The attractive flowers on Canadian thistle belie the fact that this plant is one of the most persistent weeds that can come into the garden. A native of Eurasia, it was brought to the United States in the early 1600s and has since spread to all but seven southern states. It is a cool weather perennial and thrives in sunny areas such as meadows, prairies, fields, pastures and waste places. It tolerates a wide range of soils and can invade wet areas such as stream banks.
Description: Branched erect stems grow 1½ – 4 feet tall and are rigid, grooved, and slightly hairy. Lanceolate leaves are alternate, simple and irregularly lobed, and bear sharp spines along the edges. Beginning in early summer the plant produces fragrant, lavender or white flower heads one inch in diameter. Male and female flowers are often on separate stems or plants. Each flower head can produce up to 5,000 wind dispersed- seeds that can live for as long as 20 years in the soil. But the seeds are only a small part of the problem. The real threat comes from the extensive root system that consists of a long taproot growing to a depth of five feet or more and a massive horizontal root system higher in the soil that produces buds to create new stems. This extensive root system stores nutrients that allow the weed to survive and produce new plants even when the top has been removed.
Control: Control is very difficult. The root system is so deep and extensive that hand pulling is ineffective and tilling does not go deep enough to sufficiently damage it. Herbicides such as Round Up can be effective if applied when the plant is in active growth, that is, just before flowering and later in the fall as growth begins at the base of the plants after a summer dormant period brought on by high temperatures. Removing the flower heads before seeds form is helpful but killing the roots is necessary for control. A couple of years of spraying a herbicide may be necessary.