2. Canada thistle management strategies for sustainable and organic farming systems
Abram J. Bicksler and John B. Masiunas
University of Illinois

To see great photos of the plants, seeds and learn some strategies go to manage this week go to::  http://www.new-ag.msu.edu/#2

Canada thistle (Cirsium arvense) is a vigorous, perennial weed that spreads by a fibrous root system or wind blown seed. Canada thistle is considered a noxious weed and has become a common problem of sustainable and organic farms. What makes Canada thistle such a problem weed? It can rapidly spread, forms dense patches, suppresses growth of crops, and is poorly controlled using standard approaches. Tillage can cut the roots into small pieces, spreading patches; tillage equipment can also carry root pieces to new sites. Mowing must start at thistle flowering and be repeated numerous times. Common winter annual cover crops (i.e. cereal rye, hairy vetch, wheat) are not present during the most susceptible growth stages of Canada thistle.

What are the key factors to controlling Canada thistle? It is a long-day plant; flowering and seed production starts in July through August. Shoots must be killed to prevent seed production. Emerging Canada thistle seedlings will not survive shading from other plants. Grow competitive crops that rapidly close canopy. Thistle plants store sugars and other carbohydrates in their roots. The stored carbohydrates allow thistles to overwinter and emerge in the spring or after disturbance. Established Canada thistle is best controlled after emergence at the beginning of flowering, when root carbohydrate reserves are lowest. Depletion of these reserves will reduce the thistles’ fitness and ability to re-grow from roots.

Canada thistle biology: Knowing your enemy
Abram Bicksler, John Masiunas, and Dan Anderson
University of Illinois

Canada thistle, also called creeping thistle, California thistle, and field thistle, Cirsium arvense, is a vigorous, competitive perennial weed that can establish from seed or wide-ranging, deep creeping roots. It is native to Europe, parts of North Africa, and the Middle East, and was introduced to North America in the early 17th century and has spread extensively. Canada thistle is a problem throughout the Midwestern United States on organic, sustainable and conventional farms. Cirsium species are troublesome weeds in organic cropping systems of northern Europe.

Distinguishing Canada thistle from other thistles.  

Canada thistle can be distinguished from other thistles by three characteristics: 1) creeping horizontal lateral roots, 2) dense clonal growth, and 3) small dioecious (contain both male and female parts) flower heads (Nuzzo, 1997). Moreover, Canada thistle can be differentiated from other Cirsium and Cardus species by the following traits: 1) small dioecious flower heads <1 inch high; and 2) stems not conspicuously spiny-winged. Up to four varieties of Canada thistle have been recognized across the world, based predominantly upon their leaf morphology and pubescence. Canada thistle may change morphology in response to environmental conditions, and various ecotypes may respond differently to management practices.

Limitations to Canada thistle spread
Canada thistle is the most frequently declared noxious thistle in this country. Canada thistle requires a day-length of at least 14 to 16 hours for flowering to be induced, depending on ecotype. Canada thistle can survive winter temperatures of -17o F but is limited in its southerly distribution in the United States because it is less successful in hot, dry climates. It is generally a serious weed problem in areas receiving 18 to 36 inches of rainfall a year and it thrives on deep, productive well-aerated soils that do not become too warm. Optimum growth occurs at daytime temperatures of 77o F and nighttime temperatures of 59o F in soils with high nitrogen.

Thistle flowering and seed production
Thistles flower from July to September and sometimes into October. Seeds become viable within 8-10 days after flower opening, and an individual plant may produce from 5,000 to 40,000 seeds a year. There are conflicting reports about the number of days after flowering that a plant can be cut and still produce viable seed. Viable seed may be produced if the plant is cut down 4 days after flowering, 7 to 9 days after flowering, or 10 to 11 days after flowering. A small proportion of seeds (0.2 percent) can disperse one-half mile or more from the parent plant; thus, Canada thistle is difficult to prevent even if you are using best management practices. Canada thistle seeds float and are easily spread by flooding or in irrigation water from surface sources. Viable seeds can also be dispersed in manure. Canada thistle seeds germinate best in the top one-third inch of soil at temperatures averaging from 68o to 86o F. Approximately 60 to 90 percent of seeds germinate within one year; however, some seeds can remain dormant in the soil for up to 20 years. The deeper the seed is buried, the longer the viability.
 
Seedlings
Seedlings are sensitive to competition for light and are unlikely to survive in competition with established plants. Once established, however, Canada thistle spread is primarily vegetative in both agricultural fields and in natural communities. Within 8 to 10 weeks of emergence, seedlings develop a taproot and spreading lateral roots that can penetrate to over 2 feet after 6 months. The horizontal spreading lateral roots can grow up to 25 feet in one season, but most patches spread at the rate of 3 to 7 feet per year. At the base of these spreading lateral roots, adventitious buds develop, rendering the plants able to regenerate if hoed or cultivated.

Roots and vegetative reproduction
Edges of shoots can pinpoint the spread of the patch, because roots do not spread far beyond aboveground shoots. Most regeneration occurs from roots within or just below the plough layer. Most root buds are produced in the center of the Canada thistle patch, and each foot of root can average 4 to 8 root buds. The greatest Canada thistle shoot density corresponds to regions with the most underlying root biomass and adventitious root buds. The densities of adventitious root buds are three to five times greater in late summer than in spring. Cultivation stimulates the growth of horizontal roots, which increases the number of new vertical shoots borne by the chopped horizontal runners. Root fragments smaller than 1 inch in length may not re-grow, whereas root fragments 2 to 3 inches long readily regenerate shoots. Adventitious root buds are inhibited when the main shoot is present. If the main shoot is removed by tillage or mowing new shoots can emerge rapidly. In outdoor boxes, a single C. arvense plant was able to produce 26 emerged adventitious shoots, 154 adventitious root buds, and 360 feet of roots after just 18 weeks of growth. Even in systems that use herbicides, such as glyphosate, dose-response and time-course experiments have shown that the herbicide is more likely to reduce root bud numbers and secondary shoot re-growth potential than overall root biomass, still rendering Canada thistle able to regenerate.

Control strategies should aim to deplete carbohydrate reserves. Carbohydrate reserves are stored in roots rather than in shoot bases or root buds, and that these reserves can range from as low as 3 percent of root fresh weight in spring to as high as 26 percent of root fresh weight in the fall. Canada thistle carbohydrate reserves were lowest from May through August, increased in the fall, and then began decreasing in April. In growth chamber experiments, carbohydrate movements to and from the roots correspond to environmental conditions typical of fall and early spring. Moreover, environmental cues seem to supersede growth stage control of carbohydrate movement. Development of root buds is highest in the autumn when short days and moderate temperatures dominate, and root bud elongation is greatest with the long days and high temperatures of summer.

In addition to regeneration from root buds, Canada thistle shoots can grow from lateral buds at internodes on stem segments. These shoot pieces then survive if partially buried in the soil. This capacity to regenerate from root and stem fragments is particularly troublesome, as cultivation has been used for Canada thistle control.

Canada thistle impact on crops.
Canada thistle causes extensive crop yield losses through competition, and perhaps, allelopathy (release of inhibitory chemicals). The prickly mature foliage increases harvest difficulty and deters livestock from grazing. A density of 2 Canada thistle shoots per 1 ft2 caused yield losses of 34 percent in barley, 26 percent in canola, 36 percent in winter wheat, and 48 percent in alfalfa grown for seed. In addition to deterring livestock grazing and competing with crops, Canada thistles’ shoots, roots, and leaf litter can reduce shoot and root growth of other plants through allelopathy.

Canada thistle and other perennial weeds are very difficult to control once established, so any management strategy must also prevent their introduction and spread. Canada thistle spreads as a contaminant in crop seed, hay and packing material. Additionally, it can be spread in soil attached to vehicles and farm equipment. Thus, using clean seed, hay, and packing material and cleaning equipment are important for preventing Canada thistle from being introduced or from spreading once on a farm.

There are a few key places in the Canada thistle life cycle that are most susceptible to management. Thistle seedlings are very sensitive to competition for light and cannot survive in dense competitive stands of established plants. Canada thistle seedlings were very sensitive to plant stand density, light, soil aeration, and soil moisture conditions. The seedling to rosette and the rosette to flowering transitions, when root food reserves are at a minimum, are particularly important times for reducing Canada thistle populations. During July to October, carbohydrate reserves accumulate in thistle roots while from May to July the carbohydrate reserves are at their lowest. The lowest root carbohydrate reserves in Canada thistle occur just before flowering. If insect (such as stem-mining weevil) feeding on Canada thistle shoots occurs just before flowering, levels of carbohydrates in the root system tend to be reduced. Moreover, if shoots are tilled between the 7-10 leaf stage (for plants originating from 5 cm and 20 cm root fragments, respectively), minimum re-growth is observed. Drought stress on Canada thistle also increases the efficacy of mechanical control. Several years of drought can reduce perennial root biomass and decrease adventitious bud production, limiting adventitious shoot production.

In conclusion, you should focus on maximizing early spring rosette and seedling mortality. Reducing seed production and inhibiting the survival of newly shed seeds is also important, but persistence of seeds that are already in the soil seed bank has little effect on population growth because of the importance of vegetative propagation in Canada thistle patch growth.

Selected Sources
Bond, W. and R. Turner. 2006. The biology and non-chemical control of creeping thistle (Cirsium arvense). HDRA online publication: www.organicweeds.org.uk

Doll, J. D. 1997. Controlling Canada thistle. University of Wisconsin-Madison: North Central Regional Extension Publication. No. 218.

Donald, W. W. 1994. The biology of Canada thistle (Cirsium arvense). Rev. Weed Sci. 6:77-101.

Doyle, S., M. Morgan, and S. K. McDonald. 2005. Organic noxious weed management. Canada thistle, Cirsium arvense Family Asteraceeae. www.cepep.colostate.edu/factsheets/CanadaThistle.pdf

Graglia, E. and B. Melander. 2005. Mechanical control of Cirsium arvense in organic farming. 13th European Weed Research Society, #4600. http://orgprints.org/4600/01/4600.pdf

Graglia, E., B. Melander, and R. K. Jensen. 2006. Mechanical and cultural strategies to control Cirsium arvense in organic arable cropping systems. Weed Res. 46:304-312.

Gustavsson, A-M. D. 1997. Growth and regenerative capacity of plants of Cirsium arvense. Weed Res. 37:229-236.

Haderlie, L. C., S. Dewey, and D. Kidder. 1987. Canada thistle biology and control. University of Idaho Cooperative Extension Service: Bulletin No. 666.

 [HDRA] Henry Doubleday Research Association. 2006. Creeping Thistle Management Strategies in Organic Systems. Available:www.organicweeds.org.uk.

Hogdson, J. M. 1968. The nature, ecology, and control of Canada thistle. U.S. Department of Agriculture Technical Bulletin: 1386.McAllister, R. S. and L. C. Haderlie. 1985. Seasonal variations in Canada thistle (Cirsium arvense) root bud growth and root carbohydrate reserves. Weed Sci. 33:44-49.

Nuzzo, V. 1997. Element Stewardship Abstract for Cirsium arvense. The Nature Conservancy. Arlington, VA.Sullivan, P. 2004. Thistle control alternatives. ATTRA NCAT www.attra.ncat.org/attra-pub/PDF/thistlecontrol.pdf