WEEDS CANADA THISTLE (Cirsium arvense L. Scop.) INTRODUCTION Canada thistle, also known as creeping thistle or field thistle, is native to southern Europe and the eastern Mediterranean. This aggressive perennial has creeping horizontal roots from which arise aerial shoots. It can be found throughout Canada and is locally abundant and widespread in all areas of BC. It grows in cultivated fields, pastures, roadsides, and idle areas, but also thrives in natural areas such as grasslands, forest openings, streambanks and shorelines. This thistle grows best in open, well-drained, sunny sites where soils are deep, moist and loamy clay. Canada thistle infests crops of all kinds throughout BC. It reduces forage yields of pastures by decreasing moisture and nutrients. This weed may also release toxic chemicals into the soil when it is decaying. These chemicals are believed to inhibit germination and consequently reduce competition. This ecologically harmful weed is extremely hard to control. Control of this thistle species must be consistent, with emphasis on the destruction of the creeping root system. IDENTIFICATION Canada thistle is a member of the aster family (Asteraceae). It is a unisexual, perennial herb, branching out hollow aerial shoots 0.3-1.2 m (1-4 ft) tall, which arise from 15-30 cm (6-12 in) deep roots. It grows in dense patches of either female or male plants. The alternate leaves are oblong, slightly clasping, and segmented at the tips. Canada thistle can be distinguished from other thistles, by the combination of green stems without spiny wings, small almost spineless heads and creeping roots. The root system is horizontal and supports vegetative reproduction. It contains enough food reserves to bud new plants for up to one and a half growing seasons. This root system is also very easily established; a seedling has the ability to grow a horizontal root system over 6.5 m (21 ft) long in one season! Purple-white flowers are relatively small (compared to other thistles) ranging from 15-25 mm (0.6-1.0 in) across. The male flower heads are globular in shape and are smaller than the flask-shaped female flower heads. The seeds produced by female plants are tan in color, 3 mm (0.1 in) long and slightly curved with downy white hairs (pappus) that are lightly attached. These hairs act like parachutes for wind dispersal.
BIOLOGY Canada thistle reproduces by seed and by horizontal roots. Weed spread is facilitated by the production of abundant pappus, or cluster of hairs. The seed can be transported by people, animals, water and wind. Canada thistle produces an average 1,500 seeds per plant annually. Ripe flower heads contain close to 100 seeds, and have the ability to lie dormant in the soil for almost 20 years and remain viable for four years in fresh water. Ninety percent of all seeds germinate within the first year. Optimum conditions for germination are high temperatures (~ 30 o C) and seed in the upper 1 cm of the soil. Pollination is dependent on the distance between the two sexes of plants. In fact, the highest seed production is accomplished when female and male plants are almost 17 m (56 ft) apart. When male and female plants are separated by as much as 400 m (1312 ft), virtually no pollination occurs. Although Canada thistle is a prolific seed producer, it also reproduces vegetatively. New shoots can develop from both original vertical roots and from buds along the horizontal root. These roots are also able to regenerate new plants from fragments 3-6 mm (0.1-0.2 in) thick and 8 mm (0.3 in) long. This growth is due to Canada thistle s high regenerative potential. B Canada thistle A. Plant; B. Seed with pappus A
PREVENTION The most effective way to ensure that your lands do not become infested with Canada thistle is by prevention. Here are some recommendations to prevent Canada thistle invasion on your rangelands/pastures: Maintain grasslands in a healthy, vigorous condition to ensure a productive plant community; competitive perennial grasses and forbs utilize water and nutrients that would otherwise be readily available to Canada thistle. Follow a well-designed grazing plan; excessive livestock grazing reduces competition from grass and favours weeds. Regularly patrol your property for Canada thistle plants and immediately treat new infestations. Cooperate with adjacent landowners and encourage them to control Canada thistle and other weeds. Immediately revegetate disturbed, bare soils with a suitable seed mix that provides dense, early colonization to prevent weed invasion. Clean your vehicles and machinery of plant material and soil before leaving a thistle infestation. PHYSICAL CONTROL Cultivation to a depth of 1 cm (0.4 in) in the spring followed by regular cultivation every 21 days is an effective control. This must be done continually with an aggressive and dedicated approach. This cultivation method works on depleting the nutrient reserve in the roots; therefore, if cutting is delayed, the roots will replenish themselves. In a crop situation, Canada thistle may be controlled by competition. The seedlings are very sensitive if they are shaded. A good source of competition for thistle would be alfalfa. In addition, grain crops such as barley and fall rye are excellent for this type of competitive control. The late spring growth of thistle allows for crops to have an advantage over the plant, thereby possibly preventing the plant from flowering. BIOLOGICAL CONTROL There is a tephritid fly, Urophora cardui, which burrows into the stem tissue. So far, it has been established well in Canada after being released in 1974. Another tephritid fly, Terellia ruficauda, is locally abundant and destroys seeds. There are also two weevils available for biological control of Canada thistle. A seedhead weevil, Larinus planus, consumes the contents of the flower head, thereby lowering seed production. Another weevil, Ceutorhynchus litura, mines the main stems of the plant. Being a perennial plant with creeping horizontal roots, Canada thistle is not effectively controlled by pulling or digging out. Aggressive mowing for several years will deplete the root reserves but this strategy requires long-term commitment. While there are a number of insects available for the biological control of Canada thistle, effective biocontrol for this weed is a remote possibility at this time.
CHEMICAL CONTROL Herbicide treatment, with appropriate timing, can significantly reduce thistle seed production; however, permanent, long-term control of Canada thistle cannot be achieved with herbicide treatment alone. The herbicides Tordon 22K, Transline, 2,4-D, Banvel and Roundup/Touchdown are considered effective for controlling this weed. When infestations are large (>10 ha or 25 acres), spraying should be limited to spot treatment and perimeter spraying to slow the rate of spread and the ecological impact of Canada thistle. Spot treatment of 2,4-D Amine 500 at 1.75 to 4.5 L/ha or Banvel at 2.1 L/ha is effective against young, actively growing rosettes and seedlings. Annual treatments may be required with 2,4-D. Tordon 22K also provides excellent control when spot-sprayed during the seedling, bud or early flower stage at rates of 4.5 L/ha. If late summer or early fall rains are sufficient to initiate regrowth from stem bases, Canada thistle can alternatively be sprayed with Tordon 22K in the fall. Seedlings may re-appear within 3 to 4 years of a Tordon 22K application and require re-treatment. Transline, another selective herbicide, can be applied during the rosette, bud or early flower stages at rates of 0.56 L/ha. At the mid-late flower stage, Transline application rates should be increased to 0.83 L/ha. Unlike Tordon 22K, Transline can be applied to thistle plants growing under conifer trees. Annual treatments may be required with Transline. An alternate treatment for Canada thistle is a 1:2 Roundup/Touchdown:water mixture with surfactant in a wick-type applicator to wipe the leaf and stem surfaces. The best timing is late bud to early bloom stages. Retreatment should be made at 3 year intervals if resprouting occurs. Caution must be taken with this herbicide due to its non-selective nature. Consult your local Ministry of Agriculture and Food office for the most current information on appropriate chemical controls. As with all pesticide treatment, herbicide applicators must handle and apply herbicides in a safe and responsible manner, as dictated by legislation and guidelines under the Pesticide Control Act. INTEGRATED MANAGEMENT The best overall method of control for Canada thistle is an integrated program. For agricultural lands, a combination of cultivation, herbicides, and competitive crops is effective and for natural areas (limited infestations), pulling, cutting and cautious spot-application with herbicide could be used. POST-TREATMENT GRAZING MANAGEMENT During early spring, perennial bunchgrasses are growing from root reserves and are more susceptible to competition from invasive weeds. Native grass seedlings are often poor competitors with weeds so it is important to maintain weed control until the grass stand is vigorous and competitive. It is essential that enough leaf area remains after grazing to allow re-growth when soil moisture is available. Failure to alter land management practices to allow the native plant community time to recover and resist competition from weeds will lead to a reduction of forage available for grazing animals. Management practices that will assist the recovery of the native plant community include:
delay spring grazing until range readiness has been achieved 1 avoid compaction of wet soils avoid congregations of livestock whenever possible, to ensure there are no areas of high soil disturbance employ appropriate grazing systems that maintain the vigour and health of the range plants The recovery potential of areas that have been cleared of Canada thistle is very high. Plant communities that are in good condition may recover without replanting of desirable species. However, replanting competitive native grasses and forbs can help accelerate recovery of the area. 1 Range Management Handbook for BC. 1998. Available from the BC Cattlemen s Association (250) 573-3611 Brochure funded by: Information compiled by: Lisa Scott and Kristina Robbins Revised September 1999 Editorial comments and advice provided by: George Powell (BC Ministry of Forests) and Roy Cranston (BC Ministry of Agriculture and Food)