More acres of Canada thistle in 1999. Perennial sowthistle has increase fivefold in ten years. Biennial thistles have doubled in ten years. Musk, plumeless and bull thistle exploded in 1999. Nearly 500,000 acres were infested with musk and plumeless thistle in 1997.
Biennials produce abundant seed crop.
Individual musk thistle flowers can produce up to 1500 seeds. The 1999 thistle seed crop was larger. Many infestations in fence lines and pasture went to seed. Thistles in low, wet areas will increase if water levels drop. These moist, marshy sites are ideal for thistle seedlings.