Canada Thistle


My garden is being taken over by Canada Thistle. I have pulled and sprayed and tried planting to crowd them out to no avail. What is the best way to get rid of them?


Here is some information which may help.

Canada thistle (Cirsium arvense) can be a persistent perennial weed, depending on where it grows. It reproduces through both seed and root regeneration, but the latter is the most successful. It stores food energy in an extensive root system, both to survive the winter and to fuel the plant’s reproductive drive the following season. Because of its contribution to survival, Canada thistle’s extensive root system would be the target of an effective control program.

Canada thistle plants are from one to five feet (30 cm to 1.5 m) tall with several branches. Leaves are shiny, a deep green colour and arranged alternately on the stem and branches. Some bio-types have thin, flat leaves with short, fine spines, arranged sparsely around a slightly toothed edge, but most plants have leaves that are thick, leathery, with deep and irregular serrated margins and long sturdy spines. Stems have few, if any, spines.

When pollination is successful, 1,500 seeds per plant are produced on average, but production can be greater than 5,000 per plant. Viable seeds can be present 8 to 10 days after flower emergence.

The seed is topped by a feathery tuft of hair called a pappus, that is used by other plants as a means of transporting seed great distances from the mother plant. In Canada thistle however, this small “hang-glider” often drops its passenger close to the parent plant and many seeds never leave the seed head as their “glider” leaves without them.

Only a very small percentage of seeds travel a large distance from their origin. Most Canada thistle seed will remain viable for about three to six years, but some seed may survive for up to 21 years.

Warm, moist soils (20 to 30o C) and full light are favoured for seed germination. Studies have shown that Canada thistle does not establish well from seed under highly competitive conditions, but once established it quickly develops an extensive root system.

It is important to note that although thistles are often a nuisance both in gardens and  farmers’ crops, thistles are a main food source for many birds and are a very valuable plant for bees and monarch butterflies. Rich in essential nutrients like protein, fat, and oil, these tiny seeds are a favoured food source for a variety of bird species, including goldfinches, house finches, American goldfinches, pine siskins, and migratory birds like snow buntings and horned larks. This plant is also a rich source of nectar that sustains many insects, including migrating monarchs and many native bees.

The thick, fleshy roots of Canada thistle can be difficult to control once established. The plant reproduces very successfully through vegetative “cloning” of itself from the root. Within 19 days of emergence (two-leaf stage) seedlings can regenerate after top-growth removal. During its establishment phase, Canada thistle initially produces a taproot that penetrates to depths in the soil with consistent moisture. Once moisture is found, lateral roots can extend as much as 6 m. in a single season and produce a colony of genetically identical clones. Root depths of 2 to 5 m. are not uncommon,

Lateral roots contribute to the spread of the weed vegetatively, with the assistance of humans. New plants can be produced from root cuttings as small as 3 to 6mm thick, and 8 mm long.

Since one Canada thistle plant needs to cross-pollinate with another to produce seed, those plants that establish from seed generally exhibit a large amount of variability. However, because vegetative reproduction from the roots is much more successful than by seed, there are usually only a very few unique individuals in a given area. The majority of the plants establish from root pieces. Understanding the life cycle of Canada thistle is important when developing strategies to control growth via its persistent root system.

As the soil temperature rises in the spring, buds on Canada thistle roots are stimulated to grow into new shoots. New shoots emerge during late May and June, and long day-length triggers the immediate drive toward sexual reproduction and the energy reserves in the roots are the fuel for this process. Once the reproductive stage is complete the thistle plant switches to survival mode.

Shortening day lengths, with the approach of winter, trigger the movement of energy to the root for storage. Above-ground parts produced by Canada thistle die back completely each winter and new shoots are produced from root buds in the spring. Old root tissue is continually being replaced by new, resulting in a particular root living only two years.

Research shows that Canada thistle will continue to draw from root reserves during the flowering period. You may want to consider mowing top-growth, just prior to flower buds break. This can be done as many times as needed until the plant transitions to the rosette stage where it is moving energy to the root. Then mow again within a week of rosette emergence until freeze-up. Using this strategy, you may be able to deplete root reserves.

A drawback of mowing is the potential to move root pieces and increase the spread of Canada thistle within your garden. If mowing is the only method of control used, a second year of top-growth removal may be necessary to starve the root completely. Sanitation is an important part of any cultural weed control program: it is important to clean the mower blades and other implements each time.

Using herbicides may be another option. The use of pesticides in Ontario for cosmetic purposes is strictly controlled by law. Please take care to refer HERE to find out which herbicides are allowed in Ontario. Many herbicides are only useful for top-growth control of Canada thistle, and even if applied will not hinder its long-term survivability.

Systemic herbicides are usually most effective because they circulate through the entire Canada thistle plant including the root and therefore provide better long-term control. Various techniques have been developed to maximize effectiveness of herbicide applications on Canada thistle. This link contains an allowable list of active ingredients that may be used in Ontario.

Most important, as with many uninvited plant guests, the Canada thistle may be a bane to gardeners, yet are a hugely important food source for a significant number of bird species, monarch butterflies and native bees.

Thank you for writing to TMG.