Common Name: yellow starthistle Scientific Name: Centaurea solstitialis Symbol: CESO3 Group: Dicot Family: Asteraceae Growth Habit: Forb/Herb Duration: Annual

What does it look like?

Yellow star-thistle grows one to three feet tall from its taproot, and has rigid stems that are winged and covered with short, soft hairs. The leaves at the base are deeply lobed, while the upper leaves have no teeth or lobes and are sharply pointed. Yellow flowers grow at the end of the stems with 3/4 inch “straw colored” thorns. The outer seed is dark brown without bristles and the inner seed is a mottled light brown with a tuft of white bristles (approximately 1/8 inch long). Yellow star-thistle produces flowers from May to October (HCWMA 1999)

Seed germination begins in Humboldt County in the spring (March – April). Star-thistle begins to bolt by June and is generally mature in late summer (as the flowers begin to fade and seeds start maturing). Inner seeds have hairy bristles and are transported by animals, water, humans, and vehicles. The outer leaves have no bristles and fall to the ground as flowers break apart in the winter. Each flower contains approximately fifty seeds, with most germinating the following year, but seeds buried deeply in the soil can stay viable for ten or more years.

Where does it grow?

Yellow starthistle is native to southern Europe and western Eurasia and was most likely introduced in the United States the mid-1880s. Grows in variety of areas and soils but prefers warm, exposed areas on fertile, drier soils including, roadsides, rangelands, hay fields, and recreational areas.

Is it in our watershed?

This species has not been observed in the Mattole watershed at this time.

What problems does it cause?

Yellow star-thistle is one of California’s worst invasive weeds and continues to spread into uninfested counties. Yellow star-thistle grows throughout Humboldt County, but most heavily in the southern and eastern areas of the county. It invades a site slowly, without increasing for several years until it becomes genetically adapted to that site. The population then explodes and spreads very rapidly. It is essential to eradicate small populations of this particular species as soon as it appears (BLM 1995). Yellow star-thistle also depletes soil moisture reserves in annual grasslands in California and can cause “chewing disease” in horses from long-term ingestion (Bossard et al. 2000).

How do you get rid of it?

Plans to eradicate starthistle must include persistent, systematic treatment over several years. Each infestation is unique and requires evaluation for the best control method. Consider the type of infestation (just roadsides, or extensive fields of it) before attempting any removal technique.

Controlling new invasions by spot eradication is the key to preventing large-scale infestations. In established stands, any successful control strategy will require elimination of new seed production, years of site management, and follow-up treatment and/or site restoration to prevent reinfestations (Bossard et al. 2000).

Manual Control: (Small infestations) Remove starthistle plant prior to seed maturity. Pull plants by hand with gloves. Hoeing or shovel digging will eventually decrease the seed band and reduce infestations. Post-pulling: lightly burning plants or tightly bagging them prior to disposal (BLM, Field Guide).

Mechanical Control: Mowing is an effective removal technique if timed correctly. It can reduce plant density, and seed production. Do not mow during early growth and bolting stages, for this will increase infestation by stimulating growth and reducing competition for water and sunlight by removing desirable plants. If mowed too late, after seed maturity, there is high risk of increasing seed dispersal and plant distribution. Mow before seed maturity, when the majority of flowers have begun to bloom (HCWMA, 1999).

Prescribed burning: Controlled burns can be effective before yellow starthistle produces viable seed (June-July) and after native species have dispersed their seeds. It is vital to continue monitoring projects after the burns to prevent the seedbank from increasing (Bossard, 2000).

Grazing: Yellow starthistle can be a good forage species for sheep, goats, and cattle. Intensive grazing can reduce infestations and seed production of species but must happen before the spiny stage and after bolting. A large number of animals must graze for short periods to be most effective (Bossard, 2000).

Habitat Manipulation: Using annual legumes to revegetate removal sites can be an effective control mechanism. Determining which species are best depends on the specific sites. Subterranean clover (Trifolium subterraneum), rose clover (T. hirtum), and various perennial grasses have been effective in certain areas. Revegetation combined with repeated mowing increased success in some project areas (Bossard, 2000).

Biological Control: Humboldt County Department of Agriculture, in cooperation with the California Department of Food and Agriculture and the United States Department of Agriculture, has released four biological species: the bud weevil, Bangasternus orientalis; the hairy weevil; Eustenopus villosus; and the gall fly to control infestations. The larvae of these control reduce seed production by feeding on the flower heads of the yellow starthistle. These insects are also host specific and feed on yellow starthistle only. They will not harm agricultural crops or native plants (HCWMA, 1999).

Author:
Unity Peterson, Invasive Plant Program Coordinator, Mattole Restoration Council, Petrolia, CA.

Photograph:
California Academy of Sciences, 2000

References:
BLM, Field Guide. Selected Noxious Weeds of Northeastern California: A Field Identification Guide.

Bossard, CC., J.M. Randall and M. Hoshovsky. (eds.) 2000. Invasive Plants of California’s Wildlands. University of California Press, Berkeley, CA. Online version: http://groups.ucanr.org/ceppc/Invasive_Plants_of_California's_Wildlands/

HCWMA. 1999. Control of Yellow Starthistle; In Humboldt County. Humboldt County Weed Management Area. Humboldt County Department of Agriculture. 5630 So. Broadway. Eureka, CA 95503. (Brochure).