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N A T U R A L R E S O U R C E S ~ S E R I E S ~NGE Diffuse and Spotted Knapweed no. 3.710 Quick Facts... Diffuse knapweed is a short- lived, non creeping perennial, a biennial, or occasionally an annual that reproduces and spreads solely from seed. Spotted knapweed is a short- lived, noncreeping perennial that reproduces from seed and forms a new shoot each year from a taproot. Diffuse and spotted knapweed are readily controlled with herbicides. Unless cultural techniques are used, however, the weeds will reinvade. . Ihri~vsin ExC~irau a Putting xnt~w'Icxtye W wort: ® Cdaratlo State University Cooperative Extension. 3/03. viww.e#.colostate.edu by K.G. Beck' Diffuse knapweed (Centaurea diffusa) is a short-lived perennial, a biennial, or occasionally an annual. It reproduces and spreads from seed. The plant develops a single shoot (stem), I to 2 feet tall, that is branched towazd the top. Grazed plants may produce multiple stems. Rosette and lower shoot leaves are finely divided. Leaves become smaller toward the top of the shoot and have smooth margins. Many solitary flowering heads occur on shoot tips. They aze about 1/ 8 inch in diameter and 1/2 to 9J3 inch long. Flowers usually aze white but may be purplish. Involuae bracts aze divided like teeth on a comb and tipped with a slender spine that makes them sharp to the touch. Sometimes the bracts are dark-tipped or spotted like spotted knapweed. The long terminal spine differentiates diffuse from spotted knapweed. Spotted knapweed (Centaurea macutosa) looks like diffuse knapweed with some notable exceptions. Spotted knapweed is ashort-lived, noncreeping perennial that reproduces from seed (primary means of spread) and fonns 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 ]eaves can be 6 inches long and deeply Lobed. Leaves aze similaz to diffuse knapweed. Lavender to purple flowers are solitary on shoot tips and about the same size as diffuse knapweed flowers. Involucre bracts are stiff and black-tipped. The tip and upper brect margin have a soft, spine-like fringe and the center spine is shorter than others. 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 size necessary to reproduce by seed. Diffuse knapweed is native to degraded noncropland (waste places) and seashores from southern Europe to north-central iJkraine. It generally is found on dry, light, porous soils in Europe. Diffuse knapweed appears to occupy similar azeas in the United States. Diffuse knapweed will not tolerate .flooding or shade and thrives in the semiarid west (generally in 9- to 16-inch precipitation zones). Environmental disturbance (e.g., overgrazed pastures or rangeland, roadsides, rights-of--way, gravel piles, etc.) promotes its invasion. In Colorado, the worst infestations occur along the Front Range in Larimer, $oulder, Douglas and El Paso counties. Severe infestations also occur in Archuleta and La Plata counties. A 2002 survey conducted by the Colorado Department of Agriculture found 145,148 infested acres of diffuse knapweed and 1,093 infested acres of spotted knapweed.