Thistle is the common name of a group of flowering plants characterised by leaves with sharp prickles on the margins, mostly in the plant family Asteraceae. Prickles often occur all over the plant - on surfaces such as those of the stem and flat parts of leaves. These are an adaptation to protect 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.
The term thistle is sometimes taken to mean exactly those plants in the tribe Cynareae (synonym: Cardueae), especially the genera Carduus, Cirsium, and Onopordum. However, plants outside this tribe are sometimes called thistles, and if this is done thistles would form a polyphyletic group.
Genera in the Asteraceae with the word thistle often used in their common names include:
Plants in families other than Asteraceae which are sometimes called thistle include:
In the language of flowers, the thistle (like the burr) is an ancient Celtic symbol of nobility of character as well as of birth, for the wounding or provocation of a thistle yields punishment. For this reason the thistle is the symbol of the Order of the Thistle, a high chivalric order of Scotland.
Another story is that a Viking attacker stepped on one at night and cried out, so alerting the defenders of a Scottish castle. Whatever the justification, the national flower of Scotland is Scots Thistle, Onopordum acanthium. It is found in many Scottish symbols and as the name of several Scottish football clubs.
Thistle flowers, along with bugle and brambles flowers, are favourite nectar sources of the Pearl-bordered Fritillary, Small Pearl-bordered Fritillary, High Brown Fritillary, and Dark Green Fritillary butterflies.