Canada and Bull Thistle commonly invade of seed production since its primary means of infested area to keep shoots from sprouting land managers experienced in the use of Invasive Thistles disturbed ground, including roadsides, spread is through vegetative propagation. around the edge. agricultural pesticides. These are selective for By Tom Heutte, Michael Shephard, and Cyndi pastures, riparian areas, beach meadows, Typical control methods include introduction Herbicides such as glyphosate are the best broadleaf weeds allowing competing grasses Snyder, USDA Forest Service, Alaska Region, and sand dunes. New infestations are often of insects or diseases that attack the plant choice for killing Canada thistle. Glyphosate to quickly move in, but require more care in State and Private Forestry and Jamie Snyder, UAF found on the edge of wet sites including (biocontrol), grazing, hand removal, mulching, is absorbed through foliage and translocated their handling for health and environmental Cooperative Extension Service. Revised by Jamie streambanks, lakeshores, muskegs, and mowing, tillage, and herbicides. Although through the entire plant, roots and all. reasons. Nielsen, UAF Cooperative Extension Service, Invasive Plants Program, August 2007. ditches. Although they grow along the edges utilized in other regions of North America, Glyphosate should always be applied to of forests, they do not fare well under dense biocontrol agents have not been introduced plants that are in a vigorous state of growth. Additional information on this plant can be tree cover. obtained from your local UAF Cooperative Invasive Thistles to Alaska. Sheep and goats may eat young Glyphosate is a nonselective herbicide, which Caution: Herbicides can be dangerous to the Extension Service office, Alaska State Forestry seedlings but will not eat established plants. will kill all plants, so care should be taken user and the environment unless used according office, or from: Guidelines for Control Hand removal is usually impractical for to avoid damage to desirable vegetation. to the label directions. Federal law requires that in Alaska Due to its vigorous underground growth habit, Canada thistles due to the difficulty of Herbicides may be applied by spraying or the user read, understand, and follow all label USDA Forest Service ability to store energy in its roots, and thick removing the entire root system, but may by direct application with a wick or brush to directions. Consult with a UAF Cooperative State & Private Forestry coating of dense hairs, which can protect work for very small colonies if care is taken to minimize off-target effects. Effects may take Extension Service office near you for more Forest Health Protection plants from herbicide application, Canada dig up all roots and rhizomes. Bull thistles are two weeks to become noticeable, longer information on use of herbicides. Mention of a 3301 “C” Street, Suite 202 thistle is notoriously difficult to eradicate (see easier to control by hand pulling than Canada under cool or cloudy conditions herbicide in this publication does not constitute Anchorage, Alaska 99503 Figure 4). When newly established plants thistles. Because each plant has a single a recommendation for use by the USDA, nor Phone: (907) 743-9455 Proper timing and dosage are a must; does it imply registration of a product under are found in an area every reasonable effort taproot, the entire plant may be pulled up 2770 Sherwood Lane, Suite 2A otherwise the plants recover, sometimes in Federal Insecticide, Fungicide, and Rodenticide should be made to eradicate them quickly. Juneau, Alaska 99801-8545 easily. Mulching is ineffective unless spread 6 better shape than before because competing Act, as amended. Mention of a proprietary In Alaska today, we have relatively few Phone: (907) 586-8811, ext 283 feet deep over an area 30–40 feet in diameter. vegetation was killed. Mature thistle plants product does not constitute an endorsement by locations infested with Canada or bull thistle. If it is necessary to pull thistles after seed will not absorb herbicides well enough to get the USDA. 3700 Airport Way By treating these few sites now, we can keep set, cover the plant with a bag to contain the a complete kill. But if the plants have been Fairbanks, Alaska 99709 these species from becoming permanently wind-blown seeds before pulling. Mulching forced into the rosette stage by a mid- to Phone: (907) 451-2799 established along our rivers, streams, and The U.S. Department of Agriculture (USDA) prohibits can suppress the growth of competing plants late-summer mowing they will readily absorb Or: roadways. Whatever the treatment method discrimination in all its programs and activities on the and favors the growth of thistles. Mowing will the herbicide if sufficient time is allowed for used, repeated effort and monitoring for basis of race, color, national origin, age, disability, and www.fs.fed.us/r10/spf/fhp not kill thistles unless repeated at 1-month the plants to resume growth as rosettes, but where applicable, sex, marital status, familial status, http://akweeds.uaa.alaska.edu several years are required to eliminate intervals for a period of 4 years or more. before stem elongation. For best results, mow parental status, religion, sexual orientation, genetic www.dec.state.ak.us/eh/pest/index.htm these plants from a site. Often a short-term information, political beliefs, reprisal, or because Tillage will often break up Canada thistle plants in mid-summer, then apply glyphosate decline may be seen, followed by a complete colonies into smaller pieces, which may all or part of an individual’s income is derived from to regrowing plants in the early fall. Allow at any public assistance program. (Not all prohibited recovery by the plant. Entire colonies should recover and increase infestation density and least four weeks for initiation of active growth bases apply to all programs.) Persons with disabilities be treated, as they may not be physically spread. However, mowing and tillage may and rosette development. Apply before a who require alternative means for communication of connected underground. Treating just a few program information (Braille, large print, audiotape, be used to encourage the plants to assume killing frost. Herbicide will then be transported plants in a colony may leave the others to etc.) should contact USDA’s TARGET Center at (202) Photography Credits a growth stage better suited for control by along with the photosynthetic products from 720-2600 (voice and TDD). To file a complaint of take over. Any treatment method should All photos by USDA Forest Service. herbicides. Covering with a barrier such as the leaves to the roots. discrimination, write to USDA, Director, Office of Civil include a plan to reestablish native vegetation black plastic can be effective if the plastic is Rights, 1400 Independence Avenue, S.W., Washington, Cover photo: Canada thistle plant found along or desirable plants in its place. Eradication Selective herbicides such as clopyralid, D.C. 20250-9410, or call (800) 795-3272 (voice) or Lynn Canal. left in place for a period of 2 years. Make sure United States Prepared By R10-TP-132 methods for Canada thistle should focus on 2,4-D, and metsulfuron methyl are also (202) 720-6382 (TDD). USDA is an equal opportunity the plastic spreads several yards beyond the provider and employer. Department of Forest Service Revised killing the entire colony instead of prevention recommended for use on thistles by Agriculture Alaska Region August 2007 Thistle is an old English name for a large occurred in Delta. The plant was first found Life History flower heads. Later in the season, the flower group of plants with an unpleasant reputation. in the Delta Junction area in 1982. Due to heads develop into a mass of cottony tan Canada thistle, a long-lived perennial, In ancient history the thistle represented part the aggressive action of local residents, none “parachutes” that carry the seed in the wind. reproduces by seed and vegetatively by of a primeval curse placed on the earth, and has been seen since 1987, leaving a sense of While Canada thistles produce an ever- extensive horizontal roots. A single plant can on humans in particular. cautious optimism that the plant has been expanding network of rhizomes underground, produce up to 5,000 seeds per year, but the eradicated from the area. Unfortunately, bull thistles arise from a stout taproot. Thistles in agriculture have a reputation as growth of rhizomes and underground root Canada thistle is still spreading in Fairbanks a sign of neglect. However, this unfortunate fragments is the principal means of spread. Its Like Canada thistle, bull thistle will also revert and south-central Alaska. Canada thistle characteristic is only true of a few invasive massive underground network of roots can to a rosette when mowed. Therefore, mowing has recently arrived in southeast Alaska; species and is not accurate for the vast expand more than 20 feet in one year, forming or cutting of bull thistle may extend its life “hitchhiking” as seed in the rootballs of majority of native thistles, which have many large patches with many plants connected to span beyond the normal two years, until the ornamental trees and shrubs or in hay bales useful traits. Europeans, Native Americans, the same root system. Underground stems can plant can successfully produce seed. Seed used for erosion control on construction sites. and early settlers found many thistle species produce 2–3 new buds per foot of growth, of both thistle species can be carried great to be edible as well as useful for medicinal Bull thistle has been found at a number of each bud growing into new aboveground distances on the wind, and these seeds can purposes. Thistle seeds, though bitter when locations around Alaska, including Fairbanks, remain viable in the soil for up to 21 years. Figure 2. Bull thistle flower head. stems. The resulting densely packed colonies eaten raw, are much more palatable when Anchorage, Ketchikan, and Haines. It is spread out over time, growing up to 100 feet roasted. Young leaves of Canada and bull especially abundant on Prince of Wales Island Figure 4. Canada thistles can form dense colonies Impacts form, with prickly-toothed irregularly lobed in diameter. Root fragments as small as 0.5 that are difficult to eradicate. thistle (with the spines removed) can be along roadsides. edges, and are smooth above and smooth to inches can develop into new plants. As Canada thistle colonies grow, they served as a salad or cooked like spinach. gain a foothold in new territory. New shoots displace desirable native vegetation, degrade Description hairy on the underside. The plants grow to 5 Canada thistle favors deep, well-aerated soils Canada thistle (Cirsium arvense) is an invasive feet tall with ridged, branching stems. with moderate moisture availability, but can emerge in spring when temperatures average wildlife habitat and recreation sites, and plant found throughout much of the United Canada thistle is identified by its prickly above 40º F. Growth is fastest when daytime compete with crops and pasture. Thistles are Bull thistle has thicker, more robust stems, grow in a wide variety of conditions. The States and Canada. Its common name is stems and leaves, and purple-pink flowers temperatures reach the high seventies. The problematic in rural areas because the dense, up to five feet in height, with many spreading seeds require some soil disturbance, such deceptive, as it is native to Europe and was about one inch in diameter (Figure 1). Leaves plants grow as clumps of leaves (rosettes) prickly spines can irritate and infect skin of branches. It has larger flower heads than as tillage, excavation, or moose activity, to introduced to North America in the early are oblong, 2–6 inches long, alternate on the close to the ground until day length exceeds livestock animals. In general, the sharp spines Canada thistle, with leaf surfaces prickly-hairy 1600s. Farmers have long recognized it as an stem, and arise directly from the stem without 15 hours, then the stems “bolt,” elongating of Canada and bull thistles deter livestock above and cottony below (Figure 2). Leaf aggressive weed, even in its native range. The a distinct leaf stalk. Leaves have a curled wavy and producing flowers. If mowed late in the and wildlife from grazing. Horses and cattle bases have irregular spiny “wings” that clasp State of Vermont legally listed Canada thistle summer, the plant resumes growth as a rosette may ingest the seeds of thistles and deposit the stem. as a noxious weed in 1795. Canada thistle is until a killing frost, after which it overwinters them later with an added dose of fertilizer. In listed as a noxious weed in at least 35 states Only one species of thistle, the edible thistle underground resuming growth the next spring. extreme cases, thistle may take over an entire (Cirsium edule) is native to Alaska. It is pasture. To avoid introducing thistle and other and much of Canada, and is on Alaska’s Bull thistle is a biennial (taking two years restricted to the furthest southern reaches of weeds, livestock owners, dog musher’s, and prohibited noxious weed list. Bull thistle to complete its life cycle). Reproducing by southeast Alaska. This species is rare, so if you gardeners should use only certified weed-free (Cirsium vulgare) is also a weedy native of seed only, a mature bull thistle plant can find a thistle in an undisturbed alpine meadow hay and straw. Seeds also move in soil stuck Europe, which has now invaded all 50 states. produce up to 4,000 seeds per year. In the in this small area of the state, don’t disturb it, to vehicles, farm machinery, and excavation Canada thistle was first collected in Alaska first year, Bull thistle appears as a rosette but report it to Forest Service botanists. equipment. Be sure to wash equipment in 1946 in Palmer (University of Alaska, of closely spaced leaves (see Figure 3). In thoroughly before moving from a weed- Fairbanks Herbarium). Discoveries of thistles Figure 1: Male flower heads (left) and female the second year, a many-branched stem contaminated site. have increased dramatically in the 1990s. flower heads (right) of Canada thistle are borne on Figure 3. Bull thistles appear as a basal rosette in grows and elongates, bearing a handful One example of a proactive eradication effort separate plants. the first year of their biennial life cycle. of large, distinctively urn shaped, purplish