Bright purple thistle blossoms attract butterflies and bumblebees to the garden in spring and summer and provide a favorite food of birds in summer and fall. Thistles thrive in harsh, dry conditions, but not all thistles are considered beneficial. Some thistles are invasive weeds that can crowd out native plants.
Milk thistle (Silybum marianum) can be identified by the white veins on the leaves. Milk thistle is cultivated in Europe and Asia. The seeds are roasted and eaten or used to make a tea or tincture to treat diseases of the liver, gallbladder and kidney. The plant is also sometimes eaten as a vegetable. Though not a member of the thistle (Circium) family, milk thistle resembles other thistles in many ways.
Russian thistle (Salsola tragus), when dried, forms the tumbleweeds seen along roadsides in some parts of the American West. Russian thistles can grow from 1 to 3 feet tall and up to 5 feet across. As they mature, they become very stiff and spiny. These annual plants die in winter. They dry and are eventually uprooted, to be blown about by the wind. Russian thistles also aren't really thistles. Their blossoms don't resemble thistles at all, but they do have deeply lobed, spiky foliage typical of thistles.
Canadian thistle (Circium arvense) is an invasive weed that spreads rapidly and can infest pastures and gardens. A mature plant can reach up to 4 feet in height. The purple blossoms are small, but each contains hundreds of seeds that float on the wind, allowing the plant to reproduce over a broad range. The roots of Canadian thistle are very deep, so mowing or cutting it isn't an effective way to control it. The herbicide glyphosate is one effective control, though it may have to be applied more than once to kill the plant.
Bull thistle (Cirsium vulgare) is a common roadside thistle with dark bluish-green, deeply lobed prickly leaves and pinkish-purple blossoms. Like other true thistles, it's a biennial, blooming in its second year. Where it become invasive it can often be controlled by repeated mowing before the plant forms blossoms or seeds.
Scotch thistle (Onopordum acanthium) was imported into the United States from Europe. It has sharp, spiny leaves covered with thick, cottony hairs and deep purple blooms. Though originally grown as an ornamental, Scotch thistle has become an invasive weed in many areas. Tilling or application of glyphosate are two methods of control.