Thistle, Canada


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Pest Information
Common Name: Thistle, Canada
Family Name: Asteraceae (Sunflower Famiy)
Latin Name: Cirsium arvense (L.) Scop.
Other Names: California thistle, Creeping thistle, Corn thistle, Perennial thistle, Field thistle
Provincial Designation: Noxious
Life Cycle: Perennial
Mode of Spread: Seed, Vegetative

Detailed Information
Provincial Situation: Canada thistle is found in all areas of the province with infestations ranging from low to severe.
Where it Grows (Habitat & Ecology): Canada thistle is found in annual and perennial crops, rangeland, riparian areas, non-crop areas and industrial sites. Canada thistle is best adapted to open areas in temperate regions that satisfy its’ long day requirement for flowering. Ideal habitats for Canada thistle are climates where temperatures range between 0 C and 32 C, have an average rainfall of 400 mm to 750 mm and a day length of 14 to 18 hours. Canada thistle grows readily in a wide range of soils types but prefers well drained, clay soils. It is able to survive drought conditions due to an extensive creeping root system that commonly reaches depths of 2 to 3 metres and may spread horizontally up to 6 m per year. Canada thistle seedlings require open areas with lots of sunlight and little competition in order to establish successfully but, once established, Canada thistle is very competitive and aggressive in spreading through its’ creeping root system. .
How it Spreads (Mode of Spread): Primary mode of spread is creeping roots. Seed is a secondary mode of dispersal. Seed is important in establishment of Canada thistle into new areas, but main mode of spread is via creeping roots.

Seed production
Canada thistle plants are dioecious; male and female flowers are borne on separate plants. Cross pollination is necessary for seed production, therefore, both male and female plants must be present at a site for seed production to occur. The flowers are insect pollinated with honeybees as the most common pollinators. Pollination is most successful and the greatest number of seeds produced when the male and female populations are within 33 m of each other. Canada thistle plants produce an average of 1530 seeds per plant, when pollination is successful, and may produce up to 5000 seeds per plant. The seeds, called achenes, have a small, feathery umbrella like structure called a pappus attached to them that can be picked up by the wind and carried distances of a half-mile or more. Generally, the seeds fall within several meters of the parent plant when dispersed by the pappus. At distances of 10 m from the parent plant, only 10% of pappi had seeds attached, at 1 km, only 0.2% of the pappi had seeds attached and at 2 km 0% of the pappi had seeds attached. The seeds may also be carried long distances in running water, for example streams and irrigation ditches. Most of the seeds germinate within one year but can remain viable for up to 20 years if buried deeply. The seed germinates best at high temperatures, between 25 and 30 C, where there is high light intensity and from shallow depths (optimum depth for emergence is 1 cm but will emerge from depths up to 6 cm). Seedlings only survive if competition is limited and light intensity is high.

Dispersal
Dispersal by seed is important in establishment of Canada thistle into new areas, but once established vegetative propagation is the main method of spread. The plant develops an extensive root system that spreads both vertically and horizontally. Vertical roots commonly reach depths of 2 – 3 meters and have been found at depths of up to 6.75 m while horizontal roots can spread out up to 6 m (in one season) from the mother plant. The plant can send up new shoots from either the vertical or horizontal root system. New clones can be formed every 6 – 12 cm along the horizontal root system. The plant is also able to regenerate itself from very small pieces. Root segments as small as 12.5 mm can produce new shoots and become established.

Reproduction (Dispersal): Dispersal by seed is important in establishment of Canada thistle into new areas, but once established vegetative propagation is the main method of spread.  The plant develops an extensive root system that spreads both vertically and horizontally.  Vertical roots commonly reach depths of 2 – 3 meters and have been found at depths of up to 6.75 m while horizontal roots can spread out up to 6 m (in one season) from the mother plant.  The plant can send up new shoots from either the vertical or horizontal root system.  New clones can be formed every 6 – 12 cm along the horizontal root system.  The plant is also able to regenerate itself from very small pieces.  Root segments as small as 12.5 mm can produce new shoots and become established.
Economic Importance (Beneficial Aspects): Canada thistle is used as a source of nectar for honey and young plants can be grazed by animals. It has been used as a food source in Russia and for the American Indians.
Toxicity and Other Concerns: Canada thistle reduces crop yields by competing for light, moisture and nutrients. In pasture situations, it reduces forage yields and is not grazed as its’ spiny leaves irritate the mouths of grazing animals. It reduces bio-diversity by competing with other species for light, moisture and nutrients and is allelopathic (it produces a toxin that inhibits the growth of other species). Canada thistle can also harbor destructive insects and act as an alternate host for pathogens. It can also be a contaminant in canning crops. Canada thistle can also decrease land values due to the potential cost of control and loss of recreational activity due to its’ spiny characteristics.
Origin: Native to Southeast Europe, eastern Mediterranean and Scotland.  Introduced to Canada in the early 17th century as a contaminant in farm seed.
Seed Production: Canada thistle plants are dioecious; male and female flowers are borne on separate plants.  Cross pollination is necessary for seed production, therefore, both male and female plants must be present at a site for seed production to occur.  The flowers are insect pollinated with honeybees as the most common pollinators.  Pollination is most successful and the greatest number of seeds produced when the male and female populations are within 33 m of each other.  Canada thistle plants produce an average of 1530 seeds per plant, when pollination is successful, and may produce up to 5000 seeds per plant.  The seeds, called achenes, have a small, feathery umbrella like structure called a pappus attached to them that can be picked up by the wind and carried distances of a half-mile or more.  Generally, the seeds fall within several meters of the parent plant when dispersed by the pappus.  At distances of 10 m from the parent plant, only 10% of pappi had seeds attached, at 1 km, only 0.2% of the pappi had seeds attached and at 2 km 0% of the pappi had seeds attached.  The seeds may also be carried long distances in running water, for example streams and irrigation ditches.  Most of the seeds germinate within one year but can remain viable for up to 20 years if buried deeply.    The seed germinates best at high temperatures, between 25 and 30 C, where there is high light intensity and from shallow depths (optimum depth for emergence is 1 cm but will emerge from depths up to 6 cm).  Seedlings only survive if competition is limited and light intensity is high.
How to Control: Integrated weed management (IWM) considers the overall management of a weed species with the objective of preventing the establishment of the weed from ever occurring, to prevent the spread or to minimize the impact. IWM relies on the combination of a variety of methods such as chemical, biological, mechanical, and cultural controls as well as overall preventative measures. Using IWM creates an opportunity to use herbicides more selectively, which reduces the impact on the environment as well as slow the development of weed resistance to herbicides.

Prevention
Canada thistle seedlings are not very competitive so maintaining healthy, dense vegetative cover will help to prevent establishment of new colonies. Minimizing soils disturbances and promptly re-vegetating disturbed areas with competitive plants will also prevent establishment of Canada thistle. Prevent overgrazing of pastures and use weed free hay, fill dirt and seed to prevent Canada thistle from entering into weed free sites.

Cultural Control
Hand pulling for small infestations.

Competition
Tall fescue has been reported to reduce Canada thistle density by 60-78% (Thrasher et al. 1963; Wilson and Kachman 1999; Ang et al. 1994; Reece and Wilson 1983). Hybrid wheatgrass, derived from a cross of bluebunch wheatgrass (Agropyron spicatum (Pursh.) Scrib. and Smith) with quackgrass (Elytrigia repens (L.) Neuski.), has been found to reduce Canada Thistle density by an average of 85% over 3 years (Wilson and Kachman 1999). Thick stands of the native grass switchgrass (Panicum virgatum L.) was able to reduce Canada Thistle invasion (Jewett et al. 1996). Seeding smooth brome (Bromus inermis Leyss.) and mowing for three years suppressed Canada Thistle by 90% (Derscheid et al. 1961). Dense plots of alfalfa (Medicago sativa L.) were reported to reduce Canada Thistle densities from 33 to 11 plants/m2 (Schreiber 1967) and seemed better than grasses at controlling Canada Thistle (Detmers 1927).

Fertility
Chemical and non-chemical thistle control is enhanced when fertilization makes surrounding vegetation more competitive.

Cultivation
With some exceptions, cultivation has been generally successful at reducing thistle densities, especially in regions with high rainfall. Starting tillage early in the spring is important, as delaying tillage until thistles were in bloom required two seasons for complete eradication instead of one. Weekly cultivation or alternate week cultivation gave about equally good results (Tingey 1934).

Some ecotypes can withstand more disturbance, and therefore respond differently to cultivation. Cultivation can increase the potential for soil erosion, is unable to reach deep roots, can be very costly, and may spread small fragments of roots, increasing weed distribution.

Mowing
Frequent mowing during the growing season can substantially reduce Canada Thistle populations in forage stands, and 3 years of mowing treatments severely reduced Canada Thistle in most studies. However, a single annual mowing can increase populations as a single shoot is replaced by multiple shoots resulting from the loss of apical dominance (Grekul, 2003).

Grazing
Grazing is effective in reducing thistle stands if applied for 2 - 3 years. Fence off a small section where the weed is a problem and leave the animals in this area until they graze the thistle (fenced areas need to be relative to herd size so that grass cannot regrow before they graze the thistle). Then leave pasture to rest for about 8 weeks (depending on growing conditions) so grass stays competitive. Try to graze the thistle twice a year (depending on moisture). This may not be effective on dry areas, but it is very effective in the Parkland Ecoregion (De Bruijn, 2004).

Fire
Controlled burning may only damage the above ground portion of the thistle allowing rapid regrowth from the root section or from seed. Fire should be used only in combination with other control measures.

Biocontrol
Rust fungus (Puccinia punctiformis (Strauss) + Stem weevil (Hadroiplontus litura (F.)). - a systemic rust infection that can occur in most regions of Alberta as long as moisture is not limited. Rust alone does not usually reduce stands unless combined with stem weevil. Where used in combination, these two biological control agents can reduce thistle populations by 95%.

If you interested in using this agent, please contact Dr. Rob Bourchier, AAFC Lethbridge by e-mail: bourchierR@afr.gc.ca or telephone: (403) 317-2298.

You can also visit the biological control program section of Agriculture and Agri-Food Canada website: http://res2.agr.ca/lethbridge/weedbio/index_e.htm#toc.

Integrated weed management
An integrated weed management approach is essential for control of Canada Thistle. 2,4-D amine at 0.9 L/ac applied at the bud stage for two consecutive years with mowing 2 months after each spray application provided longer term control in heavy infested grass hay land than spraying alone (Cole 1998).

Prevention: Canada thistle seedlings are not very competitive so maintaining healthy, dense vegetative cover will help to prevent establishment of new colonies.  Minimizing soils disturbances and promptly re-vegetating disturbed areas with competitive plants will also prevent establishment of Canada thistle.  Prevent overgrazing of pastures and use weed free hay, fill dirt and seed to prevent Canada thistle from entering into weed free sites.
Cultural Control: Hand pulling for small infestations.
Competition: Tall fescue has been reported to reduce Canada thistle density by 60-78% (Thrasher et al. 1963; Wilson and Kachman 1999; Ang et al.  1994; Reece and Wilson 1983).
Hybrid wheatgrass, derived from a cross of bluebunch wheatgrass (Agropyron spicatum (Pursh.) Scrib. and Smith) with quackgrass (Elytrigia repens (L.) Neuski.), has been found to reduce Canada Thistle density by an average of 85% over 3 years (Wilson and Kachman 1999).
Thick stands of the native grass switchgrass (Panicum virgatum L.) was able to reduce Canada Thistle invasion (Jewett et al. 1996).  Seeding smooth brome (Bromus inermis Leyss.) and mowing for three years suppressed Canada Thistle by 90% (Derscheid et al. 1961).
Dense plots of alfalfa (Medicago sativa L.) were reported to reduce Canada Thistle densities from 33 to 11 plants/m2  (Schreiber 1967) and seemed better than grasses at controlling Canada Thistle (Detmers 1927).
Fertility: Chemical and non-chemical thistle control is enhanced when fertilization makes surrounding vegetation more competitive.
Cultivation: With some exceptions, cultivation has been generally successful at reducing thistle densities, especially in regions with high rainfall.  Starting tillage early in the spring is important, as delaying tillage until thistles were in bloom required two seasons for complete eradication instead of one.  Weekly cultivation or alternate week cultivation gave about equally good results (Tingey 1934).
Some ecotypes can withstand more disturbance, and therefore respond differently to cultivation.  Cultivation can increase the potential for soil erosion, is unable to reach deep roots, can be very costly, and may spread small fragments of roots, increasing weed distribution.
Mowing: Frequent mowing during the growing season can substantially reduce Canada Thistle populations in forage stands, and 3 years of mowing treatments severely reduced Canada Thistle in most studies.  However, a single annual mowing can increase populations as a single shoot is replaced by multiple shoots resulting from the loss of apical dominance (Grekul, 2003).
Grazing: Grazing is effective in reducing thistle stands if applied for 2 - 3 years.  Fence off a small section where the weed is a problem and leave the animals in this area until they graze the thistle (fenced areas need to be relative to herd size so that grass cannot regrow before they graze the thistle).  Then leave pasture to rest for about 8 weeks (depending on growing conditions) so grass stays competitive.  Try to graze the thistle twice a year (depending on moisture).  This may not be effective on dry areas, but it is very effective in the Parkland Ecoregion (De Bruijn, 2004).
Fire: Controlled burning may only damage the above ground portion of the thistle allowing rapid regrowth from the root section or from seed. Fire should be used only in combination with other control measures.
Bio-Control:

Rust fungus (Puccinia punctiformis (Strauss) + Stem weevil (Hadroiplontus litura (F.)).
- a systemic rust infection that can occur in most regions of Alberta as long as moisture is not limited.  Rust alone does not usually reduce stands unless combined with stem weevil.  Where used in combination, these two biological control agents can reduce thistle populations by 95%.

If you interested in using this agent, please contact Dr. Rob Bourchier, AAFC Lethbridge by e-mail: bourchierR@afr.gc.ca or telephone: (403) 317-2298.
You can also visit the biological control program section of Agriculture and Agri-Food Canada website:
http://res2.agr.ca/lethbridge/weedbio/index_e.htm#toc.

Chemical Control: An integrated weed management approach is essential for control of Canada Thistle. 
2,4-D amine at 0.9 L/ac applied at the bud stage for two consecutive years with mowing 2 months after each spray application provided longer term control in heavy infested grass hay land than spraying alone (Cole 1998).
Life Cycle: Perennial


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Last Modified: 2012-01-16 11:38:32.44

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