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An Assessment of Exotic Plant Species of Rocky Mountain National Park

Cirsium arvense (L.) Scop. (Serratula arvensis L., Cirsium incanum)
Canada thistle, creeping thistle (Asteraceae)

Current level of impact
Known locations in RMNP: Moraine park, Beaver Meadows, East end of Grand Lake. Common in open meadows (including wetlands) and ponderosa pine savannas.
Assessment: Occurs throughout RMNP, over 45 acres known from 533 locations.
Origin: Introduced from Eurasia and North Africa.
Ecological distribution: Throughout northern half of U.S., north into Canada from Quebec to British Columbia. Distributed in Colorado from 4000 to 9500'.
Ecological distribution: Common in open meadows, including wetlands, ponderosa pine savannas. Roadsides, fields, pastures, meadows, and other disturbed areas, but also invading native plant communities. Tolerates temperatures of -35 to 40 C, optimal annual precipitation is 400-750 mm.
Soils: Mostly on rich, heavy soils. Can tolerate saline soils (up to 2% salt) and wet or dry soils, but grows best in dry soils. Does not tolerate water-logged or poorly aerated soils.

Perennial forb, reproduces by seeds and creeping roots. Roots may also remain dormant for many years if deeply buried. Flowers July to October. Insects or weather conditions may prevent maturation of seed. However, some insects are attracted to Canada thistle and may pollinate different patches up to 200' apart (Beck 1991).
Vegetative: Horizontal roots may extend up to 15', and may grow 6-15' deep in soil. By asexual reproduction, it is possible for a colony of entirely male plants to maintain itself. Generally, reproduction from root system contributes to localized spread, while seeds contribute to spread over longer distances.
Seed production: One plant has the potential to produce up to 5200 seeds in a season, but the average is about 1500 seeds/plant.
Seed longevity: Most studies indicate that seeds do not remain viable after 3 years of burial. However, some studies have found seeds remain viable up to 21 years and up to 4 months in water.
Seed dispersal: Dispersed by wind, contaminated crop seed, feed, manure, packing straw and irrigation water. Some studies indicate that the feathery pappus may break off, leaving the seed attached to the parent plant.
Germination: Seeds mature July-September. Seeds germinate most readily in mid-spring. Seeds do not tolerate drought stress or moist, poorly aerated soils, optimal pH range is 5.8-7.0.

Disturbance, minimal competition, and adequate light is required for initial establishment. However, once established Canada thistle may readily spread by rhizomes and seed to form single stands. One plant can occupy an area of 3-6' in diameter in two years.
Response to shade: Not generally shade tolerant. Growth is reduced when light falls to 60-70% of full daylight, and death occurs when light falls to 20% of full sun.
Level of impact: Currently listed as noxious in Colorado. Semi-colony forming.

Every piece of the root system is capable of forming a new plant. Because of root stocks, breaking up roots may only lead to more plants. Seeds also have the potential to remain viable in the soil for up to 20 years. In general, Canada thistle is able to recover from most stress by utilizing its root reserves. However, Beck ( 1991 ) pointed out that the key to controlling perennials such as Canada thistle is to stress the plant to reduce the plant's nutrient reserves.
Cultural: Grasses and alfalfa can compete with Canada thistle if moisture and fertility are at optimal levels. However, cultural control generally is insufficient to adequately control Canada thistle.
Mechanical: Frequent mowing over a number of years will help control Canada thistle. However, mowing by itself is not effective unless it is done with a high frequency. Mowing at two week or monthly intervals may be needed to keep root reserves low. Fall herbicide treatments that follow mowing can be effective management system (Beck 1991).
Chemical: Research at Colorado State indicates that the effectiveness of herbicides is best when combined with cultural or mechanical methods. Beck (1991) recommends the following methods for chemical control of Canada thistle. Tordon (0.5 lb. to 1 lb. ai/A) is effective whenever Canada thistle is actively growing. However, fall applications are especially effective. Tordon can be used in permanent grass pastures, rangeland, and non-crop areas. Banvel (2.0 lb. ai/A) and 2,4 D (2.0 lb. ai/A) can also be used together to help control this plant. 2,4-D should be applied in spring when Canada thistle is 10-15" tall (in pre-bud to early bud stages). Banvel applications in the fall can then be applied to areas of re-growth. Both 2,4-D and Banvel can be used in pastures, rangelands, and non-crop areas.
Biological control: A few species have some potential for biological control. However because of the presence of native thistles in the RMNP biological control is likely not a feasible option. The four biological control agents with the greatest threat are two beetles, one fly and the painted lady butterfly. Ceutorhynchus litura is a weevil currently used as a biological control agent in Colorado. This weevil can stress Canada thistle, and may cause death of the plant if populations are large enough. However, Beck (1991) points out that use of the weevil alone is not enough to control Canada thistle. Another biological control agent which shows some promise is a fungus (Puccinia) that is used in conjunction with 2,4-D.
Other: Canada thistle is capable of surviving flooding, but subsequent root and shoot growth is reduced.


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Beck, K.G. 1991. Canada thistle: biology and management in pastures and rangelands. 
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