HPIPM:Spotted knapweed

Taxonomy

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

Scientific Name

Centaurea stoebe australis

Scientific Name Synonyms

Centaurea stoebe ssp. micranthos

Centaurea biebersteinii

Acosta maculosa

Common Name

spotted knapweed

Spotted knapweed (Centaurea stoebe)

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

Spotted knapweed (Centaurea stoebe) is a short-lived, non-creeping perennial in the sunflower family (Asteraceae). The weed reproduces primarily from seed. Lavender to purple flowers are solitary on shoot tips and about the same size as diffuse knapweed flowers. Involucral bracts are stiff and black-tipped. The tip and upper bract margin have a soft, spine-like fringe and the center spine is shorter than others. Seeds are oval, brown to black with copious whitish bristles (pappus). Rosette leaves can be 6 inches long and deeply lobed. Leaves become smaller toward the top of the shoot and have smooth margins. Spotted knapweed forms one or more new shoots each year from a taproot. The shoots are branched and 1 to 3 feet tall. Spotted knapweed has a stout taproot. Seedlings have deeply divided leaves in a rosette.

Habitats

Spotted knapweed occupies dry meadows, pastures, stony hills, roadsides, and the sandy or gravelly floodplains of streams and rivers. It tolerates dry conditions, similar to diffuse knapweed, but survives in higher moisture areas as well.

Impacts

Spotted knapweed is very competitive and once established can reduce livestock and wildlife forage and desirable plant diversity.

Biology and Ecology

Spotted knapweed germinates in spring or fall. Perennial plants resume growth in early spring and bolt at approximately the same time as diffuse knapweed. Flowering occurs through the summer into fall. Seed production can be as high as 3,700 seeds/foot2. Seed dispersal distances are generally short (less than 4 feet). Seeds can remain viable in the soil for several years.

Management Approaches

Spotted knapweed is readily controlled with herbicides. However, it will reinvade unless cultural techniques are used.

Biological Control

It is believed it will take a complex of insects to reduce spotted knapweed populations. Currently eight flower-head insects and five root-boring insects have been approved and released for biological control of the weed in the U.S.

Mechanical and Cultural Control

If desirable grass competition is evident in spotted knapweed stands, judicious herbicide application that does not injure grasses may allow them to compete effectively with the weed. 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. Livestock (sheep, goats, cattle) will eat spotted knapweed and can reduce its biomass and density.

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 spotted 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=spotted%20knapweed