HPIPM:Diffuse knapweed

Taxonomy

Domain	Eukarya
Kingdom	Plantae
Phylum	Magnoliophyta
Class	Magnoliopsida
Superorder	Asteranae
Order	Asterales
Family	Asteraceae
Subfamily	Carduoideae
Tribe	Cardueae
Genus	Centaurea

Scientific Name

Centaurea diffusa

Scientific Name Synonyms

Acosta diffusa

Common Name

diffuse knapweed

Diffuse knapweed (Centaurea diffusa)

Author: George Beck, Colorado State University Cooperative Extension

Adapted from the Colorado State University Extension Natural Resources Fact Sheet " Diffuse and Spotted Knapweed"

Identification and Life Cycle

Diffuse knapweed (Centaurea diffusa) is a short-lived perennial, a biennial, or occasionally an annual in the sunflower family (Asteraceae). Flowers usually are white but may be purplish. Many solitary flowering heads occur on shoot tips. They are about 1/8 of an inch in diameter and 1/2 to 2/3 of an inch long. Involucral bracts are 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 (C. stoebe). The long terminal spine differentiates diffuse from spotted knapweed. Seeds are light brown to black and 0.08 to 0.1 inches long. Rosette and lower shoot leaves are finely divided. Leaves become smaller toward the top of the shoot and have smooth margins. The plant develops a single stem, 1 to 2 feet tall, that is branched toward the top. Grazed plants may produce multiple stems. Seedlings have finely divided leaves covered with short hairs.

Habitats

Diffuse knapweed occupies dry, light, porous soils. 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.

Impacts

Diffuse knapweed is highly competitive and can exclude desirable species by forming dense, single species stands. It reduces productivity of grazing lands and wildlife habitat. Although diffuse knapweed is palatable in early growth stages, livestock will not feed on the plant after flowering occurs.

Biology and Ecology

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. It reproduces and spreads only from seed. A single plant can produce up to 18,000 seeds. Seed dispersal is primarily by wind.

Management Approaches

Diffuse knapweed is readily controlled with herbicides; however, it will reinvade unless cultural techniques are also used.

Biological Control

It is believed it will take a complex of insects to reduce diffuse knapweed populations. At least nine insect species have been released as biological controls for diffuse knapweed; however, none appear to effectively control it. The seedhead flies Urophora affinis and U. quadrifasciata cause plants to produce fewer viable seeds and abort terminal or lateral flowers. Root-feeding insects may have a more detrimental effect on knapweed populations than seed-feeding insects. Larvae of the diffuse knapweed root beetle (Sphenoptera jugoslavica), the yellow-winged knapweed moth (Agapeta zoegana), and the knapweed root weevil (Cyphocleonus achates) feed in the roots of diffuse knapweed.

Mechanical and Cultural Control

If desirable grass competition is evident in diffuse knapweed stands, judicious herbicide application that does not injure grasses may allow them to compete effectively with the weeds. Irrigation (where possible) may help stimulate grass competition in these cases. However, infested rangeland or pastures often are degraded, allowing knapweed invasion, and herbicides alone will not restore the land to a productive state. Seeding suitable perennial grasses is necessary to prevent weed reinvasion.

Chemical Control

Chemical control is most effective when plants are in the seedling or rosette stage. Control of established infestations is greatest with fall-applied herbicides, but follow up spring applications may be required to control seedlings.

Examples of herbicides that can be used to manage diffuse knapweed

Consult herbicide labels for additional rate, application, and safety information. Additional herbicide information can be found at http://www.greenbook.net.

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.

References

For more information and images please visit IPM Bugwood. http://www.ipmimages.org/search/action.cfm?q=diffuse%20knapweed