Canadian Thistle - Cirsium arvense

Common Name: 
Canadian Thistle

A number of other names are used in other areas or have been used in the past, including: Canada thistle,[8] Canadian thistle, lettuce from hell thistle, California thistle, corn thistle, cursed thistle, field thistle, green thistle, hard thistle, perennial thistle, prickly thistle, small-flowered thistle and way thistle. The first two names are in wide use in the United States, despite being a misleading designation (it is not of Canadian origin).

Creeping thistle is a herbaceous perennial plant growing up to 150 cm, forming extensive clonal colonies from thickened roots that send up numerous erect shoots during the growing season.

Underground network: Consists of four types of structures, 1) long thick horizontal roots, 2) long thick vertical roots, 3) short fine shoots, and 4) vertical underground stems. Though asserted in some literature, creeping thistle does not form rhizomes. Root buds form adventitiously on the thickened roots of creeping thistle, and give rise to new shoots. Shoots can also arise from the lateral buds on the underground portion of regular shoots; particularly if the shoots are cut off through e.g. mowing or when stem segments are buried.

Shoots and leaves: Stems 30–150 cm, slender green and freely branched, smooth and glabrous (having no trichomes or glaucousness), mostly without spiny wings. Leaves alternate on the stem with their base sessile and clasping or shortly decurrent. The leaves are very spiny, lobed, up to 15–20 cm long and 2–3 cm broad (smaller on the upper part of the flower stem).

Flowers and seeds: The inflorescence is 10–22 mm diameter, pink-purple, with all the florets of similar form (no division into disc and ray florets). The flowers are usually dioecious, but not invariably so, with some plants bearing hermaphrodite flowers. The seeds are 4–5 mm long, with a feathery pappus which assists in wind dispersal.1–5 flower heads per branch, with plants in very favourable conditions producing up 100 heads per shoot. Each head contains an average of 100 florets. Average seed production per plant has been estimated to 1530. More seeds are produced when male and female plants are closer together as flowers are primarily insect-pollinated.

The species is widely considered a weed even where it is native, for example being designated an "injurious weed" in the United Kingdom under the Weeds Act 1959.  It is also a serious invasive species in many additional regions where it has been introduced, usually accidentally as a contaminant in cereal crop seeds. It is cited as a noxious weed in several countries

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