Northern Prairie Wildlife Research Center
Canada thistle is a dioecious, perennial forb (up to 1.5 m tall). Its erect stem is highly branched above, green, and glaberescent to covered with dense cobweb-like hairs. Canada thistle usually occurs in small to large patches with numerous individuals arising from horizontal, lateral roots bearing adventitious shoots. Leaves are simple and placed alternately on the stem. Lower cauline leaves are oblong to oblanceolate (5-18 cm long and 1.5-6 cm wide) and entire or shallowly to pinnately lobed. Each lobe has few to many spines (up to 5 mm long). Both leaf surfaces may be glabrous, or the upper surface may be lightly pubescent while the lower surface is densely pubescent. Cauline leaves are reduced in size upwards and less lobed. Leaves may have a petiole (up to 1 cm long) or be sessile, clasping, or short decurrent. Heads are numerous, and occur in terminal corymb-like clusters. Each head is discoid and unisexual or incompletely dioecious. Pistillate flowers are 1-2 cm high and 0.5-1 cm wide, and staminate flowers are somewhat shorter. Bracts are imbricate, in five to six rows, ovate to lanceolate (2-6 mm long and up to 1.2 mm wide), spine-tipped with a spine up to 1 mm long, and glabrous to covered with a dense cobweb-like hair. The corolla is tubular and pink or purple in color (occasionally white). Staminate corolla tubes are 12-14 mm long, and anthers are 3.5-4 mm long and occasionally have vestigial pistillate parts. Pistillate corollas are longer (19-24 mm long) and may have vestigial anthers. Achenes are light brown to straw-colored (2-4 mm long and up to 1.5 mm wide). Each achene has a pappus of numerous white to grayish plumose bristles reaching up to 2.5 cm in length. The four recognized varieties of this species are var. vestitum Wimm. & Grab., var. integrifolium Wimm. & Grab., var. arvense (L.) Scop., and var. horridum Wimm. & Grab.
Canada thistle is a highly competitive, noxious weed. It was apparently introduced from Eurasia into North America in colonial times as a contaminate of agricultural seed. Now a naturalized weed, Canada thistle is most commonly found in agricultural lands, pastures, and rangeland. It has become established in forests, riversides, roadsides, lawns, gardens, abandoned fields, and ditchbanks. Canada thistle can now be found in all of the lower 48 states and all of the Canadian provinces.
Canada thistle is most common in open, mesophytic areas. It has a temperature tolerance of -35 to 40°C. Optimal annual precipitation is 400 to 750 mm. It grows in a wide variety of soils, including sand dunes, but it is most abundant in clayey soils. It can tolerate saline soils and wet or dry soils, but it grows best in dry soils. Also, Canada thistle does not readily tolerate water logged, poorly aerated soils. However, it may be found growing under these conditions in a lowered condition. Disturbance is necessary for initial establishment, however, once established it may rapidly spread by both rhizomes and seed. Generally, Canada thistle is not shade tolerant. Its growth is reduced when light falls to 60-70% of full daylight, and death occurs when light is reduced to 20% of full sun. This may explain why Canada thistle does not persist in tallgrass prairies in good to excellent condition.
Extensive rhizomes of Canada thistle make it unique among the thistles. Rhizomes develop at depths far below the zone of rhizome development for most species. Most rhizome development occurs in the first 75 cm of the soil, but they have been reported to occur nearly 7 m deep. Lateral root growth of up to 6 m in one growing season has been recorded. Root buds are produced on lateral roots at 6- to 12-cm intervals. With these closely placed buds, root fragments as small as 8 mm in length and 3-6 mm thick have produced new shoots, and root fragments 13 cm in length nearly always produce new shoots. Root fragments can produce viable shoots in as few as 5 days. Root/shoot elongation increases with temperature and photoperiod. Elongation is greatest at 25/15°C day/night temperatures, soil temperatures of 30°C, and a photoperiod of 15 hours. Root reserves are lowest just prior to flowering, and are the greatest in early fall when aboveground growth stops.
Shoots begin to emerge in the early spring when soil temperatures reach about 5°C. Development of rosette leaves occurs first, followed by vertical elongation early summer. Flowering is generally from June to September when day length reaches 14 to 18 hours. Canada thistle is incompletely dioecious, with the staminate and pistillate flowers usually borne on separate plants. Therefore, natural patches are usually of one sex. Flowers are pollinated by insects, primarily honey bees and wasps. Each plant produces from 30 to 100 heads in a season. One plant has the potential to produce up to 5,200 seeds in a season, but the average seed production is about 1,500 seeds per plant. Seeds are dispersed primarily by wind. Seed size is variable averaging 650,000 to nearly 1,500,000 per kg.
Germination rates of between 50 and 95% have been observed. An average of 90% of the yearly seed production germinates within 1 year. Studies have shown that some seed can remain viable in the soil for up to 21 years and up to 4 months in water. Optimal germination in the laboratory occurs with temperatures at a constant 30°C or where temperatures alternate between 20 and 30°C or 30 and 40°C. Germination is restricted with osmotic pressures above 0.5 megapascals. Optimal germination is between pH 5.8 and 7.0. Each crop of seed produces a succession of seedlings. Some will germinate that fall and produce a rosette. These will then flower the next summer. Other seeds will not germinate until the next spring (or later) and may or may not flower that year.
Some evidence indicates that Canada thistle may have an allelopathic effect, however no specific compound has been isolated. Autotoxicity has been hypothesized in some instances.
Several widespread and dense populations of Canada thistle plants occur at Scotts Bluff National Monument (SCBL). They generally grow in patches that when combined cover an area of 11 to 50 hectares. Canada thistle plants are found on sites disturbed within the last 10 years. Populations were located along the canal and in the bottom areas along the North Platte River. In addition, Canada thistle was found in scattered patches throughout the park north of Highway 92, which intersects the park. This survey did not locate Canada thistle plants south of Highway 92, however potential exists for populations to exist in the bottom areas. Canada thistle has the potential to invade and modify existing native plant communities and are an endangerment to the areas' primary resources. The plants have a significant negative visual impact on the vegetation of SCBL.
Biological, chemical, cultural, and mechanical methods have all been used to control Canada thistle with varying levels of success. An important consideration in controlling Canada thistle is that the seeds have the potential to remain viable in the seed bank for at least 20 years. Thus, removal of living plants may not totally eliminate the problem.
Prior to applying any control method, it is important to determine if enough desirable plants are present to replace the Canada thistle. If desired vegetation is scarce or absent control will be of little value. Most control methods harm other plants. The resulting disturbances favor reinvasion by Canada thistle or other exotic species. It is also important to note that native thistles are present in the area, and should not be subjected to control. Proper identification is important.
Frequent mowing over a number of years will control Canada thistle. Most studies indicate the need to mow patches of Canada thistle at least twice a year to prevent seed dispersal and reduce root reserves. Systematic monthly mowings may be necessary to prevent lateral flower bud development and to keep root reserves depleted. Tillage may be used to control Canada thistle. However, tillage may result in an increase in abundance due to the spreading of rootstocks and the creation of a disturbance. Tillage should be to a depth of 10 cm when the elongated shoots are 8- to 10-days-old. Tillage should be repeated at a minimum of 21-day intervals. Canada thistle has a relatively high light requirement, and smother crops may provide some measure of control by shading. Smother plants that have been used include sweetclover, alfalfa, millet, sorghum, hemp, and small grains.
No prescribed burning studies have been conducted to specifically control Canada thistle. Supplementary information has shown that repeated burning in May or June reduced thistle abundance in grasslands. It should be noted that Canada thistle showed an initial increase in abundance, followed by a notable reduction in abundance in most of these studies.
A number of chemical control options exist for Canada thistle. Many herbicides are not specific to Canada thistle or may not be specifically licensed for this use. It is important to read and follow all label directions. Prior to the utilization of "modern" herbicides, compounds such as sodium chloride, sodium arsenite, calcium arsenite, sodium chlorate, and carbon bisulfide were all used in attempts to control Canada thistle. Numerous herbicides are now available for control of Canada thistle. Tordon (picloram) is probably the most effective. Tordon may give a 95% control in the first year when applied in the spring prior to flowering or in the fall during active rosette growth. Banvel (dicamba) or 2,4-D amine will suppress or control Canada thistle. However, more effective control may be achieved by combining the two herbicides in a 1:1 mixture with the label recommendation of water. This mixture should be applied in the spring prior to flowering or in the fall when the rosettes are actively growing. Roundup (glyphosate) applied at the bud stage or during the active growth period in the fall will also control this thistle. Amitrol (amitrole) applied when the plants are in the bud stage has yielded 70% control in the first year. Most herbicides, except Tordon, should not be applied while the plants are in a moisture stressed condition. Other herbicides that have shown potential to control Canada thistle are Buctril (bromoxynil), Curtail (clopyralid plus 2,4-D), and Stinger (clopyralid).
Biological control of Canada thistle has received some attention. Over 80 native species of insects and over 50 species of animals and fungi utilize Canada thistle to some extent. A few have the potential for providing some measure of control. The four with the greatest threat are two beetles [Cassia rubiginosa Muell. (Coleoptera:Chrysomelidae) and Cleonus piger (Coleoptera:Curculionidae)], one fly [Orellia ruficauda Fab. (Diptera:Tephritidae)], and the painted lady butterfly [Vanessa cardui L. (Lepidoptera:Nymphaidae)]. Only Orellia ruficauda appears to do significant damage to Canada thistle, and this level of damage is not sufficient for control. Five European insect species [Ceutorhynchus litura F. (Coleoptera:Curculionidae), Rhinocyllus conicus Froelich (Coleoptera:Curculionidae), Altica carduorum Guerin-Meneville (Coleoptera:Chrysomelidea), Lema cyanella L. (Coleoptera:Chrysomelidae), and Urophora cardui L. (Diptera:Tephritidae)] have all been released in North America for Canada thistle control. To date, only Ceutorhynchus litura has become established, spread, and begun to suppress this plant.
Fungus species of the genus Puccinia hold some promise as control agents. Puccinia punctiformis (Strauss) Roehling (Fungus:Uredinales) has been tested in Europe and New Zealand and has been found to only reduce plant vigor. The best biological control of Canada thistle has come when this fungus has been used in conjunction with either 2,4-D or Ceutorhynchus litura. Plants treated with the fungus followed by introduction of the weevil had over a 50% increase in damage over nontreated plants.
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