Ohio Perennial and Biennial Weed Guide

Family:

Sunflower Family (Asteraceae)

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. It spread so rapidly that legislation requiring landowners to control Canada thistle was drafted in Vermont in 1795 with similar legislation enacted by other states at later dates. Beginning in 1844, Ohio landowners were required to mow land infested with the weed. 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 such inhospitable sites as sand dunes, but the conditions it prefers are clay loam soils, ample moisture, and full sun.

Plant Description:

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.

• Root System:

  The species has a vigorous root system that grows up to 3 feet deep as well as 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 that elongates later in the season. Leaves attach to the stem by way of clasping bases.

• Stems:

  The erect green stems are grooved, much branched, and lack spiny wings. Stems can grow up to 4 feet tall.

• Leaves:

  Leaves are alternate (1 leaf per node) and 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.

• Flowers:

  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 flowers 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 similar in shape to a chile pepper. A plume of tannish silky hairs (pappus) is attached to one end.

Similar Species:

Just below the flower heads of most thistle-like plants (Carduus species, Centaurea species, Cirsium species) are bracts with spines, but Canada thistle flower heads have bracts that lack spines. Despite this difference, it is still difficult to separate Canada thistle and bull thistle (Cirsium vulgare). Bull thistle is a biennial that forms a leafy rosette during its first year of growth and a flowering stem in the second while Canada thistle is a perennial with a creeping root system that gives rise to new stems. Also, bull thistle stems have spiny wings that are absent in Canada thistle.

Biology:

Plants produce flowers from June until August. Flowers are primarily insect pollinated and, since the species is dioecious, both sexes must be present and close enough for pollination to occur in order to form viable seeds. In shade, an average shoot produced 18 flower heads per year. Under more favorable conditions, a single shoot can produce over 40 flower heads and 1500 seeds in the coarse of a season. Although the pappus is fragile and easily separated from seeds, its presence indicates that wind dispersal of seeds is possible. Water is another means by which seeds disperse. Buds on creeping roots give rise to new plants. A study monitoring plant growth reported that new shoots began to form in January, grew 1 to 3 inches in February while still below the soil surface, developed rapidly during March, and were well established by mid-April. Because the species spreads by creeping roots, Canada thistle often forms dense patches. The most successful control strategies are combinations of various methods. Several herbicides are effective against this deep-rooted perennial. Repeated mowing for several years can provide control if the infestation is not severe. Cultivation that is too shallow or not repeated may only break up roots, which then sprout resulting in an even larger infestation. There are numerous naturally-occurring insect and disease pests of Canada thistle that are being studied as possible biological control agents.

Toxicity:

None known.

Facts and Folklore:

• A survey of Ohio growers ranked Canada thistle as the most troublesome perennial weed in corn-soybean rotations.

• The Latin name 'Cirsium' is a corruption of the Greek 'kirsos', which roughly translates to 'swollen vein' and describes the result of being pricked by this spiny plant

• Canada thistle seeds can withstand months of immersion in water.

• Birds such as the American goldfinch relish thistle seeds.