Canada thistle, Circium arvense, isn't from Canada at all. That's not to say its places of origin in Europe, western Asian, and northern Africa would want to claim it: the species is an agricultural pest wherever it's found. Livestock won't eat it. It reduces pasture production and hay quality. And it just won't go away. Vermont was the first state to outlaw its distribution in 1795 (that's right, 1795!), but it is hugely common today along roadsides, in fields and pastures, and at the drier edges of low spots throughout the region.
Photo: Mary Ellen (Mel) Harte, Bugwood.org
Canada thistle is especially obvious now. Long days trigger flower production, and plants are covered now with pretty purple flowers. (Bull thistle, a much more robust plant, is about to bloom too. Its flower stems are winged; Canada thistle flower stems are smooth.) Flowers will be followed by thousands of seeds per plant, but the plant doesn't spread much by seed. Instead, it spreads by root buds on roots that can stretch laterally up to twenty feet a season. Most of the plants in a colony of thistle are in fact the same plant: the shoots are clones arising from a single set of shared roots. Even tiny pieces of root can send up new shoots, which is why the plant persists in tilled plots and fields.
Since 1795, Vermont has been joined by more than forty states and Canadian provinces in outlawing the movement of this plant. That it has so many governmental enemies is a sign of how few natural enemies it has and of just how aggressive, therefore, it can be. A number of beetles, weevils, and flies have been introduced to control it, and the native painted lady butterfly caterpillars feed on it, but these organisms haven't made a dent in thistle populations. A native rust and a fungus attack the plants (it's the rust, I believe, that I've seen lately), but they aren't lethal either.
Photo: Alec McClay, McClay Ecoscience, Bugwood.org
What's left for gardeners and farmers? Mechanical and chemical controls. Some studies have found that monthly mowing throughout the growing season can exhaust root reserves after two or more years. Tilling again and again, again for years, will also eradicate it. For small infestations, mowing then covering the plants with black plastic or cardboard or layered newspapers under hay mulch will eventually kill the colony.
For those willing to use herbicides, Roundup™ applied in August about two months after mowing or tilling (after the plants have been somewhat weakened bur are again vigorously transporting food and water to their roots) has given good control. Using a spray device that pinpoints the application of the chemical will help avoid damage to desirable plants. (Roundup™ kills nearly all actively growing herbaceous plants.)