Canada Thistle, Cirsium arvense
Other names: Carduus arvensis, California thistle, creeping thistle, cursed thistle, field thistle, green thistle, perennial thistle, small-flowered thistle
Origin and distribution
Canada thistle is a native of southeastern Eurasia that immigrated to North America in the early 1700's, probably as a contaminant of crop seed. Canada thistle is naturalized in 58 of the 88 counties in Ohio. The plant grows in cultivated fields, pastures, rangelands, roadsides, waste places, and other open areas. It is capable of growing in inhospitable sites such as sand dunes, but the conditions it prefers are clay- loam soils, ample moisture, and full sun.
Canada thistle can be distinguished from other spiny thistles by its creeping perennial roots, which extend downward as well as horizontally, and its relatively smooth spineless stems. Also, it has small lavender flower heads that arise singly or in groups of 2 to 5 at the ends of stems and axillary branches. The plant reproduces by seeds and dense patches of shoots emerge from creeping roots.
The species has a vigorous root system that grows up to 3 feet deep as well as grows horizontally.
Seedlings and shoots
The first two leaves to emerge from a seed (cotyledons) are oval, crisp, thick, dull green, and united at the base forming a shallow cup. Subsequent leaves are egg-shaped and have bristly hairs on the upper and lower surfaces. Edges of young leaves are wavy and irregularly toothed with a sharp prickle at the end of each tooth. Initially, seedlings consist of basal leaves attached to a compressed stem with elongates later in the season. Leaves attach to the stem by way of clasping bases.
The erect green stems are grooved, much branched, and lack spiny wings. Stems can grow up to 4 feet tall.
Leaves follow an alternate pattern (1 leaf per node) and are generally oblong with edges that are irregularly lobed and spiny. Leaves are dark green and smooth on top and light green and often hairy beneath. Leaves attach to stems by way of a clasping base.
Canada thistle is dioecious meaning flowers that are functionally male and female are produced on separate plants. However, there is little difference in the appearance of the two flower types. Flowers consist of as many as 100 lavender (rarely white) tubular florets clustered onto a head and surrounded by scale-like leaves (bracts). Flower heads are flask-shaped, 1/4 to 3/4 inch wide, and produced singly or in groups of 2 to 5 at the ends of stems and axillary branches.
Fruits and seeds
Mature seeds are brown, 3/16 inch long, and are similar in shape to a chile pepper. A plume of tannish silky hairs (pappus) is attached to one end.
The key principle to Canada thistle control is to stress the plant and force it to use stored root nutrients. Canada thistle can recover from almost any stress, including control attempts, because of root nutrient stores. Therefore, returning infested land to a productive state occurs only over time. Success requires a sound management plan implemented over several years.
Colorado State University – Extension