Flowering occurs from June to September, provided that adequate moisture is 

available and temperatures are mild. Pollination is usually via insects; however, plants are selfcompatible. Individual plants can produce over 700 seeds and population seed production has 

been recorded as high as 40,000 seeds/m2

. 

Spread 

Diffuse knapweed flowerheads have effective mechanisms for short- and long-distance dispersal. 

After seed production, the majority of heads remain closed with seeds intact until fall. Once the 

heads are fully dry, involucres on the sides of the heads open and seeds drop for short distance 

dispersal. For long-distance dispersal, plants break off at ground level and tumble in the wind, 

dispensing seeds individually through small openings at the tip of the heads. Seeds also spread 

long distances when the tumble weeds attach to vehicles or equipment. Rivers and irrigation 

systems can also carry entire plants and their seeds long distances. Colonization and spread of 

diffuse knapweed is enhanced when the competitiveness of the plant community is compromised 

NRCS−Montana−Technical Note−Invasive Species−MT-20 2
by disturbance or limiting resource shortfalls. Prescribed grazing to maintain competitive 

grasses is important to reducing the spread of diffuse knapweed. 

Habitat 

Diffuse knapweed is native to the grasslands, shrub steppes, and fringe forest of eastern 

Mediterranean and western Asian locales with a climate characterized by arid summers. This 

species has demonstrated wide ecological amplitude in colonizing western North America. In 

North America, it is most invasive in shrub steppe with light, well-drained soils that support 

bitterbrush/bluebunch wheatgrass communities. It does not grow in poorly-draining soils or 

intense shade, but can dominate in open ponderosa pine and Douglas fir community types. Dry 

pastureland and meadows dominated by Kentucky bluegrass are susceptible to invasion. In 

Montana, the largest infestations are found on native grasslands and fringe forest areas near Big 

Timber, Ennis, and Helena. 

Impacts 

Significant environmental and economic impacts have been associated with diffuse knapweed 

infestations. A Washington State economic analysis estimated losses of nearly $1 million in 

pasture and rangeland grazing revenue and $2.9 million in lost hay revenue in 1988. A study on 

the Colorado Front Range found that species richness decreased as diffuse knapweed cover 

increased, and that total vegetation cover decreased and bare ground increased with increasing 

diffuse knapweed cover and density. These results support reports that diffuse knapweed 

infestations reduce biodiversity through their ability to form near monotypic stands. Also, 

apparent increases in soil erosion and reduced water infiltration and cycling attributed to 

increased bare ground and reduced bunchgrasses in diffuse knapweed infestations are supported 

by this study. Diffuse knapweed increases the cost of roadside maintenance, threatens natural 

area preserves, and reduces land values. 

Management

Biological Control 

Larinus minutus. Reductions in diffuse knapweed populations in Montana, Oregon, and 

Washington have been observed following the release of the lesser knapweed flower weevil, 

Larinus minutus. In Colorado, diffuse knapweed cover declined from 8.3% to 1.9% over one 

year after the insects were released, and in one area, diffuse knapweed disappeared entirely. 

Diffuse knapweed cover increased from 14.5% to 17% in a nearby area where the insects were 

not present. Reductions of 95% or greater in diffuse knapweed stand density have been reported 

from L. minutus release sites in Washington state. 

NRCS−Montana−Technical Note−Invasive Species−MT-20 3
Figure 2. The exit hole of an adult Larinus minutus weevil from the pupal cell in a diffuse 

knapweed flowerhead (photo by Bob Richards, USDA-APHIS). 

Adult weevils emerge from overwintering sites approximately four weeks before the diffuse 

knapweed flower buds begin to form. Due to their sensitivity to high temperatures, adult weevils 

are primarily active (and most apparent) during the relatively cooler morning or evening hours, 

from May through mid-September. Feeding damage to diffuse knapweed foliage, shoots and 

immature flower buds in spring or early summer when newly-emerged adult weevils are 

aggregated and feeding in high densities can result in significant shoot stunting, flowerhead 

abnormalities, or even plant mortality. Over-wintered female weevils must feed on the host plant 

before ovariole maturity can occur; oviposition then begins on newly-opened flowers. Eggs are 

deposited among individual pappus hairs on plants typically free of significant adult feeding 

damage. Larvae complete three instar stages over a four week period; at this time they actively 

feed on florets, developing seeds and pappus hairs and are capable of consuming all seeds 

produced in a head. Mature larvae (one per flowerhead) construct pupal cells from seed debris 

and pappus hairs. The pupal cell is a domed, straw-colored structure filling the capitulum 

(bottom) of an otherwise empty seedhead. Pupal color changes from white to brown just before 

adult emergence. Empty pupal cells have a large hole in the center marking the adult weevil’s 

exit point (see Figure 2). The reproductively immature adults over-winter protected in the soil 

and litter near the base of plants. Adult weevils can be collected by aspirator (see Figure 3) 

when they congregate in large numbers around the root crown in spring, or they can be swept 

from the diffuse knapweed foliage before 50% bloom is attained. After 50% bloom the females 

are more likely to have laid their full compliment of eggs and will not effectively spread the 

insect population. In laboratory cultures of the weevil, the adult lifespan ranged from five to 14 

weeks. Larinus minutus prefers hot, dry areas and does poorly at high elevations and in areas 

with prolonged rainfall. Livestock grazing while plants are bolting is detrimental to L. minutus

population buildup because the females will disperse to find flowering plants. 

NRCS−Montana−Technical Note−Invasive Species−MT-20 4
Figure 3. A simple aspirator used to collect adult Larinus minutus in the spring 

from diffuse knapweed root crowns (photo by Sharlene Sing). 

Diffuse knapweed is currently host to 12 other insects and one nematode released in North 

America for the control of knapweed species (see Table 1). In addition, Sclerotinia sclerotiorum

is a common native soil fungus that infects the crowns of diffuse knapweed, and Puccinia jaceae

var. diffusea attacks the leaves. These fungi are not cleared as bio-control agents and 

pathogenetic control of diffuse knapweed using fungi, bacteria, or viruses has had limited 

consistent success in field applications. 

Table 1. Biological control insects for management of diffuse 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 Aspirate or sweep net 

before 50% bloom 

Larinus obtusus Weevil Flower 

head 

Adults/flowering Sweep net 

Metzneria 

paucipunctella 

Moth Flower 

head 

Larvae-pupae/seed heads 

(fall-winter) 

Whole plant bouquets 

Pelochrista 

medullana 

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 

Sphenoptera 

jugoslavica 

Beetle Root Adults/flowering Sweep net 

NRCS−Montana−Technical Note−Invasive Species−MT-20 5
Table 1 (continued). Biological control insects for management of diffuse 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 

Subanguina picridis Nematode Stem 

leaves 

Larvae Galls 

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 25 years ago in Montana and are now well 

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

species have been observed to reduce seed production by 50% or more. Competition inhibits 

overall bio-control efficacy when multiple seed-feeding agent species are present in the same 

release sites. Other flower-head feeding insects are not as well distributed, but may be as 

effective as the Urophora flies. The other flower-head weevil, Larinus obtusus, and the 

Metzneria seed-head moth are established in Montana and believed to be effective in reducing 

seed production. Larinus weevils prefer hot, dry sites while Metzneria does best on sites with 

winter snow cover. The Bangasternus seed-head weevil is not widely established in Montana. 

However, it is reported to consume up to 100% of the seeds per flowerhead where it has 

established. 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 due to the locally dramatic control of diffuse 

knapweed by Larinus weevils. 

 

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 generally better. The knapweed infestation for insect release and establishment should 

be at least two acres in area and moderately dense to very dense (three or more knapweed plants 

per square yard). Bio-control releases should only be made on land that will not be treated with 

herbicides in the following three to five years, and where relatively slow evidence of control will 

be acceptable. 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. Retain collected insects in large coolers 

equipped with “blue ice” wrapped in towels so that direct contact is not made with the insect 

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

where condensation will not form. Non-waxed ice cream containers work well, paper bags will 

work if they are not crushed or saturated when chilled, and pillow cases are convenient for 

NRCS−Montana−Technical Note−Invasive Species−MT-20 6
collecting and transport. Do not use plastic bags. Including some fresh knapweed in the 

container reduces crowding and confinement stress, and gives the insects something to 

congregate and feed on. 

 

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

you take the time to complete a release record for each release site; the record should include: 

date of release, species and number of insects released, site GPS coordinates or township/ 

range/section, and descriptive directions to the release point. 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. Dump insects out of containers – this avoids attracting the 

attention of predators and curious wildlife or livestock, and prevents accidental drowning in the 

event of a significant rainfall! The post can be used as a photo point to sequentially 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 may 

reduce the time between establishment and significant impact. 

Herbicidal Control 1/

Herbicides are effective for short-term control of diffuse knapweed. Table 2 lists herbicides, 

rates, soil half-life, and eco-toxicity for chemicals commonly used to control diffuse 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 diffuse knapweed and 

can provide 90% 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 halflife in Table 2). 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 environmentally-sensitive areas or where the use of picloram is prohibited. The 

timing of 2,4-D application is important to maximizing control because this herbicide has brief 

residual soil activity (see Table 2). To have the greatest reduction of diffuse 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 

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-20 7
production. Repeated annual applications of 2,4-D may be necessary to maintain control of 

stands effectively regenerating from the soil seed bank. However, re-application will depend on 

the degree of seedling suppression by competitive plants. 

Other herbicides available for control of diffuse 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 attractive option where preserving the forb community is important to management 

objectives. These chemicals have residual activity greater than 2,4-D but less than that of 

picloram (see Table 2). Timing of application for these herbicides for optimum control of 

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

Aminopyralid is a recently developed chemical that is effective for controlling diffuse 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 short soil 

half-life, 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 risks associated with herbicidal control of diffuse 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 adverse 

effects on 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 eco-toxicity evaluations (see Table 2). 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 re-applications. This will also reduce 

economic risk. Maintaining competitive grasses can reduce the number of herbicide applications 

needed to suppress diffuse knapweed, thereby reducing environmental risk. 

Table 2. Chemical and product name, recommended application rate, soil residual half-life, 

and eco-toxicity of herbicides commonly used to control diffuse knapweed. 

Chemical name Product name Rate 

per Acre 

Half-life 

(days) 

Eco-toxicity (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®, 

Picloram22® 

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-20 8
Grazing Control 

Diffuse knapweed is more likely to be grazed when it is green and succulent. Therefore, the 

timing of grazing management of diffuse knapweed using sheep or goats is important and 

repeated grazings are needed to affect populations. The first grazing should be timed to the 

bolting stage of development and before spiny flowerheads form. Subsequent grazings will be 

determined by local conditions and the re-growth of the knapweed. Utilization levels, duration, 

and frequency should be keyed on the desirable forage species to maintain vigor in competition 

with diffuse knapweed. Controlling diffuse knapweed using sheep or goat grazing is not 

compatible with seed-feeding bio-control insects because the grazers consume the flowerheads 

the insects need for larval development. 

Hand Pulling 

Hand pulling that extracts the root crown is an effective method to temporarily reduce diffuse 

knapweed on small-scale infestations, scattered plants either as new invaders or those persisting 

after herbicide treatments. 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 regeneration. One experiment showed that when 

rosettes were cut just below the ground, 38% of the crowns re-grew compared to 4% of those cut 

two to four inches below the ground. Pulling should be done three times during the year; first in 

spring, second in June when plants bolt, and third before seed dispersal to target plants missed 

the first two times. Flowering plants should be sealed in plastic bags and disposed of in the trash 

to prevent seed spread. Wearing gloves while pulling diffuse knapweed will protect against 

potential skin irritation from spines and phytochemicals. 

 

Mowing 

Mowing will not control diffuse knapweed but it can be used to reduce seed production. When 

diffuse knapweed was mowed each month from April through October, 22% of the plants were 

still growing fours years after mowing began. Viable seeds can be produced after cutting if 

flowers have developed. Plants mowed in the early flowering stages can re-grow and produce 

late-season seeds. Mowing can alter phenological development, resulting in late-season 

flowerheads that will be out of sync with the seasonal timing of attack by biological control 

insects. Mowing can also stimulate new growth. Mowing when plants have bolted to produce a 

flower, but before flowering, will reduce root-energy reserves and seeds produced. Repeated 

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

equipment after mowing will help prevent spread of this weed where 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 

diffuse knapweed populations. This treatment may, however, increase regeneration 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 infestations and 

before it is used on weed-free areas to prevent weed spread. 

NRCS−Montana−Technical Note−Invasive Species−MT-20 9
Irrigation 

Although there is no information on using irrigation in management of diffuse knapweed, it may 

be applied in conjunction with other management tactics, similarly to the management of spotted 

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

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

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

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

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

insects, the competitive pasture plants may suppress establishing seedlings. 

Prescribed Burning 

Diffuse knapweed rosettes re-sprout from root crowns following fire, including intense wildfire, 

and fire may stimulate seed release from dormancy in the seed bank. In addition, fire may 

damage perennial grasses and populations of seed-feeding biological control insects, depending 

on the timing of the burn. Burning plant litter prior to an herbicide application may increase 

herbicide contact with re-sprouting plants and improve herbicide efficacy. Prescribed burning 

should be combined with herbicide management on diffuse knapweed infestations. Wildfire may 

prepare an infested site for rehabilitation using a combination of herbicide application and 

perennial grass seeding treatments. 

Fertilization 

Experimental addition of nitrogen and phosphorous increased the biomass and seed production 

of diffuse knapweed on the Colorado Front Range prairie. Fertilization on native rangeland will 

likely favor diffuse knapweed over native plants and is therefore not recommended. On 

cultivated pastures and hay meadows, nutrient management is important to maintaining the 

competitiveness of desired perennial grasses over diffuse knapweed. Nutrient management 

combined with judicious use of herbicides and crop rotation is recommended where diffuse 

knapweed invades tame pastures and hay meadows. 

Re-vegetation 

Sustainable suppression of diffuse knapweed populations is less likely without desirable 

perennial plants to compete for water, nutrients, and light. On disturbed sites, and sites where 

the competitive vegetation has been lost, re-vegetation may be necessary. 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 diffuse knapweed infestations should be 

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

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

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

soil texture and depth, slope and aspect, will affect species establishment. On native rangeland, a 

diversity of perennial grass and forb species that occupy many niches over time and space will 

most fully utilize available resources and compete effectively with diffuse knapweed. Refer to 

Montana Plant Materials Technical Note 46, ‘Seeding Rates and Recommended Cultivars’ and 

NRCS−Montana−Technical Note−Invasive Species−MT-20 10
Extension Bulletin EB19 ‘Dryland Pasture Species for Montana and Wyoming’ for seeding rate 

guidance and re-vegetation species selection. State, area, and field resource specialists can help 

determine the most appropriate, site-specific species mix and timing of seeding for local 

conditions. 

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. Although following this seed-bed preparation protocol may be practical on 

agricultural land, a no-till drill may be the best method to seed into diffuse knapweed infestations 

that have been suppressed on uncultivated rangeland. 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. Refer to 

Montana Plant Materials Technical Note MT-58, ‘Seedbed Preparation and Seeding’ for more 

information. 

 

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

lands. The herbicides listed in Table 2 will control diffuse knapweed and reduce competition 

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

provides the most persistent diffuse knapweed suppression of the herbicides listed in Table 2, 

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

important for species that are slow to establish, such as many of the native grasses. However, 

where herbicides have been applied, chemical carryover should be assessed prior to planting 

permanent vegetation, particularly if forbs, shrubs, or trees are included in the planting plan. 

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 risks associated with herbicidal application. Keys to 

successful IPM include understanding the biology of the weed, understanding the ecology of the 

system, and careful planning. The biology of diffuse knapweed important to IPM has been 

outlined above and can be used to time control actions targeting specific life stages of the weed: 

1) to reduce early summer rosette density, 2) to increase mortality during the transition from the 

rosette stage to the flowering plant stage, 3) to reduce flowering plant survival, and 4) to reduce 

the number of seeds produced per flowering plant. Knowledge of the ecology of the system 

including climate, soil, and biotic community are important factors to consider when designing 

herbicidal, biological, grazing, and cultural management. 

 

Careful planning is based on three factors: overall management goals, measured threats or risk 

to management goals, and measured progress toward reaching those 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 diffuse knapweed spreads and its requirements for establishment. Early detection and 

small-scale eradication is achieved through persistent scouting and well-timed herbicide 

application. Diffuse knapweed populations are contained by herbicidal control of population 

borders 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 

NRCS−Montana−Technical Note−Invasive Species−MT-20 11
NRCS−Montana−Technical Note−Invasive Species−MT-20 12

control measures that reduce diffuse knapweed population fitness, such as biological control by 

the Larinus weevil, and vegetation management options that increase the fitness of desirable 

plant populations, such as prescribed grazing. 

References 

Berube, D.E. 1980. Interspecific competition between Urophora affinis and U. quadifasciata

(Diptera: Terphritidae) for ovipositional sites on diffuse knapweed (Centaurea diffusa: 

Compositae). Journal of Applied Entomology 90: 299-306. 

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 University Press, Corvallis. Pages 

196-232. 

Kashefi, J.M. and R. Sobhian. 1998. Notes on the biology of Larinus minutus Gyllenhal (Col., 

Curculionidae), an agent for biological control of diffuse and spotted knapweeds. Journal of 

Applied Entomology 122: 547-549. 

LeJeune, K.D., K.N. Suding, and T.R. Seastedt. 2006. Nutrient availability does not explain 

invasion and dominance of a mixed grass prairie by the exotic forb Centaurea diffusa Lam. 

Applied Soil Ecology 32:98-110. 

Norton, A.P., A.C. Blair, J.G. Hardin, S.J. Nissen and G.R. Brunk. 2008. Herbivory and novel 

weapons: no evidence for enhanced competitive ability or allelopathy induction of Centaurea 

diffusa by biological controls. Biological Invasions 10: 79-88. 

Piper, G.L. 2004. Biotic suppression of invasive weeds in Washington state: A half century of 

progress. In: Cullen, J.M., D.T. Briese, D.J. Kriticos, W.M. Lonsdale, L. Morin and J.K. Scott 

(eds). Proceedings of the XI International Symposium of Biological Control of Weeds, 

Canberra, Australia, 27 April – 2 May 2003. Pages 584-588. 

Roché, B.F. and C.T. Roché. 1999. Diffuse knapweed. In: Biology and Management of 

Noxious Rangeland Weeds. Oregon State Press. Pages 217-230. 

Seastat, T.R., N. Gregory, and D Buckner. 2003. Effect of biocontrol insects on diffuse 

knapweed (Centaurea diffusa) in a Colorado grassland. Weed Science. 51:237-245. 

Smith, L. and M. Mayer. 2005. Field cage assessment of interference among insects attacking 

seed heads of spotted and diffuse knapweed. Biocontrol Science and Technology 15: 427-442. 

Watson, A.K. and A.J. Renney. 1974. The biology of Canadian weeds. 6. Centaurea diffusa

and C. maculosa. Canadian Journal of Plant Science. 54: 687-701. 

Wilson, L.M. and C.B. Randall. 2002. Biology and Biological Control of Knapweed. FHTET2001-07. USDA, U.S. Forest Service, Forest Health Technology Enterprise Team. 

Morgantown, WV.