| Yellow starthistle Centaurea solstitialis |
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yellow starthistle: Annual, sometimes biennial, to 2 m tall. Plants are highly competitive and typically develop dense, impenetrable stands that displace desirable vegetation in natural areas, rangelands, and other places. Yellow starthistle is considered one of the most serious rangeland weeds in the northwestern U.S. It has spread rapidly since its introduction into California around 1850. In 1995, it was estimated to infest 10-12 million acres in the state, with the heaviest infestations primarily in the northern and central-western regions. Seeds often contaminate grains, lowering the value and quality of harvests. Yellow starthistle contains an unidentified compound that causes nigropallidal encephalomalacia or chewing disease in horses. The compound only affects horses and permanently damages the area of the brain that controls fine motor movements, including mouth and lip movements. Toxicity effects are cumulative. Horses must consume a 50-150% of an animal’s weight in dry-weight plant material over a period of 1 to 3 months to produce symptoms. Because of its bitter taste, horses usually avoid grazing yellow starthistle. However, the disease can occur when horses are allowed to graze infested pastures, especially those that lack adequate amounts of suitable green forage, or are fed contaminated hay over a period of time. Once the toxicity threshold has been reached, symptoms occur rapidly. Symptoms include fatigue, lowered head, an uncontrolled rapid twitching of the lower lip, tongue-flicking, involuntary chewing movements, and an unnatural open position of the mouth. Without intervention, affected horses are unable to eat or drink and eventually die from starvation or dehydration. The impact of yellow starthistle is not always negative. Bees foraging on yellow starthistle flowers produce a flavorful high quality honey. Several biocontrol agents have been introduced from the Mediterranean region to control yellow starthistle. Currently established in California are the yellow starthistle bud weevil (Bangasternus orientalis), hairy weevil (Eustenopus villosus), flower weevil (Larinus curtus), gall fly (Urophora sirunaseva), peacock fly (Chaetorellia australis), and false peacock fly (Chaetorellia succinea). These insects have yet to provide significant reduction in yellow starthistle populations in most areas. Yellow starthistle was introduced from Southern Europe.
SEEDLINGS: Cotyledons oblong to spatulate, base wedge-shaped, tip +/- squared. First few leaves typically oblanceolate. Subsequent rosette leaves oblanceolate, entire to pinnate-lobed. Terminal lobes largest. Later rosette leaves to 15 cm long. Hair characteristics are visible with 10-14x magnification. yellow starthistle: Cotyledons 6-9 mm long, 3-5 mm wide. Later rosette leaves typically deeply lobed +/- to midrib, appear ruffled. Surfaces +/- densely covered with fine cottony hairs that +/- stiff thick hairs and leaf surfaces. Lobes mostly acute, with toothed to wavy margins. Terminal lobes +/- triangular to lanceolate. Leaves of rosettes under reduced light levels are larger and more erect. MATURE PLANT: Stems stiff, openly branched from near or above the base or sometimes not branched in very small plants. Stem leaves alternate, mostly linear or +/- narrowly oblong to oblanceolate. Margins smooth, toothed, or wavy. Leaf bases extend down the stems (decurrent) and give stems a winged appearance. Rosette leaves typically withered by flowering time.yellow starthistle: Largest stem wings typically to ~ 3 mm wide. Lower stem leaves sometimes +/- deeply pinnate-lobed. Foliage grayish- to bluish-green, densely covered with fine white cottony hairs that +/- hide thick stiff hairs and glands. FLOWERS: Heads ovoid, spiny, solitary on stem tips, consist of numerous yellow disk flowers. Heads sometimes closely 2-3-clustered in Malta starthistle. Vigorous individuals of Malta and yellow starthistle may develop flower heads in branch axils. Phyllaries palmately spined, with one long central spine and 2 or more pairs of short lateral spines. Insect-pollinated. | ||
