• IMPORTANCE TO LIVESTOCK AND WILDLIFE
• PALATABILITY
• NUTRITIONAL VALUE
• COVER VALUE
• VALUE FOR REHABILITATION OF DISTURBED SITES
• OTHER USES AND VALUES
• MANAGEMENT CONSIDERATIONS

IMPORTANCE TO LIVESTOCK AND WILDLIFE:

PALATABILITY:

NUTRITIONAL VALUE:

COVER VALUE:

VALUE FOR REHABILITATION OF DISTURBED SITES:

OTHER USES AND VALUES:

MANAGEMENT CONSIDERATIONS:

Canada thistle has the potential to rapidly form dense infestations through vegetative reproduction, and the spread of these clones may continue indefinitely, crowding out and displacing native grasses and forbs through shading, competition, and possibly allelopathy [85,105]. Its spread can change the structure and species composition of natural areas and reduce plant and animal diversity [105,153]. Infestations of Canada thistle may contribute to the elimination of endangered and/or endemic plant species such as the Colorado butterfly plant in Wyoming [38]. Canada thistle can decrease or limit forage and livestock production on rangelands and can limit the use of recreational areas, as it is annoying to hikers [85,153,238]. Natural communities that are threatened by Canada thistle include nonforested plant communities such as prairies, barrens, savannas, glades, sand dunes, fields, and meadows, especially those that have been impacted by disturbance as well as those undergoing manipulative restoration management [105,215]. In addition, Canada thistle can spread from adjacent disturbed sites into sedge meadows, wet prairies [105], and disturbed forests. Surveys in the Northwest indicate acreage infested by Canada thistle increasing at an annual rate of 10%, causing an average 42% reduction in range carrying capacity on infested lands [49].

Maintaining a healthy native community is the best defense against Canada thistle invasion, and can help to shade and weaken Canada thistle plants on sites already infested [56,105]. Canada thistle should be removed from lightly infested natural areas when first observed, since it is very tenacious and difficult to control once well established [162]. Priorities for controlling infestations must be developed when planning a Canada thistle management program, with actions ranging from prevention, to reduction and containment, to eradication. Control measures for Canada thistle have been developed primarily for agricultural systems, and may not be feasible in natural areas due to comparatively low forage values, rough topography, and/or large areas. Furthermore, some control measures can negatively impact wildlife and native plant populations [162,253]. Eradication of established Canada thistle in natural areas is, therefore, not often a practical goal. Reducing infestations to manageable levels is a more viable objective [162]. Some state noxious weed laws require the implementation of control measures [153]. Many native species of thistle occur in the U.S.; some of them are rare. Because of the possibility of confusion with native species, Canada thistle must be accurately identified before any control efforts are attempted [215].

Effective long-term control of Canada thistle includes killing the roots and root buds, and preventing seed production and reinfestation by seedlings [53,85]. Because Canada thistle has root nutrient stores, it recovers readily from most types of stress, including control attempts. Therefore, control is optimized by stressing the plant enough to force it to use all of its root-stored nutrients. New seedlings must be killed within 2.5 weeks of emergence so they will not become perennial [85]. A buffer zone between uninfested areas and external sources of thistle can help prevent vegetative invasion, and Canada thistle plants within wind-dispersal range must be controlled or kept from seeding. Treatment to control the perennial plants must be thought out in advance and followed until control is complete [53,85]. Control may take several years and the treatment area must be monitored annually for presence of Canada thistle [53,162]. It takes at least 2 growing seasons to determine whether a particular control method is effective. Degree of control is influenced by clonal structure, growth stage, season of treatment, weather conditions, ecotype, soil type, and control methods used [53,55,223]. Timing of control methods to coincide with the most susceptible phenological stage of the plant is critical in optimizing effects. A single control method is rarely effective, and it is often necessary to use 2 or more methods at any given site [55]. Treatments or combinations that are effective at one site may be ineffective at others [53]. Donald [53] provides a comprehensive review of cultural and chemical control practices for Canada thistle. Management of Canada thistle may be achieved through hand cutting, mowing, controlled burning, chemical poisoning, or some combination of these treatments, depending on the level of infestation and the type of area being managed.

Integrated weed management: Control programs with a series of well calculated and properly timed combinations of treatments are likely to be the most effective means of controlling Canada thistle, and may require 5 to 10 years of effort [53]. Haderlie and others [84] suggest optimizing control of Canada thistle by using late summer tillage to increase the number of foliar shoots and leaf area, thereby creating more surface area for subsequent herbicide application. In natural areas with limited infestations, Canada thistle can be pulled and/ or cut several times during the growing season to weaken roots, and carefully treated with spot application of herbicide in the fall. Combinations of mechanical (hand pulling and mowing) and chemical methods improved control of Canada thistle in non-crop situations in Colorado [198]. At another Colorado site, however, disking after herbicide application did not influence Canada thistle control for any of the herbicides tested, regardless of the timing of application (spring vs. fall) [255]. In native prairies, a spring burn followed by seeding of native species is a possible option [162]. Drought stress reduces the effectiveness of most herbicides against Canada thistle but increases the effectiveness of mechanical controls. Therefore, mowing and burning may be preferred strategies under drought conditions [162]. Tilling and planting competitive grasses was as effective as yearly applications of clopyralid for Canada thistle control in a Nebraska pasture [247]. The interaction of defoliation (by Cassida rubiginosa) and plant competition (by crownvetch (Coronilla varia)) and tall fescue (Festuca arundinaceae)) can seriously reduce productivity of Canada thistle [5,6,7]. Similarly, the weevil Ceutorhyncus litura alone will not effectively control Canada thistle, but may be more successful when combined with cultural techniques that allow for maximum desirable plant competition [182]. Canada thistle stands were treated with mechanical, chemical, and biological measures in Mesa Verde National Park, with some degree of success. Results are still forthcoming [64,65,66].

Prescribed fire: For information on prescribed fire as a management tool for Canada thistle control, please see the "Fire Management Considerations" section of the "Fire Effects" frame in this report.

Competition: Canada thistle is susceptible to shading and grows best when no competing vegetation is present. In a study comparing possible control methods for perennial pepperweed, Canada thistle established, along with non-native cheatgrass (Bromus tectorum), after disking and herbicide treatments that reduced cover of native forbs and grasses [112]. Overgrazing is a major cause of perennial weed invasion, because grasses can compete effectively with and inhibit the establishment of non-natives if grass growth is favored by good management. Canada thistle growth may be discouraged in disturbed natural areas if suitable native species are seeded densely enough to provide sufficient competition [83]. To be effective against Canada thistle the seeded species must come up before Canada thistle, grow rapidly during the early summer in order to shade out the thistle, and retain vigor until frost [162]. In a riparian site in north-central Washington, areas seeded to grass kept noxious weeds out and areas which were missed during grass seeding operations were rapidly colonized by annual and perennial forbs including Canada thistle [34]. An attempt at controlling Canada thistle with competitive species in Minnesota failed because the grasses failed to establish [19]. On a reclaimed parking lot in Illinois that was planted by broadcast seeding and seedling transplanting, then burned 5 years later and on an annual basis thereafter, Canada thistle decreased over time and was virtually absent by year 8 [114].

Cultivation: Repeated tillage or mowing gives long-term Canada thistle control by depleting root carbohydrate reserves. A typical seedling, 14 days after emergence, does not yet have a perennial root, so it can be killed by cultivation. However, a 3-week-old seedling can regenerate after the top-growth is clipped. Tillage exposes roots to drying and freezing at the soil surface, and can be an effective method of control [85,153]. Because the roots of Canada thistle tend to be deeper than plow depth, cultivation may affect only a small part of the plant and must be started at flowerbud time, repeated every 10 to 14 days through the season, and repeated again the following year [85,158]. Tilling is generally limited to cropland [85], and is not recommended in natural areas since it is likely to severely damage natives and may make an infestation worse by spreading root pieces [162]. Tillage disturbance of soil may also provide ideal conditions for erosion, reinvasion, and introduction of other exotics.

Mowing: Mowing or cutting Canada thistle plants temporarily reduces aboveground biomass and can prevent flowering and seed production, but does not kill plants unless repeated at 7- to 28-day intervals for up to 4 years [83,162]. Repeated and frequent pulling or hand cutting of individual plants will eventually starve underground stems. Cutting or pulling should be at least 3 times each season, in June, August, and September. This treatment is feasible for light and moderate infestations, but may be prohibitively time consuming in heavy infestations [105,153]. Stems with flowers that have been open for 8-10 days can develop viable seeds and must be removed from the site in order to prevent germination. The best time to cut is in the very early bud stage when food reserves are at their lowest point, and cutting must be repeated until the starch reserves in the roots are exhausted [215]. When the primary stem of Canada thistle is removed, rootbuds are stimulated to produce new shoots that might otherwise be suppressed. Therefore, plants must be cut high enough to leave more than 9 leaves/stem or more than 8 inches (20 cm) of bare stem tissue. In high humidity, root buds are stimulated to grow regardless of cutting method, and cut plants produce twice the length and weight of new shoots after 7 days under high humidity than they produce in low humidity [104]. After several years of mowing a hay meadow in Illinois and then stopping, native prairie species reemerged and dominated the site [190].

Biological control: Biological control may be a viable option for national parks and other natural areas that have restrictions on herbicide use [208]. Wilson and McAffrey [245] provide a discussion of considerations and safety issues in developing and implementing a biological control program.

There are more than 130 species, including pathogens, birds, and over 80 insects known to attack Canada thistle [137,162]. Some of the biocontrol agents that have been tested and references pertaining to them are listed below:

To date, biological control of Canada thistle has not been successful, although some biological control agents (e.g., Ceutorhynchus litura and Cassida rubiginosa) suppress it to a limited extent and may be effective when combined with other control methods [5,6,7,53,69,85,181]. Even in its native range in Europe, Canada thistle is a serious weed in spite of the large number of insects that are found in and on the plant [83,162]. Most potential biocontrol organisms are not adequately synchronized with Canada thistle's life cycle in North America to induce high mortality. Management that delays Canada thistle maturation, such as mowing or burning, may help synchronize the susceptible thistle growth stage to the biocontrol agent life cycle [69]. Biological control measures may also be more effective when coupled with factors that favor the competitive ability of native forbs and grasses [111].

Grazing is not likely to be an effective control method for Canada thistle since cattle and horses avoid it and graze competing vegetation which results in gradual dominance by Canada thistle. Furthermore, heavy grazing breaks up sod and reduces cover of native vegetation, which encourages the spread of Canada thistle [162]. Some evidence suggests that domestic goats, sheep, and cattle may prevent Canada thistle from flowering by grazing it prior to the bud stage in early spring [129,153]. Domestic sheep have been reported to graze and trample plants that have been treated with salt [83], and intensive pasture grazing by sheep in Australia reduced the spread of Canada thistle compared to an ungrazed pasture [153].

Chemical: Most reports and studies of herbicide use for reduction of Canada thistle apply to agricultural areas and are not directly applicable for use in natural areas. Many herbicides cannot be used in natural areas because of their potential to harm non-target plant and animal species, including soil organisms, aquatic species, humans, and other vertebrates, and the potential to contaminate water resources and set back the succession of natural communities [105,111,162]. Dioxin toxicity is also a concern in herbicide use since these chemicals are often found as impurities in herbicides [227]. Herbicides usually require repeated applications to achieve moderate control, and their continual use may lead to herbicide resistance, soil sterilization, and erosion [111]. Herbicides seldom provide long-term control of weeds when used alone and outside the context of an integrated weed management plan. They may control existing weeds, but the lack of a healthy community of desirable plants usually allows that weed or another invasive species to become established after the residual effects of the herbicide have dissipated [31]. 

Donald [53] provides a comprehensive review of chemical control of Canada thistle. More information is available on use of herbicides on rangelands [31], on national forests in the Pacific Northwest ([244] and USDA Forest Service-PNW), and in general (EXTOXNET or NPTN). See the Weed Control Methods Handbook for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.

Results from herbicide treatment of Canada thistle can vary for several reasons including differential susceptibilities of different ecotypes, different growth stages [70], application method and rate, and the possibility of selecting a tolerant variety from the original gene pool [85,162]. If the physiological, morphological, and phenological stage of the plants and the environmental conditions under which the plants are growing are not optimal, herbicides will be ineffective and control will not be satisfactory [203]. Many herbicides recommended for control of Canada thistle are foliar applied, systemic toxins that must be translocated to the roots to be effective. These chemicals must be applied when moisture status of the plants is favorable (i.e. do not apply herbicides to old leaves or to drought stressed plants), plants are growing well, and, ideally, during the period when photosynthates are translocated to the roots [203,222]. For Canada thistle, this period is thought to correspond to the bud stage [203], but more recent evidence suggests that greater photoassimilate movement to the roots occurs during the rosette and bolt stages. Additionally, environmental conditions typical of fall, and possibly early spring, favored photoassimilate movements to the roots [223]. Spot treating Canada thistle with herbicides in the early spring or fall when native species are dormant may be less damaging to the ecosystem as well. Differences in foliar lipid extracts between Canada thistle ecotypes may have implications for chemical control, as those with greater amounts of lipids on the leaves are more resistant to foliarly applied herbicides [55,203].

Related categories for SPECIES: Cirsium arvense | Canada Thistle