Grazing reduces biomass and seed produc-tion. Adults overwinter outside the host plant, leaving their … Yellow starthistle - Centaurea solstitialis. Yellow starthistle is in the Asteraceae family and is a long-lived winter annual; Stems are erect, rough, branching and can grow up to 1 m in height. Yellow star-thistle is a listed noxious weed and is invasive in many western states. Growth Form/Reproduction: Winter annual. It has taken over vast areas of land in the western United States with estimates of over 15 million acres in California, 280,000 acres in Idaho, and 135,000 acres in Oregon. It is poisonous to horses. Legal Status Central Kootenay: Weed Control Act, Forest and Range Practices Act, Community Charters Act. However, Yellow Starthistle growth rates can decline as its density increases. In deep-soiled rangelands Yellow Starthistle roots grow fast and deep (Sheley and Petroff 1999). Goats will eat the plant even in the spiny stages. micranthos (invasive), diffuse knapweed, Centaurea diffusa (invasive), meadow knapweed, Centaurea x moncktonii (invasive), and brown knapweed, Centaurea jacea (invasive) – These knapweeds have pink or white flowers, unlike the yellow flowers of yellow starthistle and they lack the one inch long spines on their flowering heads that yellow starthistle has. Perennial grasses can compete against yellow starthistle invasions, particularly species that can initiate growth in the fall. Common names: yellow starthistle Centaurea solstitialis (yellow starthistle) is a bushy winter annual (family Asteraceae) that invades 12 million acres in California. Genetics Yellow starthistle is diploid (2n=16), with a large amount of genetic variation (Sun, 1997; Sun and Ritland, 1998). Seed. It is easily recognizable from its brilliant yellow flowers, and cactus like spines. Other Common Names: St. Barnaby’s thistle, yellow cockspur. Yellow starthistle inhabits open hills, grasslands, open woodlands, fields, roadsides, and rangelands, and it is considered one of the most serious rangeland weeds in the state. Yellow Starthistle Centaurea solstitialis L. Sunflower family (Asteraceae) NATIVE RANGE Africa, temperate Asia and Eurasia (Balkan-Asia Minor, the Middle East and south-central Europe) Mediterranean region of southern Europe and northern Africa, especially Turkey DESCRIPTION Yellow starthistle is an annual herbaceous plant. PREVENT Centaurea solstitalis Family: Asteraceae (Sunflower). Sometimes it can grow in clusters that appear shrubby. Yellow starthistle is an invasive plant found in pastures and rangelands. yellow star-thistle in the bolting stage but, before the flower’s spines develop. Origin: Europe. Overgrazing will reduce the ability of other vegetation to recover and shade out the starthistle. Yellow starthistle is an invasive herbaceous annual or biennial of the Asteraceae family that spreads by seeds, forming dense stands in open, sunny areas (Maddox, 1981). Impacts:Agricultural: Infestations can reduce forage quality and forage availability in pastures and in hay. Adults cause minor defoliation, with the majority of the damage caused by larval feeding. Invasive plants should never be considered as forage. Yellow starthistle, Centaurea solstitialis, is a particularly nasty type of invasive knapweed in the Aster family. Consult the PLANTS Web site and your State Department of Natural Resources for this plant’s current status (e.g., threatened or endangered species, state noxious status, and wetland indicator values). Dense populations of this plant result in lowered yield and quality of forages and occasionally crops. ... For more details check out the Invasive Species Compendium datasheet on Centaurea solstitialis (yellow starthistle). 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