Canada Thistle - Weed of the Week

Thistles have been cited in both ancient and modern literature. They were referred to as “weeds whose cure is often worse than the disease itself” by Jethro Tull in 1751 in his famous book, “Horse Hoeing Husbandry,” and that “weeders cut back thistles only to find out that they sprout up again like Hydra, with more heads than before!” Scots were warned by loud cries of Viking attackers who stepped on thistles that were piled along the beaches as a first line of defense.

Furthermore, taxonomist Carl Linnaeus described thistles as the “greatest pest of our fields” in 1753. Despite advances in modern agriculture, thistles continue to enjoy notoriety as one of the most troublesome and difficult weeds to control today. Thistles are not considered to be toxic and also are known to possess medicinal values and serve as forage for certain beneficial insects.

Cananda Thistle Identification and Growth Habits

Thistles are characterized by spines present on above-ground parts and bright purple or pink flowers that attract pollinators. They may be categorized as biennials or perennials based on their life cycle. Canada thistle (Cirsium arvense), also referred to as creeping thistle, is a perennial. Canada thistle initially invades virgin fields through wind-borne seeds or root fragments.

Over a period of three to five years, it can form a colony in the field, if not managed. Its persistent nature is due to its extensive root system capable of producing adventitious shoots along long roots that run parallel to the soil’s surface. Tillage or cultivation can break these roots into smaller fragments, spreading them and often times, aggravating the problem. Recent findings indicate that increasing levels of atmospheric carbon dioxide lead to higher root production compared to shoots, which makes it all the more difficult to control.

Canada Thistle Control

To control thistles, monitor fields for new emergence and execute timely management efforts. Established colonies may require a carefully planned management strategy carried out over a period of two to five years, depending on population levels and age. Stored reserves in the root system are lowest during early-bloom stage (late-May to early-June). Removal of top growth or application of a systemic herbicide inflicts the most damage during this stage. If mechanical control is resorted to, repeated mowing at a frequency of two to three weeks is recommended.

Chemical control using herbicides containing the active ingredients, aminopyralid (Milestone) or clopyralid (Stinger), are somewhat effective among those labeled for use in pasture or hayfields. Proper personal protective equipment should be worn, especially if the thistles are over shoulder height at the application stage. If the early-bloom stage is missed, fall is the second best window to apply a systemic herbicide. Sequential applications are also recommended to manage established populations. In such cases, cut the thistles back two weeks after the initial application, and allow them to regrow prior to second application. Biennial thistles, such as bull thistle (Cirsium vulgare) (pictured) and musk thistle (Carduus nutans), are most vulnerable to herbicides or cultivation during the rosette stage of the first year, usually in early spring. Commonly used broadleaf herbicides such as 2,4-D are effective at this stage. Thistle weevils (Rhinocyllus conicus) have been documented as effective biocontrol agents, especially for musk thistles.