More info for the terms: competition, cover, density, fire management, interference, invasive species, litter, natural, restoration, root crown, succession, tree
Impacts: Bull thistle is a problem in pastures because it competes with and decreases desirable forage and has no significant nutritive value for livestock [106]. Sharp spines deter livestock, and presumably wildlife, from grazing. One adult bull thistle plant per square yard decreased spring or summer live weight gains of sheep by about 3.8 lb (1.7 kg) per animal in New Zealand pastures [64]. Bull thistle is a range weed in 20 countries and is more frequent in grazed than in ungrazed pastures. It is regarded as a serious pest in protected areas and parks such as Yosemite, Yellowstone, Teton and Glacier National Parks [50]. Bull thistle may also interfere with growth of Douglas-fir transplants in the Oregon Coast Range as indicated by results presented by Gourley and others [55], where tree growth was improved by control of various weeds, including but not limited to bull thistle.
Bull thistle often dominates recently clearcut forest areas in the Sierra Nevada of California, and infestations may limit growth of replanted tree seedlings. Work in a replanted Sierra clearcut forest indicated that stem growth of ponderosa pine saplings was negatively correlated with density of thistles within about 7 feet (2 m) of pines [123]. Bull thistle also colonizes and maintains high population densities for up to 6 years in clearcuts in redwood and mixed evergreen forests in northwestern California [122].
Control: Bull thistle should be accurately identified before attempting any control measures, since several native species of thistles have a similar appearance. See Klinkhamer and de Jong [80] and General Botanical Characteristics for information on proper identification.
The key to successful management of bull thistle is to prevent seed production. Combining control methods into an integrated management system will result in the best long-term population decreases. Control data suggest that viable seed production by biennial thistles must be eliminated to achieve long-term population decreases, although zero seed production may not be a realistic goal. The transition from seedling to rosette in bull thistle may be the most precarious stage in its life cycle. Seedling and rosette growth stages are the most logical to target for control efforts in biennial thistles.
Desirable plant competition to deter establishment of bull thistle seedlings is a critical part of any biennial thistle management strategy. Recovery of infested areas should not be considered complete until a diverse population of desirable plants has replaced invasive biennial thistles, and bull thistles are a minor to nonexistent component of the plant community. Always monitor and evaluate weed management programs to determine whether and when to repeat and/or modify control treatments [15].
Prevention: Prevention is the most effective method for managing invasive species, including bull thistle [15,139]. Preventing or dramatically reducing seed production will help decrease the spread of infestations. This is accomplished by cleaning mowers, vehicles, and tillage equipment after operation in an infested area. When seeding is necessary, use clean, certified weed-free seed and mulch to ensure that bull thistle or other weeds are not being sown. Preventing the establishment of weeds in natural areas is achieved by maintaining healthy natural communities and by conducting aggressive monitoring several times each year. Monitoring efforts are best concentrated on the most disturbed areas in a site, particularly along roadsides, parking lots, fencelines, and waterways. When an infestation is found, the location can be recorded and the surrounding area surveyed to determine the size and extent of the infestation, so these sites can be revisited on follow-up surveys. For more on monitoring see Johnson [74]. Place a priority on controlling small infestations so they do not expand [15,74].
Good grazing management will stimulate grass growth and keep pastures and rangelands healthy. Healthy pastures and rangeland may be more resistant to biennial thistle invasion. Bare spots caused by overgrazing are prime habitable sites for biennial thistles. In many instances, grazing lands will have to be rested from grazing for grasses to recover. This should be coupled with precipitation cycles, so adequate soil moisture will be available to stimulate grass growth. Grazed pastures that are managed carefully may enhance grass competition and deter thistle survival from seedlings to rosettes [15].
Weed prevention and control can be incorporated into all types of management plans, including logging and site preparation, management of grazing allotments, recreation management, research projects, road building and maintenance, and fire management [152]. See the "Guide to noxious weed prevention practices" [152] for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions.
Integrated management: The goal of any management plan should be not only controlling invasive plants, but also improving the affected community, maximizing forage quality and quantity and/or preserving ecosystem integrity, and preventing reinvasion or invasion by other invasive species, in a way that is complementary to the ecology and economics of the site [40,73]. Effective long-term control requires that invasive plants be removed and replaced by more desirable and weed-resistant plant communities [73]. Once the desired plant community has been determined, an integrated weed management strategy can be developed to direct succession toward that plant community by identifying key mechanisms and processes directing plant community dynamics (site availability, species availability, and species performance) and predicting plant community response to control measures [138]. This requires a long-term integrated management plan [15].
Most often, a single method is not effective for controlling an invasive plant, and many possible combinations of methods can achieve the desired objectives. Methods selected for removal or control of bull thistle on a specific site will be determined by land use objectives, desired plant community, extent and nature of the infestation(s), environmental factors (nontarget vegetation, soil types, climatic conditions, important water resources), economics, and effectiveness and limitations of available control techniques [126]. Killing thistles and decreasing weed populations must be followed by the establishment of desirable vegetation in the newly opened niches; herbicide applications in spring followed by dormant seeding of competitive perennial grasses in the fall is an example of an effective management system for biennial thistles in the western U.S. Similarly, integrating herbicides and biological control agents is likely to be more effective than insects alone [15] (see "Biological control" below, for more information). For information on integrated weed management without herbicides, see the Bio-Integral Resource Center (BIRC) website.
Some examples of combined approaches and considerations for managing bull thistle infestations are presented within the following sections. Managers are encouraged to use combinations of control techniques in a manner that is appropriate to the site objectives, desired plant community, available resources, and timing of applications.
Physical/mechanical: Any mechanical or physical method that severs the root below the soil surface will kill bull thistle plants. However, it is essential to re-vegetate the site with desirable plants to compete with bull thistle that may reinvade from seeds left in the soil. Tillage, hoeing, and hand pulling may provide effective control, providing these operations are done before the reproductive growth stages to prevent seed production. Mowing alone is not an effective control measure for biennial thistles, because some seed will still be produced. Mechanical methods may not be practical on rangeland and natural areas, but could be useful in improved pastures or roadsides [15]. The long duration of flowering in bull thistle increases the importance of timely control operations and may make repeated treatments necessary [49,122].
A single mowing will not control a bull or musk thistle infestation, because infestations often consist of plants of various ages, and stands therefore have nonuniform development and flowering. Bull thistle plants mowed just before seed dispersal do not produce seed or recover well [50,80,122]. If mowed too early, bull thistle plants resprout and flower. About 4% of bull thistles cut 2 to 4 inches (5-10 cm) above the soil surface a month before flowering resprout [121,122].
Bull thistle will not withstand cultivation; however, tillage is not appropriate in wildlands and rangelands since it can damage important desirable species, increase erosion, alter soil structure, and expose the soil for rapid reinfestation by bull thistle and other invasive species [88]. Slicing off the root crown of bull thistle plants is time consuming, but very effective [129]. At Yosemite National Park, less than 5% of adult bull thistles cut at the soil surface resprouted, while over 80% of adult bull thistles in control plots survived and flowered [120,121]. Of the bull thistle plants that resprouted, mean height and number of inflorescences were lower (25 inches or 63 cm and 3.7 flowerheads ) than for adults in control plots (33 inches or 85 cm and 15.8 flowerheads) [121,122]. Plants that were cut at the root crown a few days after their 1st flowers appeared and then laid on the ground produced abundant viable seed, so removing cut stems from areas being cleared may be important [50,122].
Even if bull thistle plants resprout after mechanical control, populations may be reduced by limiting seed production [122]. Removal of adult bull thistle plants must be repeated annually for 4 years or more, since some plants will stay in the rosette form for up to 5 years [121]. Mechanical control may be labor intensive; however, sometimes volunteer groups are available. The Salmon River Restoration Council (SRRC) provides an example of watershed-scale weed control using primarily mechanical control methods.
Fire: See Fire Management Considerations.
Biological: Biological control of invasive species has a long history, and there are many important considerations to be made before the implementation of a biological control program. The reader is referred to other sources [126,164] and the Weed Control Methods Handbook [151] for background information on biological control. Additionally, Cornell University, Texas A & M University, and NAPIS websites offer information on biological control.
In its native range, number of viable seeds produced by bull thistle plants can be greatly reduced by insects feeding on the stem, flowerheads, or seeds [80]. Several agents have been considered and tested for bull thistle control, and those in the following table have been introduced in North America:
Biological control agent Mode of action Areas established References thistle head weevil (Rhinocyllus conicus) larvae eat seed-producing tissue well established in most northwestern and northern plains states; GA, TN, TX, VA [15,36,63,77,125,127] thistle crown weevil (Trichosirocalus horridus) larvae feed on the growing points of thistle rosettes and developing shoots CO, KS, MO, MT, OR, VA, WA, WY [15,125] bull thistle gall fly (Urophora stylata) larvae feed within seed producing tissues of developing seedheads CO, MD, OR, WA, BC, NS, PQ [15,26]
Rhinocyllus conicus was introduced from Europe to Montana and Virginia in 1969 to control musk thistle, but it also uses bull thistle. Rhinocyllus conicus will use Carduus, Cirsium, Silybum, and Onopordum genera as hosts but prefers the musk thistle group [127]. In areas where the plant and insect life cycles are synchronized, R. conicus is extremely effective in reducing seed production in musk thistle [125]. It is unclear if it is as effective on bull thistle. Several strains of R. conicus have been identified and they vary in their utilization of various thistle species. At least 1 of these strains does attack some native Cirsium species [89,125], and reviews by Randall [122], Beck [15] and Wilson and McCaffrey [164] indicate that it is known to attack native and rare thistles. Therefore, before releasing insects in a new area containing native Cirsium species, investigate whether any of the local species may be attacked [125]. A detailed discussion of the biology of R. conicus is given by Harris and Shorthouse [63].
Urophora stylata feeds on developing seeds in bull thistle flowerheads and decreases seed production up to 60% [26]. Trichosirocalus horridus was introduced to the U.S. in 1974. This weevil uses thistles of the subtribe Carduinae, including bull thistle, musk thistle, plumeless thistle (Carduus acanthoides), Italian thistle (C. pycnocephalus), Canada thistle, and Scotch thistle. Reports of suppression vary from slight to great. Trichosirocalus horridus is more effective when used in conjunction with R. conicus [125]. In areas of Missouri where R. conicus and T. horridus have been present for over 15 years, an 80 to 90% reduction in thistle populations has occurred [142].
Chemical: Herbicides are effective in gaining initial control of a new invasion or a severe infestation, but are rarely a complete or long-term solution to weed management [23]. Herbicides are more effective on large infestations when incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations. Control with herbicides is temporary, as it does not change conditions that allow infestations to occur [169]. See the Weed Control Methods Handbook for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.
Chemical control of bull thistle is reviewed by Beck [15], Forcella and Randall [50], and Randall [122]. Clopyralid, dicamba, MCPA, picloram, 2,4-D, metsulfuron, and chlorsulfuron will all kill bull and musk thistles. Timing of application is important. Autumn is a good time to control biennial thistles with herbicides because all live plants will be seedlings or rosettes, and plants are easiest to control in the seedling and rosette stages. Plants are, however, more difficult to locate at this stage, and cold weather may decrease the effectiveness of some chemicals. Herbicide choice and rates are influenced by growth stage, stand density, and environmental conditions (e.g. drought or cold temperatures). Check with state or county weed specialists for appropriate local use rates and timing. Bull thistle is less aggressive and easier to control than other biennial thistles [14].
In pastures and range containing appreciable quantities of broadleaf forage species, application of any of the herbicides listed above may damage valuable plants and reduce forage production and livestock weight gain as much as that caused by thistle interference [64], so it is important to prevent these and other non-target effects of chemical control.
Cultural: Bull thistle germination and establishment is favored in open areas and by disturbance [15]. No matter what method is used to kill weeds, reestablishment of competitive, desirable plant cover is imperative for long-term control. Fertilization and reseeding with competitive, adapted species is often necessary in areas without a residual understory of desirable plants [126].
Revegetation with aggressive desirable species has been shown to inhibit reinvasion of bull thistle, especially with the help of effective biological control agents and carefully prescribed grazing practices. Promoting desirable competitors is important both after weed control and before weed establishment. Choice of species to sow will depend upon climate, location, and management objectives. The Natural Resource Conservation Service and land grant universities are good sources of information about appropriate perennial grass species for a particular locale. Management that allows grasses to grow taller in spring to shade bull thistle seedlings may decrease seedling establishment and growth [15].
At Thousand Springs Preserve in Idaho, bull thistle invades native grasslands. Where healthy native grasses have re-established, they outcompete bull thistle and their litter prevents bull thistle seeds from reaching the ground and germinating [129]. On a reclaimed parking lot in Illinois that was planted by broadcast seeding and seedling transplants, then burned 5 years later and on an annual basis thereafter, bull thistle decreased over time and was virtually absent by year 7 [78].