Canada Thistle: May’s Weed of the Month
Canada thistle is a wide spread invasive weed that’s a common problem in cultivated fields and pastures. It’s a perennial plant that spreads both by seed and by creeping horizontal roots (called rhizomes). This weed is very difficult to kill.
Canada thistle has multiple small pink to purple flowers that are bristly and 1/2 inch long and wide. The plants are usually 3 to 5 feet tall with green leaves that are glossy on the upper surface and wooly on the underside. The leaves are irregularly lobed with sharp spines. Seeds have tufts of tan hair loosely attached to the tip and are dispersed by the wind. They can remain viable in the soil for up to 20 years.
This plant reproduces both from seeds and from rhizomes. A plant can send up new shoots from its extensive root system just 6 weeks after germination. After one growing season a single plant can develop a root system with a 20 foot spread. Roots that are cut can grow into new plants. Tillage and cultivation will spread infestations, but may also be carefully used to control plants, as described below.
Canada thistle grows best on open, moist, disturbed areas including roadsides, meadows, pastures, ditches, campgrounds, landslides, logged and fire areas.
Control Canada thistle by hand-pulling early in the season or by tilling the soil every 3 weeks for an entire growing season. Control can also be obtained with the use of herbicides (contact the OSU Extension Master Gardeners). Timely mowing can be used to prevent the plants from going to seed. Grazing is not an effective means of control as Canada thistle is not palatable and animals tend to avoid it.