Yellow starthistle (Centaurea solstitialis)

Author: Tim Prather, University of Idaho, Department of Plant, Soil, and Environmental Sciences

Adapted from: Idaho Weed Resource webpage on Yellow starthistle.

Identification and Life Cycle

Yellow starthistle (Centaurea solstitialis) is a winter-hardy annual in the aster family (Asteraceae). Yellow starthistle matures from 2 to 72 inches tall. Cotyledons are about 1/4 inch long and oblanceolate. A rosette of leaves up to 8 inches long forms after seed germination in the fall. First leaves are generally without lobes but older rosette leaves are pinnately lobed. Older rosette leaves also have cotton-like hairs. Early rosette stages have from 8 to 15 leaves while later stages may be 6 to 8 inches in diameter with up to 26 leaves. Stem leaves up to 4 inches long develop in early spring, their blades forming fringelike extensions down the side of the stem. Yellow flower heads develop at the tips of branched stems from late spring until fall. Flower head bracts bear stiff, sharp thorns that are 3/4 inch long. Seeds are tan with white and brown mottling, 1/8 inch long. Both plumed and unplumed seeds are borne in each flower head. Plumed seeds are not highly windborne; unplumed seeds not at all. Small plants usually have an unbranched stem and one flower head; the large plants have a stem with many branches and can have over 100 flower heads.

Habitats

Yellow starthistle grows in canyon grasslands, rangelands, pastures, edges of cropland, roadsides, and disturbed areas.

Impacts

Yellow starthistle roots continue to develop through the winter months allowing plants to capture moisture, nutrients and solar energy before annual grasses begin to grow.

Biology and Ecology

Yellow starthistle is a winter-hardy annual that normally begins growth in fall with germination and the emergence of oblong cotyledons or seed leaves. Plants bolt in late May and June, sending up a flower stalk with branches tipped with a firm flower bud. During this spring growth period, dense infestations of yellow starthistle that inhabit southern exposures of steep canyons may be identified from a distance by their characteristic blue-green color. From mid-June to early July, each flower bud appears as a small swelling enclosed by shingle-like layers of bud scales called bracts. A yellow-green spine appears at the tip of each bract and develops with the bud to become 1/4 inch to 2 inches long after the flowers fully open. Plants can mature at any height from 3 inches up to 4 feet.

Management Approaches

A variety of chemical, cultural, and biological control tactics are available for yellow starthistle. Plants in the flowering stage are easy to see, but they are too mature to control economically.

Biological Control

Five biological control agents are available for release on yellow starthistle. They include the starthistle bud weevil (Bangasternus orientalis), the Peacock fly (Chaetorellia australis), the starthistle hairy weevil (Eustenopus villosus), the starthistle flower weevil (Larinus curtus), and the starthislte gallfly (Urophora sirunaseva). All five agents reduce seed output.

Mechanical and Cultural Control

Planting competitive grasses in canyon grasslands in fall or winter can be effective.

Chemical Control

Chemical control is available for plants from the rosette to early flowering stages. Yellowstar thistle at one site in Washington was reported to be resistant to synthetic auxins in 1988. For more information on this case, please see http://www.weedscience.org/Summary/USpeciesCountry.asp?lstWeedID=44&FmSpecies=Go

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=yellow%20starthistle&Start=1&results=15

HPIPM:Yellow starthistle

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