Cirsium vulgare, the spear thistle, bull thistle, or common thistle, is a species of the Asteraceae genus Cirsium, native throughout most of Europe (north to 66°N, locally 68°N), Western Asia (east to the Yenisei Valley), and northwestern Africa (Atlas Mountains). It is also naturalised in North America, Africa, and Australia and is an invasive weed in some areas. It is the national flower of Scotland.
The plant provides a great deal of nectar for pollinators. It was rated in the top 10 for most nectar production (nectar per unit cover per year) in a UK plants survey conducted by the AgriLand project which is supported by the UK Insect Pollinators Initiative. Marsh Thistle, Cirsium palustre, was ranked in first place while this thistle was ranked in sixth place. It also was a top producer of nectar sugar in another study in Britain, ranked third with a production per floral unit of (2323 ± 418µg).
It is a tall biennial or short-lived monocarpic thistle, forming a rosette of leaves and a taproot up to 70 cm long in the first year, and a flowering stem 1ń1.5 m tall in the second (rarely third or fourth) year. It sometimes will function as an annual, flowering in the first year. The stem is winged, with numerous longitudinal spine-tipped wings along its full length. The leaves are stoutly spined, grey-green, deeply lobed; the basal leaves up to 15ń25 cm long, with smaller leaves on the upper part of the flower stem; the leaf lobes are spear-shaped (from which the English name derives). The inflorescence is 2.5ń5 cm diameter, pink-purple, with all the florets of similar form (no division into disc and ray florets). The seeds are 5 mm long, with a downy pappus, which assists in wind dispersal. As in other species of Cirsium (but unlike species in the related genus Carduus), the pappus hairs are feathery with fine side hairs.
Spear thistle is often a ruderal species, colonising bare disturbed ground, but also persists well on heavily grazed land as it is unpalatable to most grazing animals. Nitrogen-rich soils help increase its proliferation. The flowers are a rich nectar source used by numerous pollinating insects, including honey bees, wool-carder bees, and many butterflies. The seeds are eaten by goldfinches, linnets and greenfinches. The seeds are dispersed by wind, mud, water, and possibly also by ants; they do not show significant long-term dormancy, most germinating soon after dispersal and only a few lasting up to four years in the soil seed bank. Seed is also often spread by human activity such as hay bales.
Cirsium vulgare as a weed
Spear thistle is designated an "injurious weed" under the UK Weeds Act 1959, and a noxious weed in Australia and in nine US states. Spread is only by seed, not by root fragments as in the related creeping thistle C. arvense. It is best cleared from land by hoeing and deep cutting of the taproot before seeds mature; regular cultivation also prevents its establishment.
Despite this label, the plant has beneficial qualities beyond its very high nectar production. It produces seeds eaten by the American goldfinch, down from seed pods that is used by those birds for nesting material, and serves as a host plant for the Painted lady butterfly. The monarch butterfly and other larger-sized butterflies can feed easily from the flowers due to their large size. Some farmers, due to its usefulness as part of the natural food chain, may find it useful to allow it to grow in beetle banks. The supports for pollinators and pest-eating birds could outweigh its drawbacks. In areas where it does not form monocultures it may also be welcome as a part of a meadow landscape.
Other English names include bull thistle, Scots, Scottish, or Scotch thistle, and common thistle.