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Cirsium vulgare (spear thistle) is listed in the United States (where as a non-native invasive species it has been renamed "bull thistle") as a noxious weed in nine states. [6] Some species in particular are cultivated in gardens and wildflower plantings for their aesthetic value and/or to support pollinators such as bees and butterflies.
Flower heads are sometimes produced one at a time, sometimes in small groups, each head with light purple disc florets but no ray florets. [2] The flowers of Cirsium texanum provide nectar for butterflies and the foliage is used as a source of food for the larvae of the painted lady butterfly. Goldfinches also use the seeds as a food source. [5]
Cirsium horridulum, called bristly thistle, purple thistle, or yellow thistle is a North American species of plants in the tribe Cardueae within the family Asteraceae. It is an annual or biennial . [ 2 ]
The flowers are usually dioecious, but not invariably so, with some plants bearing hermaphrodite flowers. [20] The seeds are 4–5 mm long, with a feathery pappus which assists in wind dispersal. [ 26 ] [ 27 ] [ 28 ] One to 5 flower heads occur per branch, with plants in very favourable conditions producing up to 100 heads per shoot. [ 14 ]
The Cardueae are a tribe of flowering plants in the daisy family and the subfamily Carduoideae. [5] Most of them are commonly known as thistles; [6] four of the best known genera are Carduus, [7] Cynara (containing the widely eaten artichoke), Cirsium, [7] and Onopordum.
Cirsium occidentale var. candidissimum — snowy thistle [7] [8] Cirsium occidentale var. compactum — compact cobwebby thistle; a short, clumpy California endemic that grows only along the coast of the San Francisco Bay Area and the Central Coast of California [9] Cirsium occidentale var. coulteri — Coulter's thistle [10]