Considered noxious by some and loved by others, thistles are a broad group of flowering plants most often belonging to the aster family. Thistle is not a scientific name, instead given to those plants that produce flowers protected by a prickly pedestal-like growth or radiating spikes. Sometimes the entire plant is prickly. The spiny nature of thistles are meant to keep the plant safe from being eaten.
The thistle can be very aggressive in the landscape. Indeed, some consider the thistle to be a weed and fight back against thistle invasions. It can be a daunting fight: Connie Strunk, a Turner County South Dakota Extension educator, points out that five Canada thistle plants per square yard can create 2,777 seeds per square foot. Seeds can be widely disseminated through the wind, in the case of the Canada thistle, a half mile away from the plant.
Though many states classify certain thistles to be noxious and intrusive, a lot of thistles possess desirable qualities and are cultivated. Thistle roots, leaves, flowers and seeds are all edible, depending on the species. Bull thistles, plumeless thistles and Canada thistles, for instance, have edible leaves and stems.
Milk thistle and blessed thistle are both examples of thistle varieties that are considered medicinal and are cultivated for the purpose. Blessed thistle is traditionally reputed to cure the bubonic plague. Milk thistle supplements are widely sold in the U.S. to help detoxify the liver. The flowers produce the seeds from which the medicinal extract is derived.
Instead of a calyx, which is composed of sepals that protect a flower before it opens, thistles flowers sit atop involucres. These support the flower like a pedestal might, or else cup the flower as an open calyx would. Involucres are bracts, which are leaf-like structures.
Role in the Environment
Those wanting to attract birds and butterflies are well-served by thistles. As far as birds go, goldfinches, especially, like thistle flowers. Bees, too, visit thistle blossoms for food. Some insects eat the pollen. Butterflies like the monarch and black swallowtail frequent the flower, and some butterfly larvae exclusively feed on thistle.