By Josie Borlongan, Garden Guides Contributor
Canada thistle weeds are clump- or patch-forming perennials that grow to about 1m tall. They have extensive creeping roots and small unisexual flower heads. Canada thistle weeds are either male or female (dioecious). Canada thistle weeds have hairs or on the surface of their stems and leaves. They have oblong to lanceolate leaves. The flower heads have rows of spine-tipped phyllaries that are overlapping. The disk flowers interspersed with bristles on the receptacle. Canada thistle weeds consist of an extensive network of roots that are either vertical or horizontal that, sometimes, can survive even in frozen soil.
Canada thistle weeds thrive in cool temperate regions climate. The common habitat for the Canada thistle weeds are open disturbed sites, roadsides, fields, pastures, hillsides, rangeland, forest openings, stream banks, and gardens. Canada thistles can tolerate a wide range of soil types, but they grow best in moist soils. They typically do not tolerate deep shade or constantly wet soils.
Cultivation and Care
Canada thistles reproduce from creeping roots and by seed. New roots and shoot buds develop during winter, while in the spring shoots would emerge. Roots and buds are developed from subterranean shoots at any node. Root and subterranean shoot fragments, approximately 1cm long or more, then develops into a new plant. They are insect- and self-pollinated; their flowers bloom from June to October. During male to female pollination, they can produce viable seeds. When pollination occurs, viable seeds develop within 8 to 10 days after flowering. Some male flowers of the Canada thistles have functional ovaries that can develop few seeds. Seeds can germinate within 3 years of maturing. The deeply buried seeds can survive up to 10 years or more. Seed germination occurs from mid- to late spring. Heavy grazing and disturbances that create bare soil patches facilitate Canada thistle weed seedling establishment and survival.
Weed Control Techniques
- Cultural Control: Plantings that can compete with Canada thistles for resources can help in controlling their spread. For example, planting perennial grasses and alfalfa close to Canada thistles can lessen infestations. Take note that alfalfa is only effective after it is well established; it will perform poorly as a seedling, especially in dense Canada thistle infestations. * Mechanical Control: Mowing frequently can control and reduce the population of Canada thistle weeds. This method is ineffective if done infrequently, and in natural areas where you normally would not mow. Mowing must be done when flowers begin to bloom to prevent viable seeds from coming out or being reproduced. Another way for mowing to be more effective is by doing low mowing and by applying herbicides or weed killers. * Tilling can be used to control weeds; but be careful not to spread root fragments to new areas where they can cause new infestations. When tilling, be sure to do it deep enough and not shallowly, so that it would be more effective. Biological Control: Several organisms can help in the Canada thistle weed control. Two examples of insects introduced in California, Ceutorhynchus litura and Urophora cardui, are being used as biological control of Canada thistles. The female insects would lay eggs underneath the Canada thistle leaves in early spring. The eggs will then turn into larvae that bore into the main leaf vein, then down into the plant's crown area. This will then cause the Canada thistle to die when it is heavily infested. * This type of control is not effective on its own. It should be combined with other methods to be more successful in controlling the population of the Canada thistle weeds. * Prescribed Fire: Canada thistle may be controlled by burning. Proper timing of the burn can yield a positive outcome. Burning Canada thistles during their dormant season may remove the dense canopy layer and release nutrients, favoring other species and resulting in increased competition. Chemical Control: Canada thistle weeds can be controlled with the use of chemicals such as herbicides. The most effective treatments include glyphosate, clopyralid chlorsulfuron, and dicamba. Check the labels for the recommended use.