Thistle

Thistle is the common name of a group of flowering plants characterized by leaves with sharp prickles on the margins, mostly in the family Asteraceae. Prickles often occur all over the plant – on surfaces such as those of the stem and flat parts of leaves. These are anadaptation that protects the plant against herbivorous animals, discouraging them from feeding on the plant. Typically, an involucre with a clasping shape of a cup or urn subtends each of a thistle's flowerheads.

 

Thistles, even if one restricts the term to members of the Asteraceae, are too varied a group for generalisation; many are troublesome weeds, including some invasive species ofCirsium, Carduus, Silybum and Onopordum. Typical adverse effects are competition with crops and interference with grazing in pastures, where dense growths of spiny vegetation suppress forage plants and repel grazing animals from eating either the thistle plants or neighboring forage. Some species, although not intensely poisonous, do affect the health of animals that swallow more than small amounts of the material.

Conversely however, the genus Cynara includes commercially important species of artichoke and some species regarded as major weeds are commercial sources of vegetablerennet used in commercial cheese making. Similarly, some species of Silybum that occur as weeds, also are cultivated for seeds that yield vegetable oil and pharmaceutical compounds such as Silibinin.

Other thistles that nominally are weeds are important honey plants, both as bee fodder in general, and as sources of luxury monofloral honey products