Thistles are aggressive weeds in fields, pastures and croplands throughout North America. The Environmental Protection Agency has registered numerous herbicides for their control, but not all herbicides are registered for use in croplands. Herbicides need to be applied at different times on biennial and perennial thistles. Knowing the correct species of thistles is a useful guide to selecting the best herbicide.
Musk thistle (Carduus nutans L.), plumeless thistle (Carduus acan-thoides L.) and bull thistle (Cirsium vugare) are the most common biennial thistles. They spread by seed. Biennial thistles generate profuse amounts of seed. Plumeless thistles produce up to 8,400 seeds for each plant; musk thistles generate up to 120,000 seeds per plant. Seeds usually germinate in the summer and fall. The young thistles spend the winter as a rosettes, a circular arrangements of leaves at the base of a plant; they resume growing in the spring when they bolt and produce large flower-heads from May to October. After yielding seed the plants die, ending their life cycle. Biennial thistles are best controlled before they produce flowers.
Canada thistle (Cirsium arvense), Flodman thistle (C flodmanii) and wavyleaf thistle (Cundalatum) are the most common perennial thistles. Perennial thistles spread aggressively and can dominate and area if they are not stopped. They are harder to control than biennial thistles because they spread both by seeds and by their roots. The most effective herbicides attack the root system.
Herbicides for Biennials
Herbicides used to control biennial thistles should be applied late in the fall but before freezing to allow seedlings to emerge and rosettes to form. Treated seedlings that emerge the next summer will remain in the rosette stage and not bolt. For controlling biennial thistles, North Dakota State University horticulturalists recommend herbicides with the active ingredients clopyralid or dicamba. Dicamba plus dichlorophenoxyacetic acid (2,4-D) is effective when applied to thistles in the rosette stage. The EPA registers triclopyr plus clopyralid or glyphosate for controlling thistles in cropland. Metsulfuron will control biennial thistles in the bolting to bud growth stages in the spring, preventing them from producing seed.
Herbicides for Perennials
Herbicides should be applied to perennial thistles in the early summer when the buds are beginning to form or in the fall when plants are in their rosette form. For long-term control, perennials require repeat treatment for several years. For application to non-cropland, agronomists at North Dakota State University recommend herbicides with the active ingredients 2,4-D, clopyralid plus 2,4-D, dicamba, picloram, picloram plus 2,4-D or triclopyr plus clopyralid.
To kill Canadian thistles, the government of Saskatchewan recommends herbicides containing the active ingredients aminopyralid for pastures, dicamba for fallow, clopyralid and glyphosate for fallow and pre-harvest cropland. The Saskatchewan government recommends 4-(4-chloro-o-tolyloxy)butyric acid (MCPB) and 2-methyl-4-chlorophenoxyacetic acid (MCPA) or dichlobenil for shelterbelts and picloram for thistles growing in ranges and pastures.
Iowa State University horticulturalists recommend herbicides containing picloram for perennial thistles because it travels through the plant phloem to the roots. Horticulturists at Colorado State University recommend the effectiveness of herbicides containing clopyralid and 2, 4-D for perennial thistles.
For cropland, agronomists at North Dakota State University recommend herbicides containing the active ingredients 2,4-D, MCPA, clopyralid plus 2,4-D, tribenjuron, metsulfuron, triasulfuron, metsulfuron and thifensulfuron plus tribenuron, metsulfuron plus chlorsulfuron or thifen-sulfuron plus tribenuron. Of these clopyralid plus 2,4-D gives most consistent, effective suppression, but long-term control requires more than one application.