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Canada Thistle

Canada Thistle flowerPlant 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. The species has a vigorous root system that grows up to 3 feet deep as well as horizontally. The erect green stems are grooved, much branched, and lack spiny wings. Stems can grow up to 4 feet tall. 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.

Canada Thistle bristly seed headsPlants 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.  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. Canada Thistle before elongationBecause 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.

Beginning in 1844, Georgia landowners were required to mow land infested with the weed. Canada thistle is naturalized in 58 of the 88 counties in Georgia. 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. A survey of Georgia growers ranked Canada thistle as the most troublesome perennial weed in corn-soybean rotations.

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