"Thistle" doesn't just refer to a single plant. There are many plants that bear the name, including: Scotch thistle, milk thistle, Canadian thistle, star thistle and even a plant or two, like Nyjer, which aren't actually thistles. All of these plants produce seeds which are valued for various uses. Depending on the species, they may have medicinal utility, or they might just provide gourmet dining for your neighborhood wild birds.
The Scotch thistle (Onopordum acanthium), native to Europe and Asia and newcomer to the U.S., has the classic spiny leaves and globe-shaped flower heads in shades from dark pink to lavender that you probably picture when you hear the word "thistle." As its name suggests, it's the national emblem of Scotland, beloved for having purportedly warned the Scots of Viking attack when the invaders stumbled across the spiky plants and cried out in pain.
Though Scotch thistle is often considered a weed, it was introduced to the U.S. as an ornamental plant. It's still sometimes sold today for that purpose. The base of its flower, the receptacle, is edible and can be prepared much like an artichoke. The stem hairs have been used to stuff pillows. In Europe, the oil of the slender, plumed seeds has been used as fuel.
According to Washington State Noxious Weed Control Board, the Scotch thistle is associated with reports of treating cancers, ulcers and excess discharges of mucous membranes.
Native to the Mediterranean region, you can find the milk thistle (Silyburn marianum) in dry, sunny areas around the world. Flowers are red-purple and produce small, hard-skinned fruit that is brown, spotted and shiny. Like the Scotch thistle, milk thistle is covered in spines. Its wide leaves have white blotches or veins. When crushed, they ooze the milky fluid for which the milk thistle is named.
Milk thistle contains a compound called silymarin. This is in fact a group of flavonoids called silibinin, silidianin and silicristin. According to the University of Maryland Medical Center, silymarin is thought to help repair damaged liver cells and keep new ones from being destroyed by alcohol and other toxins. Additionally, silymarin reduces inflammation and is an antioxidant.
Most milk thistle products contain 70 to 80 percent of silymarin, and are produced from the seeds of the plant. Other available forms include dried herb capsules, liquid extracts and tinctures.
The Nyjer (Guizotia abyssinica) produces a black birdseed whose high oil and caloric content makes it popular for wild bird feeding. Finches especially love it, as do pine siskins, redpolls, doves, juncos and sparrows. The seeds are commonly called "thistle," but, according to eBirdseed.com, this is inaccurate. The Nyjer has yellow, daisy-like flowers quite unlike those of actual thistle plants.
Nyjer seeds are heat-sterilized before being exported to the U.S. to prevent germination of invasive weed seeds that might be hitching a ride. Some rare few do arrive viable, but volunteer seedlings don't propagate well in any part of the U. S.
Since Nyjer seeds come at a premium price due to importation and sterilization, eBirdseed.com recommends feeding birds in upside-down feeders. This not only prevents seed loss but also allows goldfinches and pine siskins to feed without competition from house and purple finches.