INVASIVE SPECIES SPOTLIGHT: CANADA THISTLE
Invasive plants take over and degrade natural ecosystems. They disrupt the intricate web of life for plants, animals, and microorganisms and compete for limited resources. One such plant is the invasive Canada thistle, also known as Canadian thistle.
The origins of Canada thistle lie in Eurasia. It was introduced into the United States in the 1600’s, most likely by accident. By 1954 it had been declared a noxious weed in 43 states. Canada thistle is now declared a “noxious” weed throughout the U.S. and is a major agricultural pest, costing millions of dollars in direct crop loss and control. Canada thistle is also harmful to native species and natural ecosystems.
Canada thistle is distributed throughout the northern U.S., from northern California to Maine and southward to Virginia. It is also found in Canada, for which it was named. Canada thistle grows in barrens, glades, meadows, prairies, crop, pastures, and waste places. It does best in disturbed upland areas but also invades wet areas with fluctuating water levels.
Canada thistle produces an abundance of bristly-plumed seeds which are easily dispersed by the wind. Most of the seeds germinate within a year, but some may remain viable in the soil for up to twenty years or more. Vegetative reproduction in Canada thistle is aided by a fibrous taproot capable of sending out lateral roots as deep as 3 feet below ground, and from which shoots sprout up at frequent intervals. It also readily regenerates from root fragments less than an inch in length. Management of Canada thistle can be achieved through hand-cutting, mowing, controlled burning, and chemical means, depending on the level of infestation. Due to its perennial nature, entire plants must be killed in order to prevent regrowth from rootstock. Hand-cutting of individual plants or mowing of larger infestations should be conducted prior to seed set and must be repeated until the starch reserves in the roots are exhausted. Because early season burning of Canada thistle can stimulate its growth and flowering, controlled burns should be carried out late in the growing season for the best results.
In natural areas where Canada thistle is interspersed with desirable native plants, targeted application of a systemic herbicide such as glyphosate (e.g., Roundup® or Rodeo®), which carries plant toxins to the roots, may be effective. For extensive infestations in disturbed areas with little desirable vegetation, broad application of this type of herbicide may be the most effective method of control. Repeated applications are usually necessary due to the long life of seeds stored in the soil. There are also some Biocontrol methods underway in North America. At least 7 insect species have been released to aide in the control of this noxious weed.
The information contained in this article comes from the two websites below. For more information on Canada thistle and other invasive species please visit the National Park Service website or the USDA Forest Service website.
To volunteer for the Weed-Warriors, a growing and important effort to remove invasive plant species from Montgomery County parks, visit http://mcparkandplanning.org/parks/ or call volunteer services at 301-495-2504.
For pesticide application contact: Montgomery Weed Control, Inc. PO box 5681 Derwood, MD 20855 Gary Barkman Sr. 301-503-6024 Lane Heimer – 410-841-5871