Canada Thistle

Managing Canada Thistle in Organic Cropping Systems

Randy L. Anderson, USDA-ARS, Brookings South Dakota, 57006

Published in Small Farmer’s Journal 33(4): 79-82 (2009)

Summary. 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.
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.

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)



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.

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.

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)


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.

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).

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.

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.

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 re-establish the stand if they are not controlled.
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).


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.

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).

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-corn-soybean 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 weed-suppressive 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 2010). 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 - soybean - corn. 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).


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