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Cultivating Questions Canada Thistle


Concerning the Bio-Extensive Market Garden

by Anne and Eric Nordell of Trout Run, PA


According to Walter Conrad Muenscher’s 1935 classic handbook, Weeds, “The Canada thistle is one of the most feared weeds in the United States.” Judging from the number of desperate requests we have received on how to deal with this prickly perennial, Canada thistle still strikes terror in the hearts of innocent farmers across the country.

We have put off cultivating the Canada thistle question because we do not have firsthand experience using our system of rotational cover cropping to control this challenging weed. In fact, the research we have seen on using annual cover crops to set back Canada thistle has not yielded consistently positive results. The only surefire way we know of to completely eradicate its extensive, resilient root system requires three years or more of intensive, soil destructive tillage. Just when we had given up hope of being able to offer a more holistic, earth friendly solution, an answer arrived in our mailbox this summer from Randy Anderson, a weed ecologist at the USDA Agricultural Research Service in Brookings, South Dakota.

I met Randy at the Professional Crop Producers Conference in State College, PA where he presented the results of his research on using a suite of complementary cultural tactics to significantly reduce weed pressure in field crops. The cultural practices he trialed included: restricting fertilizer application to the crop row; increasing seeding rates; utilizing taller cultivars; eliminating soil/residue disturbance; and, most importantly, implementing a four year rotation comprised of two different warm season crops (such as corn and sunflowers) followed by two different cool season crops (e.g. winter wheat and a mix of spring oats and peas) to reduce the seed bank of both warm and cool season weeds.

Although each of these cultural practices reduced weed competition and weed seed production to a significant degree, effectiveness increased eight fold when three or more of these tactics were employed at the same time. Conventional growers who have adopted Randy’s integrated approach to weed populations management have been able to cut weed control costs by fifty percent!

Our common interests in cultural weed control, reduced tillage and rotation design has led to an interesting exchange of articles. Randy sent us a fascinating research paper detailing a hypothetical nine year organic field crop rotation based on his no-till methods for weed management. The article was solicited by organic grain growers from the Northern Plains. Since these farmers were concerned about managing Canada thistle without tillage, Randy put together the following article.

We reprint “Managing Canada Thistle in Organic Cropping Systems” in its entirety because the thought process it illustrates is as valuable as the specific prescription for eradicating Canada thistle. To our way of thinking, his strategy for tailoring the rotation to match the weak points in the life cycle of Canada thistle could be applied to other noxious weeds – or other types of crop production. By way of example, we conclude this column with a few comments on how we would adapt Randy’s rotational plan for Canada thistle to the bio-extensive market garden.

The Canada thistle is one of the most feared weeds in the United States, having been declared noxious in the seed laws of thirty-seven states (1935), forty-three states in 1954. The noxiousness of the Canada thistle is due to its creeping root system, every piece of which can give rise to a new plant, and to the numerous seeds which are easily scattered by the wind and which have a great longevity. Along the southern border of its range, the Canada thistle does not produce seed as freely as farther north. In certain localities it is commonly believed that it does not seed. This belief, as has been pointed out by Detmers, is due to several reasons. Since this species is dioecious, that is the staminate and pistillage heads are borne on different plants, two individuals are necessary for seed production. If a patch of Canada thistle is started from one seed and spreads or its roots are dragged throughout a field, it still represents the offspring from one seed. If this happens to be a staminate plant no seed can ever be produced by its offspring; if it is a pistillage plant, its offspring cannot produce seed unless other staminate plants are near enough to supply pollen for pollination…

Control. – Small patches of Canada thistle should be destroyed promptly by digging out the roots, or by covering with heavy mulch paper. In pastures persistent cutting with a scythe or hoe will gradually destroy the thistle. The first cutting should be made just before the plants blossom. At this stage the reserve foods stored in the roots have been much reduced and no seeds have yet been formed. Additional cuttings should be made as soon as new shoots appear. On large areas clean cultivation is the most practical method of controlling Canada thistle. A short rotation of a grain crop (one year), clover (one year) and cultivated crop (two years) will permit sufficient cultivation to control the Canada thistle under most conditions. As preventive measures, no seed or manure containing viable Canada thistle seeds should be put on the land. Avoid scattering the roots from small patches of thistle to clean parts of the field with cultivators. Allow no thistles to produce seed.

  • Walter Conrad Muenscher, Weeds (second edition), Cornell University Press (1980)

Managing Canada Thistle in Organic Cropping Systems

by Randy L. Anderson, USDA-ARS, Brookings, South Dakota

Producers in organic farming are seeking to reduce the amount of tillage used in their systems. However, they are concerned that perennial weeds such as Canada thistle [Cirsium arvense (L.) Scop.] may proliferate if tillage is used less frequently. In this paper, we describe growth and development of Canada thistle, and then suggest various control tactics based on vulnerable aspects of its life cycle. Canada thistle can be managed by planning weed-suppressive rotations that disrupt population dynamics of Canada thistle across time. A key factor in rotation design is to include crops with life cycles that differ from Canada thistle, such as alfalfa and winter wheat. Perennial forages are especially effective in reducing Canada thistle density. Organic producers may be able to reduce the need for tillage to control Canada thistle with system design.


Producers in organic farming would like to reduce the amount of tillage used in their cropping systems to limit the detrimental impact of tillage on soil health (Sooby et al. 2007). However, they are also concerned about weed management, as tillage is a primary control tactic. In particular, they believe that perennial weeds such as Canada thistle will escalate in systems with less tillage.

Cultivating Questions Canada Thistle
FIGURE 1. Change in Canada thistle density after three years as affected by management tactics, compared to the initial density. Abbreviations are: SW + T, spring wheat and tillage with moldboard plow and field cultivator each year; PG, perennial grasses with mowing twice a year. Tillage included plowing in the spring, followed by cultivation every three weeks during the summer. Alfalfa was mowed two times for forage each year. (Adapted from Hodgson 1958; Derscheid et al. 1961; Wilson and Kachman 1999).

Canada thistle is prominent in both conventional and organic farming systems in the northern United States (Moore 1975). Canada thistle is difficult to control once perennial roots are established because carbohydrates stored in roots provide energy for shoot growth and long-term survival (Donald 1990). Historically, Canada thistle was effectively controlled by tilling infested land every 3 weeks during the growing season. Continuous tillage prevents Canada thistle from producing and translocating carbohydrates to the roots; consequently, roots die over time. In some studies, Canada thistle was eliminated in three years with this management (Hodgson 1958; Derscheid et al. 1961). However, this intensive tillage damages soil and prohibits crop production during the tilled interval. But, three years of alfalfa (Medicago sativa L.) production suppressed Canada thistle density similarly to tillage (Figure 1). Alfalfa growth and management also prevents Canada thistle from producing carbohydrates for survival across time; thus, alfalfa is an alternative to tillage for Canada thistle control. Perennial grasses harvested for forage also suppressed Canada thistle growth.

Yet, not all crops are effective like alfalfa. Canada thistle density increased across time when spring wheat (Triticum aestivum L.) was grown, even with moldboard plowing before planting (Figure 1). Canada thistle was able to complete its life cycle during the spring wheat growing season, increase the quantity of carbohydrates stored, and subsequently, increase plant density in following years.

Producers may be able to compensate for less tillage in managing Canada thistle by their crop choices and sequencing of these crops. In this paper, we describe ecological aspects of Canada thistle growth in relation to cultural tactics, and then suggest management strategies for organic producers to reduce density of Canada thistle in their croplands.



Emergence of Canada thistle seedlings and shoots begins in early May in the northern U.S., when daily mean temperatures average between 8 and 10 C (Moore 1975; Donald 2000). Plants start bolting three to four weeks after emergence, with flowering occurring between mid-June and early July. The life cycle is completed by early August. Canada thistle is prominent in crops such as spring wheat, corn (Zea mays L.), or soybean [Glycine max (L.) Merr.] because it can complete its life cycle during the growing seasons of these crops. Canada thistle also initiates growth in the fall, where shoots develop a rosette and produce carbohydrates for storage before dying during winter.

Cultivating Questions Canada Thistle
FIGURE 2. Seasonal movement of carbohydrates between leaves and storage organs for Canada thistle. Carbohydrate data represents quantity in storage organs. Numbers along the curved line refer to descriptions in the text. (Adapted from Moore 1975; McAllister and Haderlie 1985; Tworkoski 1992; Gustavsson 1997).

Carbohydrate Movement

A critical aspect of Canada thistle management is understanding carbohydrate movement in the plant. Carbohydrates are the energy source that ensures plant survival over winter as well as accelerates shoot, stem, and root growth during the growing season. Canada thistle exhibits a characteristic pattern with carbohydrate levels in its storage organ, the roots, that is related to plant development (Figure 2). When buds break dormancy in the spring to develop shoots, carbohydrates from roots supply energy for rapid growth of shoots (interval between 1 and 2 in Figure 2). When shoots begin to form flower buds, carbohydrates produced by leaves then are translocated back to roots (interval between 2 to 3). Translocation of carbohydrates to roots continues until seed formation (number 3), when carbohydrates move to developing seeds and the plant senesces. In the fall, a second growth period occurs where shoot growth again is supported by carbohydrates from roots (interval between 4 and 5). After shoots have four leaves, carbohydrates produced in leaves are transferred to roots and used for winter survival and future plant growth (interval between 5 and 6).

Long-term survival of Canada thistle depends on carbohydrate supply. If carbohydrate quantity in roots increases during the growing season, the plant develops more roots and infests more land. In contrast, declining carbohydrate quantity in roots across time eventually leads to plant death.

Bud Dormancy

Canada thistle can survive intensive control tactics during one year because some buds on roots are dormant, which moderates the plant response to control tactics. Dormancy in buds is related to an interaction between carbohydrate supply and plant hormones (Moore 1975; Robertson et al. 1989). Actively-growing buds accrue most of the carbohydrates being translocated; restricted supply of carbohydrates favors dormancy on neighboring buds. Bud dormancy is reduced when carbohydrate flow is stopped by control tactics; this leads to more buds producing shoots and susceptible to control.

Shoot Survival

Shoots and seedlings of Canada thistle require high light levels to survive. Shoots and seedlings die in a crop canopy when light intensity falls below 20% of full daylight, whereas growth is reduced when light intensity is 60 to 70% of full daylight (Moore 1975). Producers can manipulate light intensity by crop choice. In eastern South Dakota, Canada thistle shoots begin emerging in early May. At this time, light penetration to the soil surface is less than 30% in winter wheat but greater than 95% in spring wheat (Anderson 2008). Consequently, Canada thistle seldom establishes in winter wheat but density can be quite high in spring wheat.

Suppression of light is also one reason why alfalfa is competitive with Canada thistle. Alfalfa develops an extensive canopy before Canada thistle shoots emerge, and reduced light quantity limits shoot growth and survival. This suppression of shoot growth supplements the impact of mowing alfalfa for forage on Canada thistle survival.


Even without tillage, several cultural options are available for producers to manage Canada thistle. A key to effectiveness is to relate cultural options to aspects of Canada thistle physiology and growth patterns that are most vulnerable to management.

Cultivating Questions Canada Thistle
FIGURE 3. Impact of alfalfa on quantity of carbohydrates in Canada thistle roots. Numbers 2 and 3 signify shoot emergence in the summer, and number 4 refers to shoot emergence in the fall. The arrow highlights the difference in carbohydrate quantity in roots between the beginning (1) and end of the growing season.

Diversity of Life Cycles

Producers can gain a competitive edge by growing crops with life cycles that differ from Canada thistle. For example, alfalfa suppresses Canada thistle effectively because alfalfa initiates growth weeks before Canada thistle shoots emerge in the spring (number 2 in Figure 3). The canopy of alfalfa severely restricts shoot growth of Canada thistle to reduce carbohydrate production. Also, alfalfa suppresses shoots that emerge after the first forage harvest because of its rapid canopy development (number 3 in Figure 3). Fall growth of Canada thistle is also limited because it remains in the rosette stage whereas alfalfa grows upright (number 4 in Figure 3). At the end of the growing season, carbohydrate quantity in the roots declines compared to the level present in the spring (shown by the vertical arrow in Figure 3). Biennial clovers will suppress Canada thistle carbohydrate production similarly (Thrasher et al. 1963).

Earlier, we noted the difference between winter wheat and spring wheat in canopy development in early May. Canada thistle seldom infests winter wheat because reduced light penetration in the canopy is lethal to shoots (Moore 1975). In contrast, Canada thistle shoots survive in spring wheat or corn because shoots emerge with the crop seedlings and grow extensively before either crop can produce a competitive canopy to reduce light penetration. Another aspect of corn is its late harvest prohibits control tactics in the fall to suppress Canada thistle growth.

Crop Competition

Even though Canada thistle shoots are supplied with carbohydrates during early growth, shoots need high light levels to begin photosynthesis for survival (Moore 1975). Cultural tactics that enhance crop canopy development will help suppress Canada thistle (Ang et al. 1994). For example, light penetration to the soil surface in early May was threefold greater when winter wheat was planted October 1 compared with planting on September 10 (Anderson 2008). Canada thistle will establish and survive in winter wheat that is planted late. Also, it is critical that Canada thistle growth after small grain harvest be suppressed; otherwise, carbohydrates will be produced for root storage (interval 5 to 6, Figure 2).

Underseeding small grains with red clover (Trifolium pratense L.) reduces growth of Canada thistle in the fall because the red clover canopy minimizes solar radiation reaching the rosettes (Hoffman and Regnier 2006). If an annual species is preferred, buckwheat (Fagopyrum sagittatum Gilib.) also suppresses Canada thistle fall growth (Ekelsen and Crabtree 1995). The rapid seedling growth of this warm-season species planted after small grain harvest leads to a dense canopy that restricts light quantity for Canada thistle rosettes.

Cultural tactics can also help corn suppress Canada thistle. Light penetration in corn is reduced almost 30% when corn is planted in 38 cm row spacing compared with rows spaced 76 cm apart (Teasdale 1995). Another tactic is growing hairy vetch (Vicia villosa Roth) as a cover crop preceding corn. Creamer et al. (1995) designed an implement comprised of an undercutter and roller that kills hairy vetch by severing its roots, yet leaves the biomass on the soil surface. The dense biomass minimizes light penetration to suppress Canada thistle shoot growth.

Cultivating Questions Canada Thistle
FIGURE 4. Number of Canada thistle shoots in designated sites, measured across several years. Each year, control strategies minimized growth and carbohydrate translocation to the roots. Rotation was continuous barley. (Adapted from Darwent et al. 1994).

Need for Long-Term Management

Control tactics for Canada thistle will need to be imposed for several years because plants can survive for three or four years, even during intensive control efforts (Graglia et al. 2006). A study in Canada showed that when plant growth was prevented for three years, Canada thistle shoots were still observed in the fourth year (Figure 4). Shoot density was only 1% of the initial stand, but surviving shoots can reestablish the stand if they are not controlled.

Cultivating Questions Canada Thistle
FIGURE 5. Regrowth of Canada thistle after one year of intensive control efforts (year 1 on X axis). Stand counts occurred in the same site for four years. Rotation was continuous spring wheat. (Adapted from Carlson and Donald 1988).

Furthermore, Canada thistle can recover rapidly if control is imposed for only one year. A study in North Dakota found that three years after a control treatment, Canada thistle shoot density recovered to its original density (Figure 5). In other words, one year of excellent control did not affect long-term survival of Canada thistle.

Control Tactics

Even if organic producers reduce the use of tillage, other control tactics are available. For example, mowing can suppress Canada thistle growth. A between-row mower has been developed to control annual and perennial weeds in corn and soybean (Donald 2006). This mower eliminates the need for tillage during the growing season, and controls weeds with two operations as effectively as herbicides or tillage.

Hand weeding is another viable option. A trend noted with alfalfa is that infestation patches of Canada thistle are smaller after several years of forage harvest (Ominski et al. 1999). Producers could remove Canada thistle plants by hand in these small patches, particularly during years when crops such as corn are grown, and restrict population growth of Canada thistle.


Reducing tillage will help organic producers preserve and restore soil health (Lal 2008). However, less tillage may require that organic producers change their production system to manage perennial weeds such as Canada thistle. Cultural tactics are available, but emphasizing control with one tactic or during only one season will not achieve long-term management. For example, Canada thistle is still prominent in the corn-soybean and spring wheat-cornsoybean rotations, even with extensive use of herbicides and tillage (Gibson et al. 2006).

Crop choice and management provide alternatives to tillage for producers to manage Canada thistle. Two crops especially helpful for Canada thistle management are alfalfa and winter wheat; three years of alfalfa production almost eliminates Canada thistle. But, a further factor is rotation design. Canada thistle management will be enhanced by rotations that balance weedsuppressive crops like alfalfa with crops that favor Canada thistle growth, such as spring wheat, corn, or soybean. We encourage producers to avoid several years of corn and soybean in a row because Canada thistle is able to complete its life cycle in these crops and increase its area of infestation in croplands.

Recently, we suggested a rotation design to reduce density of annual weeds in organic systems for the western Corn Belt (Anderson 2009). This nine-year rotation consists of three years of alfalfa, followed by corn-soybean-oat (Avena sativa L.)/pea (Pisum sativum L.) mixture for forage-winter wheat-soybeancorn. This rotational sequence disrupts population growth of annual weeds across time by altering crop life cycles and providing more opportunities to control weed seedling establishment.

This rotation should also suppress Canada thistle population growth across time. The years of alfalfa, winter wheat, and the oat/pea mixture will minimize Canada thistle growth and carbohydrate translocation to roots, subsequently reducing plant density. Canada thistle growth can be partially suppressed in corn and soybean with cover crops or the between-row mower developed by Donald (2006). The variation in crop life cycles with this rotation design avoids a sequence of several years with crops favorable for Canada thistle growth. If Canada thistle density is extremely high, alfalfa could be grown for four years.

Our goal with this paper was to provide insight for organic producers to manage Canada thistle without relying extensively on tillage. Canada thistle can be managed with less tillage, but pivotal to success will be crop choice and systems design. Because of the need for long-term management, we encourage producers to plan rotations that are not favorable for population growth of Canada thistle across time. Such rotations would include perennial forages and crops with life cycles that differ from Canada thistle in addition to crops commonly grown. Our suggested rotation provides an example, but other rotation designs can also be effective. Conventional producers have used this approach to manage other perennial weeds such as quackgrass [Elytrigia repens (L.) Nevski]. Integrating rotation design and crop competition with control tactics has reduced density of this weed in reduced-till cropping systems (Loeppky and Derksen 1994).


  • Anderson, R. L. 2008. Growth and yield of winter wheat as affected by preceding crop and crop management. Agron. J. 100:977-980.
  • Anderson, R. L. 2009. A rotation design to reduce weed density in organic farming. J. Sustain. Agric. (in review).
  • Ang, B. N., L. T. Kok, G. I. Holtzman, and D. D. Wolf. 1994. Canada thistle (Cirsium arvense) response to simulated insect defoliation and plant competition. Weed Sci. 42:403-410.
  • Carlson, S. J. and W. W. Donald. 1988. Fall-applied glyphosate for Canada thistle (Cirsium arvense) control in spring wheat (Triticum aestivum). Weed Technol. 2:445-455.
  • Creamer, N. G., B. Plassman, M. A. Bennett, R. K. Wood, B. R. Stinner, and J. Cardina. 1995. A method for mechanically killing cover crops to optimize weed suppression. Am. J. Altern. Agric. 10:157-162.
  • Darwent, A. L., K. J. Kirkland, M. N. Baig, L. P. Lefkovitch. 1994. Preharvest applications of glyphosate for Canada thistle (Cirsium arvense) control. Weed Technol. 8:477-482.
  • Derscheid L. A., R. L. Nash, and G. A. Wicks. 1961. Thistle control with cultivation, cropping, and chemicals. Weeds 9:90-102.
  • Donald, W. W. 1990. Management and control of Canada thistle (Cirsium arvense). Rev. Weed Sci. 5:193-250.
  • Donald, W. W. 2000. A degree-day model for Cirsium arvense shoot emergence from adventitious root buds in spring. Weed Sci. 48:333-341.
  • Donald, W. W. 2006. Mowing for weed management. Pages 329-372 in Singh, H. P., D. R. Batish, and R. K. Kohli (eds.) Handbook of Sustainable Weed Management. Food Products Press, New York.
  • Eskelsen, S. R. and G. D. Crabtree. 1995. The role of allelopathy in buckwheat (Fagopyrum sagittatum) inhibition of Canada thistle (Cirsium arvense). Weed Sci. 43:70- 74.
  • Gibson, K. D., W. G. Johnson, and D. E. Hilger. 2006. Farmer perception of weed problems in corn and soybean rotation systems. Weed Technol. 20:751-755.
  • Graglia, E., B. Melander, and R. K. Jensen. Mechanical and cultural strategies to control Cirsium arvense in organic arable cropping systems. Weed Res. 46:304-312.
  • Gustavsson, A. D. 1997. Growth and regenerative capacity of plants of Cirsium arvense. Weed Res. 37:229-236.
  • Hodgson, J. M. 1958. Canada thistle (Cirsium arvense Scop.) control with cultivation, cropping, and chemical sprays. Weeds 6:1-11.
  • Hoffman, M. L. and E. E. Regnier. 2006. Contributions to weed suppression from cover crops. Pages 51-75 in Singh, H. P., D. R. Batish, and R. K. Kohli, (eds.) Handbook of Sustainable Weed Management. Food Products Press, New York.
  • Lal, R. 2008. Soils and sustainable agriculture. A review. Agron. Sustain. Dev. 28:57- 64.
  • Loeppky, H. A. and D. A. Derksen. 1994. Quackgrass suppression through crop rotation in conservation tillage systems. Can. J. Plant Sci. 74:193-197.
  • McAllister, R. S. and L. C. Haderlie. 1985. Seasonal variation in Canada thistle (Cirsium arvense) root bud growth and root carbohydrate reserves. Weed Sci. 33:44-49.
  • Moore R. J. 1975. The biology of Canadian weeds. 13. Cirsium arvense (L.) Scop. Canadian J. Plant Sci. 55:1033-1048.
  • Ominski, P. D., M. H. Entz, and N. Kenkel. 1999. Weed suppression by Medicago sativa in subsequent cereal crops: a comparative survey. Weed Sci. 47: 282-290.
  • Robertson, J. M., J. S. Taylor, K. N. Harker, R. N. Pocock, and E. C. Yeung. 1989. Apical dominance in rhizomes of quackgrass (Elytrigia repens): inhibitory effect of scale leaves. Weed Sci. 37:680-687.
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  • Teasdale, J. R. 1995. Influence of narrow row/high population corn on weed control and light transmission. Weed Technol. 9:113-118.
  • Thrasher, F. P., C. S. Cooper, and J. M. Hodgson. 1963. Competition of forage species with Canada thistle, as affected by irrigation and nitrogen levels. Weeds 11:136-138.
  • Tworkoski, T. 1992. Developmental and environmental effects on assimilate partitioning in Canada thistle. Weed Sci. 40:79-85.
  • Wilson, R. G. and S. D. Kachman. 1999. Effect of perennial grasses on Canada thistle (Cirsium arvense) control. Weed Technol. 13:83-87.


Cultivating Questions Canada Thistle

Randy’s description of the life cycle of Canada thistle made it very clear to us that our weed management system of alternating annual vegetables with a fallow year of cover crops would not be a good match for this particular weed. To control Canada thistle, we would need to completely redesign the bio-extensive market garden to include at least three years of alfalfa or some other long term sod.

Our inclination would be to shift our every-other-year system to a six year rotation comprised of three years of alfalfa and three years of vegetables. This radical change would still maintain the 50/50 ratio of produce to fallow land. It should also provide similar, if not better, overall weed management and soil restoration. In fact, we know of more and more organic growers whose primary weed control strategy is to put vegetable ground into two to three years of sod whenever the fields get too weedy. These growers have even gone to the trouble and expense of renting extra land just to take advantage of this soil improving, weed reducing practice.

As for the specifics of Canada thistle management, we would follow the three years of thistle suppressive alfalfa with heavy feeding, late planted vegetables, such as fall brassicas. Beginning the vegetable section of the rotation in year 4 with late crops would allow the alfalfa one more opportunity to shade out any remnants of Canada thistle before turning under the nitrogen-rich legume toward the end of May. Secondary tillage and seedbed preparation during the month of June should coincide with the lowest carbohydrate reserves in the surviving thistle roots and buds. This period of soil cultivation and decomposition should also help reduce the number of grubs, wireworms and slugs that often build up in the soil during a long-term sod. Given our primitive secondary tillage tools, we imagine a course, clumpy seedbed following the alfalfagrass plow-down, better suited for transplants set out in July, like fall broccoli, cabbage and kale, or possibly large seeded produce, such as sweet corn and beans, rather than fine seeded crops like spinach, salad mix and carrots.

Along the same line of reasoning, we would designate year 5 for early planted produce since by this point the alfalfa tap roots and sod clumps from the previous year’s plow-down should be completely decomposed and not interfere with seedbed preparation or direct seeding even the smallest seeded vegetables. In addition, Randy Anderson’s research suggests that spring planted crops are the least competitive with Canada thistle. In our minds, it just makes sense to plant the early produce after four years of focused thistle eradication.

Starting year 5 with early planted vegetables would also open a window after harvest for establishing a cover crop of rye and hairy vetch late August to provide a new infusion of homegrown nitrogen and organic matter before planting midseason produce in June of year 6. Roughly equivalent in planting date and growth habit to the early seeded winter wheat in Randy’s field crop rotation for controlling Canada thistle, the overwintering rye and vetch combination should create plenty of shade suppression for any thistle that revives during the spring vegetables.

After the third year of produce we would cycle the rotation back to three more years of soil building, thistle suppressive alfalfa or shift to our every-other-year annual cover crop system – whichever promised to do a better job of tackling the most pressing weeds in the market garden. Either way, we would keep the following items in mind when implementing the initial six year rotation for Canada thistle:

  1. Since a vigorous, enduring stand of alfalfa is the key to this thistle eradicating plan, we would make sure to check with area dairy farmers on the best varieties, mowing schedule and fertility program for growing alfalfa in our climate and soil type. We would not be surprised by a recommendation to add a little grass seed when planting the alfalfa to insure long-term, competitive ground cover. For poorly drained or less fertile soil, a grass-clover mix might be even better.
  2. Realizing we may not need all three cuttings of alfalfa for hay, or even have time to make it, we could imagine using this perennial legume cover crop as a convenient source of high nitrogen mulch for the vegetables. To save even more labor, we might also try grazing the alfalfa, or, preferably, a pasture mix of clovers and grasses. Research in the Upper Midwest has proven that goats will eliminate Canada thistle over a several year period. Even cattle can be used for this purpose, according to livestock specialist, Kathy Voth, who taught cows to eat Canada thistle, leafy spurge and spotted knapweed in a project with Utah State University. For a step-by-step explanation of Kathy’s training program, based on positive behavior modification, see her entertaining DVD, “We’d Eat It!”
  3. Whatever the end use of the alfalfa, we would take care to time the harvest not to coincide with the flowering period of strawberries, string beans and other crops prone to tarnished plant bug damage. These insects, which will also cause cosmetic injury to lettuce, are attracted to legumes, like alfalfa, and are likely to move into the adjacent produce fields when the alfalfa is cut or grazed.
  4. Due to the long tentacles of the Canada thistle root system, we would devote as much attention to eradicating it from the headlands, farm lanes and pastures surrounding the market garden as the vegetable fields. Otherwise, we imagine the thistle reinvading the produce from the field edges. Again, maintaining a thick, competitive sod and a regular mowing or grazing schedule would be the key to taking care of the thistle in these areas surrounding the produce fields if it is not possible to give the whole site the 3 year alfalfa treatment before starting vegetable production.
  5. Since, in all likelihood, we could not afford to give all the produce fields, let alone the whole site, the 3 year alfalfa treatment before starting vegetable production, we would try to temporarily reduce thistle vigor on part of the market garden with a one year fallow using the sudex-clover routine described in the Summer CQ. We suspect that this one year thistle attack would be more effective if we delayed the sudex and clover seeding until the second week of July so that we could first setback the thistle with the shade of a rye cover crop in the spring and then use a period of intensive tillage early summer when the root system is malnourished. Weakening the Canada thistle before establishing the sorghum-sudangrass and medium red clover mid-July should allow this cover crop combo to take over thistle suppression for the rest of growing season. It should also shade out the thistle the following spring before preparing the ground for late planted produce just like the scenario described earlier for fall brassicas after three years of alfalfa.

In the case of early planted vegetables, we would be inclined to replace the sudex and red clover with the mix of buckwheat and crimson clover featured at the Annual Organic Cropping Systems field day at Cornell on August 24. The trial plot planted on July 10 was virtually weed-free due to the fast, smothering growth of the buckwheat. Based on the previous year’s research, the crimson clover in the understory takes over with weed suppression after the buckwheat is mown, growing faster than medium red clover. Our guess is that the buckwheat-crimson clover mix would be even more effective at shading out Canada thistle than the sudex and red clover with the added advantage that the crimson clover winterkills at the Freeville, NY experiment station, facilitating seedbed preparation for early planted vegetables the next year.

Please keep in mind that all of these ideas for setting back Canada thistle in one year are 100% speculative. At best, this annual cover crop approach might reduce thistle vigor to the point that aggressive cultivation and hoeing would prevent this tenacious perennial from taking over the vegetables. For complete eradication of this fearinstilling weed, sooner or later it would be necessary to dedicate all of the market garden to three years of soil destructive tillage or soil building alfalfa.

Our approach to Canada thistle management (at University of Illinois) combined tillage, summer annual cover crops, and mowing. Unmowed sudangrass alone or combined with cowpea (70:30 Sudangrass:cowpea) produced 6.2 and 5.8 tons per acre of dry biomass, respectively. Sudangrass alone or combined with cowpea caused a 96% reduction in thistle density in the first growing season. One year after planting sudangrass, thistle numbers were still below ten percent of the beginning densities. Neither buckwheat nor a summer fallow adequately suppressed Canada thistle. Mowing is less important for reducing thistle fitness and survival compared to the sudangrass cover crop.

We recommend disking thistle-infested areas several times during the spring to eliminate emerged thistle, prevent flowering, cut roots into small pieces, and create a uniform seedbed for sudangrass. In early June, drill a sterile sudangrass hybrid such as “Special Effort” at 55 lbs. per acre into the freshly prepared seedbed. The thick sudangrass canopy can shade out Canada thistle. Sudangrass may be mowed when 4 to 6 feet tall to manage cover crop growth and prevent flowering of surviving Canada thistle. Use a flail mower to create a surface mulch and encourage sudangrass regrowth and tillering. In late fall or early spring, incorporate the grass residue into the soil and plant a competitive crop. Use cultivation, hand-removal, or spot treatments with herbicides or flaming to control any remaining Canada thistle.

  • from “Canada Thistle Suppression with Cover Crops” by Abram Bicksler and John Masiunas, MSU Extension Bulletin E-3065, Integrated Weed Management: Fine Tuning the System


In July of 2005, buckwheat, hybrid sudangrass and cowpea were planted at seeding rates of 108 lbs. per acre, 35 lbs. per acre, and 45 lbs. per acre, respectively, to reduce Canada thistle populations in Maple Park, Illinois. Fallow areas in the alley ways were also left at this time to allow for comparisons. Rye was planted at 135 lbs. per acre in early November following the chopping of cowpea and sudangrass, harvest of buckwheat, and disking and cultivating of the field…

In 2006 the effect of these cover crops and the fallow treatment were examined more closely for their relative impacts on Canada thistle populations. On May 22, 2006 the rye was chopped. The rye was tilled on May 27 and plowed under on June 7. The field was then disked and twice cultivated before planting organic buckwheat in late July…


There was no significant difference in Canada thistle populations among cover crop treatments at any of the sampling dates though there may have been differences in biomass, which was not measured. Though the difference was not statistically significant, the grower was pleased with thistle suppression by sudangrass and buckwheat.

Grower Comments

My suggestions are the following: “A) Cowpeas: Do not plant due to poor germination in the summer, B) Fallow: Not very effective; not agronomically sound either, C) Buckwheat: Has some merit, especially if an entire field has a light to moderate thistle infestation. It also provided me with an income of approximately $250 per acre; this is assuming good quality and good marketing as well. Planting buckwheat in late July also allows for a couple of trips with the field cultivator during early-midsummer, D) Sudangrass – most effective! In less than 3 months the sudangrass reached 6- to 9-feet tall. There was so much sudangrass biomass generated in a short period of time that the thistle, which had already been knocked down by repeated summer cultivations, was probably both starved and smothered. I would recommend planting sudangrass for use in a highly thistle infested field.”

  • from “Cover Crops for Canada Thistle Suppression” in “On-farm Weed Management Trials” by Erin Taylor, MSU Extension Bulletin E-3065, Integrated Weed Management: Fine Tuning the System