Yellow Star Thistle (Centaurea)

Yellow Starthistle, Yellow Star-thistle and St. Barnaby’s thistle


Yellow Star-thistle

 Yellow star thistle in an invasive winter annual, rarely a biennial or short-lived perennial. Yellow star thistle populations in the United States exhibit variations in phenology (bolting and flowering time) and morphology (plant size and leaf shape). Sun found genetic diversity within yellow star thistle populations, and little divergence between populations. The following description provides characteristics of yellow star thistle that may be relevant to fire ecology and is not meant to be used for identification. Keys for identifying yellow star thistle are available.

Yellow starthistle stems are stiff and erect, 6 to 72 inches in height. Stems are openly branched, except in some very small plants. Yellow starthistle produces rosette leaves that lie close to the ground. Lower leaves are 2 to 6 inches in length, and become progressively smaller as they go up the stem. Upper leaves are 0.4 to 1.2 inches in length, narrow, and densely covered with cobwebby hairs later in the season.

Yellow starthistle inflorescences are borne in solitary flower heads on stem tips, although vigorous plants may produce flower heads in branch axils. The involucre is about 0.5 to 0.7 inches in length. Phyllaries have 1 long central spine 0.4 to 1 inch length and 2 or more pairs of short lateral spines and are densely to sparsely covered with hairs. Yellow starthistle fruits are achenes of 2 types, both glabrous and about 2 to 3 mm in length. Most of the achenes (seeds) (75-90%) have a short (2-5 mm), stiff pappus (plumed). Seeds at the periphery of the flowerhead are darker in color and have no pappus (plumeless).

Yellow starthistle has a large taproot that grows to soil depths of 3.3 feet (1 m) or more, allowing access to deep soil moisture during dry summer and fall months. Hairs and waxy coating on mature yellow starthistle leaves reflect light, thus reducing the heat load and transpiration demand, while winged stems also dissipate heat. These qualities, along with deep roots, allow yellow starthistle to thrive under full sunlight in hot, dry conditions. Vesicular arbuscular mycorrhizal fungi have been observed on yellow starthistle plants.

Once established, yellow starthistle can survive at high population densities, with estimates of 2 to 3 million plants per acre (5-7.5 million per hectare) reported. This dense cover reduces sunlight penetration to the soil surface, inhibiting germination and development of competing vegetation. Old yellow starthistle stalks are persistent and usually remain standing through the winter.

Yellow starthistle is native to southern Europe and western Eurasia (Flora Europaea). Today, yellow starthistle can be found in most temperate areas around the world. In North America, yellow starthistle now occurs in at least 41 of 50 US states and 4 Canadian provinces. Yellow starthistle is reported to be adventitious in Hawaii. Infestations of yellow starthistle in the eastern two-thirds of the United States are sporadic and localized, but apparently populations fail to establish and persist on a year-to-year basis, possibly because of unfavorable growth conditions. Infestations west of the Rocky Mountains are most severe in California, Washington, Oregon, and Idaho. Infestations in California cover more area than those in all other states combined.

It is likely that there were multiple introductions of yellow starthistle to the United States, and that contaminated alfalfa seed (Medicago sativa) was the primary vehicle for these introductions. Yellow starthistle seeds were found in adobe bricks from the period between 1824 and 1848 in California (Hendry 1931, as cited by ). The plant was first collected in Oakland, California in 1869 and was most likely introduced from Chile, while introductions from 1899 to 1927 appear to be from Turkestan, Argentina, Italy, France and Spain. Introduction of yellow starthistle to other western states began in the late 1800s, and it was first reported outside of California near Bingen, Washington, around 1900. Yellow starthistle began spreading into grasslands in the Pacific Northwest in the 1920s. By 1985 yellow starthistle had spread to about 8 million acres (3.2 ha) in California. Extensive road building, suburban development, and expansion in the ranching industry since the 1960s have contributed to the rapid and long-range dispersal of seed and the establishment of new satellite populations

Conditions of Poisoning:
Toxic to Horses after Ingestion

Toxic Principle:

Clinical Symptoms:
The plant is not usually eaten unless other forage is unavailable, or when incorporated into hay. Poisonings occur at ingestions of 50%- 200% of the animal’s body weight over a 60 to 90 day period of grazing. Clinical signs have been referred to as “chewing disease”, where horses chew but are unable to obtain food or swallow. Signs seen include a grinning appearance, open mouth with protruding, lolling tongue, yawning, head tossing, central nervous system depression, lost interest in food, difficulty breathing due to inhalation of food, submerging the head in water in an attempt to drink. Death can occur due to lack of eating and drinking, leading to dehydration and malnutrition.


 Lone Star English Setter Club provides this information
as a partial reference of the potential poisons that could harm your dog.
We are not veterinarian’s and DO NOT provide medical help.

 If you think that your animal is ill or may have ingested a poisonous substance,
contact your local veterinarian or
the ASPCA’s 24-hour emergency poisoning hotline directly