Canada Thistle
By Bessie Joyce
Canada thistle (Cirsium arvense) is a colony-forming, aggressive perennial weed. It reproduces by seed, however, predominate reproduction is through extensive lateral roots sending up new shoots each year. In one meter of root there are approximately six nodes, which can sprout new plants. Stems can grow 3.6 to 4.5 feet tall and are ridged and branched. The aboveground portion of the shoot dies during the winter, but the underground part generally survives to produce new shoots the following season. New shoots are also developed for lateral root buds. The leaves are spiny, and the edges are serrated and ruffled. The flower head is urn-shaped and may vary in color from purple, pink, or white. The number of seed produced per head will vary from 0 to 100, with a large plant capable of producing 5,000 seeds. A high percentage of the seeds will be viable from heads that have been open eight to 10 days after blooming. Flowering occurs during July and August.
This weed grows in cultivated fields, pastures, rangelands, forests, and along roadsides, ditches and river banks. It was reported in 1952 in infest more acreage than any other noxious week in the states of Idaho, Montana, Oregon, and Washington. In heavy concentration it effectively prevents grazing, this reducing a field’s potential to support livestock. It is also highly competitive with crops.
Management techniques
Mowing has been a very effective means of non-chemical control. The key to control by mowing is to keep doing it, starting in late spring or early summer, and eventually the nutrient reserves in the roots will become exhausted. The most appropriate time for mowing is just before the thistle flowers. Keep in mind that tilling of the roots encourages new growth and may allow the weed to infest other areas by machinery transport.
Crop competition can be an effective tool in Canada thistle control if a vigorous crop is established. On grazing sites, native grasses are the most effective competitors.
Several species of insects have been established and studied for biological control of Canada thistle.
- The thistle stem gall fly (Urophora cardui) has been the most effective in weakening Canada thistle populations. The gall fly deposits eggs in the stem of the thistle. The developing larvae cause the plant to form a gall, which eventually saps the plant’s nutrients making it less resistant to other insects, competitors and pathogens.
- The larvae of the Canada thistle stem weevil (Ceutorhynchus litura) mine the stem, root crown, and root. The weevil leaves the plant to over-winter in the soil allowing other organisms and or pathogens to enter the thistle stem through the weevil’s exit hole. The USDA is currently researching a rust fungus pathogen that may be very effective biocontrol agent.
- The larvae of the Canada thistle bud weevil (Larinus planus) feed on the developing tissues of the thistle bud and seeds causing the buds to become distorted and fail to open fully. The bud weevil is less significant in controlling thistle since the weed reproduces mainly by vegetative growth.
- The larvae of the thistle head weevil (Rhinocyllus conicus) feed on seed-producing tissue and sometimes the stems of a wide range of thistles.
Biological controls generally weaken their host’s population, but may not contain or eradicate them. Therefore, other control techniques should used in conjunction with biological controls. Unfortunately, most of these insects do not do well in coastal climates and areas where regular flooding occurs.
