Canada Thistle (Cirsium arvense): A Closer Look at a Common Invasive (archive)
Published on Thursday October 8, 2015
Authored by PEIISC
Canada thistle is a non-native (despite the English name) perennial weed that originates in Europe. It has sharply-barbed, deeply lobed, spear-like leaves. The flower is a purple pom-pom shape in clusters at the top of the plant (photo). When the seeds are mature, the flower will become white and fluffy, similar to a dandelion seed head. Most parts of the thistle are barbed and the thorns will come off and lightly puncture skin when handled.
Controlling Canada thistle organically is a matter of repeated treatments over several years. Few animals graze thistles, leaving plants to mature and spread seed. Because the seed travels on the wind, seed from well beyond your control area can germinate profusely even when control of local plants is done before they go to seed.
Pulling the plant is not advised as the root is deep and if it breaks it will grow a new plant. However, in some soils pulling does remove most of the root and the number of plants is reduced. When combined with snipping the plant off at the base before the seed head forms, some control is possible. Do not put Canada thistle seed heads in compost in the garden. Even if immature at the time of cutting, the seeds can mature after cutting.
Mowing after the plants has gone to the fluffy seed stage only spreads the seeds further. Check the thistle control location weekly after pulling or cutting and cut off new growth (photo). As with all invasive plants, the idea is to catch the local population before the plants spread thousands of seeds into the area.
Urophora cardui, or the Canada thistle gall fly attacks Canada thistle by forming a gall structure (photo). This gall prevents the plant’s nutrients from being sent to other areas of the plant. The plant above the gall is less vigorous, but generally does not kill it.
Adult flies emerge from galls in late spring to early summer and deposit one to 30 eggs in the thistle shoots. Larvae initiate gall development by tunneling into the stem. Multiple larvae (3 to 10) can be found within large galls. Pupation occurs within the gall in the following early spring.