Diffuse and Spotted Knapweed Fact Sheet No. 3.110 Natural Resources Series|Range by K.G. Beck* Diffuse knapweed Centaurea( diffusa) Quick Facts is a short-lived perennial, a biennial, or occasionally an annual. It reproduces and • Diffuse knapweed is a short- spreads from seed. The plant develops a single lived, non-creeping perennial, shoot (stem), 1 to 2 feet tall or more, that is a biennial, or occasionally an branched toward the top (Figure 1). Grazed annual that reproduces and plants may produce multiple stems. Rosette spreads solely from seed. and lower shoot leaves are deeply divided and covered with short hairs (Figure 2 and 3). • Spotted knapweed is a short- Leaves become smaller toward the top of the lived, non-creeping perennial shoot and have smooth margins. that reproduces from seed Many solitary flowering heads occur and forms a new shoot each on shoot tips. They are about 1/8 inch in year from a taproot. diameter and 1/2 to 2/3 inch long. Flowers usually are white but may be purplish • Diffuse and spotted (Figure 4). Involucre bracts are divided like knapweed are readily teeth on a comb and tipped with a slender controlled with herbicides. spine that makes them sharp to the touch. Unless cultural techniques Sometimes the bracts are dark-tipped or are used, however, the weeds spotted like spotted knapweed. The long will reinvade. terminal spine differentiates diffuse from spotted knapweed (Figure 6).

Figure 2: Diffuse knapweed rosettes.

Spotted knapweed (Centaurea stoebe; a.k.a., C. biersteinii and C. maculosa) looks like diffuse knapweed with some notable exceptions. Spotted knapweed is a short- lived, non-creeping perennial that reproduces from seed (primary means of spread) and forms a new shoot each year from a taproot. The weed produces one or more shoots that are branched and 1 to 3 feet tall. Rosette leaves can be 6 inches long and deeply lobed. Leaves are similar to diffuse knapweed. Lavender to purple flowers are solitary on shoot tips and about the same size as diffuse knapweed flowers (Figure 5 and 6). Involucre bracts are stiff and black-tipped. The tip and upper bract margin have a soft, spine- like fringe and the center spine is shorter © Colorado State University than others. Extension. 3/03. Revised 11/13. Figure 1: Large diffuse knapweed plant nearing www.ext.colostate.edu flowering growth stage.

*Colorado State University Extension weed specialist and professor, plant pathology and weed science. 11/2013 Phenology, Biology and Occurrence Diffuse knapweed seeds germinate in spring or fall or anytime during the growing season following a disturbance, if adequate soil moisture is present. Seedlings develop into rosettes and diffuse knapweed remains as a rosette until it grows to a critical size, then it bolts, flowers, and sets seed. It may take from one to several years for diffuse knapweed to reach the critical Figure 3: Diffuse knapweed leaves; note fine, size necessary to reproduce by seed. Diffuse short hairs on surface. knapweed plants break off at the soil surface and become tumbleweeds over winter and disperse their seeds in the process. It often gets caught in pasture fences (Figure 7), which can lead to damaged fences. Figure 7: Winter dispersal of tumbling diffuse Diffuse knapweed is native to degraded knapweed caught in pasture fence. noncropland (waste places) and seashores from southern Europe to north-central the western Montana mountains). Spotted Ukraine. It generally is found on dry, light, and diffuse knapweed infestations often porous soils in Europe. Diffuse knapweed occur together in Colorado. appears to occupy similar areas in the Spotted knapweed infestations are not United States. Diffuse knapweed will not as severe in Colorado as diffuse knapweed. Figure 4: Diffuse knapweed flowers, note fringe tolerate flooding or shade and thrives in However, this weed spreads rapidly. For on sides of bracts and long terminal spine on bract tips. the semiarid west (generally in 9- to 16- example, spotted knapweed was first inch precipitation zones). Environmental observed in Gallatin County, Montana, in disturbance (e.g., overgrazed pastures or the 1920s, but is now found in all Montana rangeland, roadsides, rights-of-way, gravel counties. Today, over 4.7 million acres piles, etc.) promotes its invasion. are infested. In Colorado, the worst infestations occur along the Front Range in Larimer, Boulder, Douglas and El Paso counties. Management Severe infestations also occur in Archuleta Diffuse and spotted knapweed can and La Plata counties. A 2005 survey be managed similarly. They are readily conducted by the Colorado Department controlled with herbicides. However, of Agriculture found over 138,400 acres the weeds will reinvade unless cultural infested with diffuse knapweed (Figure 8) techniques are used. and over 10,000 acres infested with spotted Chemical control. Research conducted Figure 5: Spotted knapweed flowers; note dark- knapweed (Figure 9). at Colorado State University indicates tipped bracts and lack of long terminal spine on tip of bract. Spotted knapweed germinates in spring that Tordon 22K (picloram), Milestone or fall. Perennial plants resume growth in (aminopyralid), Transline (clopyralid), early spring and bolt at approximately the Curtail (clopyralid + 2,4-D), or Banvel/ same time as diffuse knapweed. Flowering Vanquish/Clarity (dicamba) control diffuse occurs through the summer into fall. knapweed. Tank mixes of Banvel/Vanquish/ Spotted knapweed is native to central Clarity plus 2,4-D at 1 pt + 2 pt/A or Europe, where it is found in light, Banvel/Vanquish/Clarity control diffuse porous, fertile, well-drained and often knapweed. Refer to Table 1 for rate and calcareous soils in warm areas. It occupies timing recommendations. dry meadows, pastureland, stony hills, Tank mixes of Banvel/Vanquish/Clarity roadsides, and the sandy or gravelly plus 2,4-D at 1 pint + 2 pints/A or Banvel/ floodplains of streams and rivers. The Vanquish/Clarity plus Tordon 22K at 1 to greatest abundance of spotted knapweed 2 pints + 0.5 to 1 pint/A or Tordon plus in Europe occurs along forest/grassland 2,4-D at 0.75 pint + 2 pints/A all control Figure 6: Diffuse knapweed on left, spotted interfaces. Spotted knapweed tolerates dry diffuse knapweed. These tank-mixes knapweed on right. conditions, similar to diffuse knapweed, but may save money and reduce grass injury survives in higher moisture areas as well resulting from higher use rates of a single (e.g., it thrives in the wetter conditions of herbicide. Adding 2,4-D to a tank mix, Table 1. Herbicide used to control diffuse and spotted knapweed. competition in these cases. However, Rate Application infested rangeland or pastures often are Herbicide (Product/A) timing Comments degraded, allowing knapweed invasion, and Tordon 1 to 2 pints Spring at rosette to mid-bolt Use higher rates for older herbicides alone usually will not restore the growth stages; or fall or dense stands land to a productive state. Seeding suitable Milestone 5 to 7 fl oz Spring at rosette to bolting Use higher rate for older or perennial grasses, forbs, and shrubs is growth stages; or fall dense stands; Milestone necessary to prevent weed reinvasion. may be used to edge Biological control. Many insects are ponds or streams being evaluated for biological control Transline 0.67 to 1.33 pints Spring after all shoots have Use higher rate for older or of diffuse and spotted knapweeds. emerged, rosette to early dense stands Researchers at Montana State University bud growth stages; or fall believe it will take a complex of insects Curtail 2 to 3 quarts Spring after all shoots have User higher rate for older or (perhaps 12) to reduce diffuse and spotted emerged, rosette to early dense stands knapweed populations. bud growth stages; or fall Several insects are available in Colorado Banvel, 1 to 2 pints Spring rosette growth Use higher rate for older or from the Colorado Department of Vanquish, or stage; or in fall dense stands Agriculture. The seedhead fliesUrophora Clarity (dicamba) affinis and U. quadrifasciata have been released in many Front Range counties. These insects cause plants to produce however, with very selective fewer viable seeds and abort terminal or herbicides such as Tordon, lateral flowers. Milestone, or Transline, Root-feeding insects may have a broadens the spectrum of more detrimental effect on knapweed activity and may result in populations than seed-feeding ones. increased injury to desirable Larvae of the diffuse knapweed root beetle native forbs (broadleaf (Sphenoptera jugoslavica) feed in the plants) and shrubs, which roots of diffuse knapweed. Larvae of the are important components of yellow-winged knapweed moth (Agapeta the plant community to resist zoegana) and the knapweed root weevil re-invasion by weedy forbs (Cyphocleonus achates) feed in the roots (broadleaf weeds like diffuse of both knapweed species. The knapweed and spotted knapweed). seedhead weevil (Larinus minutus) is Spotted knapweed and an extremely aggressive and effective diffuse knapweed generally biocontrol insect for diffuse and spotted occupy the same areas in knapweed. Larvae consume knapweed Figure 8: A 2005 survey conducted by the Colorado, so the same herbicide treatments seeds and up to 100 percent of seeds may Colorado Department of Agriculture found 138,443 infested acres of diffuse knapweed. can be applied. Weed scientists at Montana be destroyed in an infested seedhead. When State University indicate that Tordon larval development is complete, larvae controls spotted knapweed pupate then emerge from the seedhead as for two to three years, but the adults and consume foliage of diffuse and weed will reinvade the area spotted knapweed until they enter the litter unless other management and soil to overwinter. techniques are used. Milestone Livestock (sheep, goats, cattle) will eat also is very effective to diffuse and spotted knapweed. Research control spotted knapweed completed by Colorado State University and Transline, Curtail, and shows that cattle grazing diffuse knapweed Banvel/Vanquish/Clarity also twice in spring decreased seed set by 50 control spotted knapweed and percent and tumbling off-site over winter these herbicides too should be by 15 percent. Cattle were managed to coupled with cultural control. achieve 50 percent utilization of pasture Cultural control. If and were allowed to graze at two, 10-day desirable plant (grasses and intervals when diffuse knapweed was forbs) competition is evident bolting and about 6 to 12 inches tall. in diffuse or spotted knapweed Figure 9: A 2004 survey conducted by the stands, judicious herbicide application that Colorado State University, U.S. Department of Colorado Department of Agriculture found 9,493 does not injure desirable plants (especially Agriculture and Colorado counties cooperating. infested acres of spotted knapweed. grasses) may allow them to compete CSU Extension programs are available to all without discrimination. No endorsement of products mentioned effectively with the weeds. Irrigation is intended nor is criticism implied of products not (where possible) may help stimulate grass mentioned.