When people talk about plant diversity, I’m not sure too many include Canada thistle into their list of cherished species. Without control, this perennial can turn a mixed pasture into a seemingly thistle monoculture. We are now entering a prime time to initiate control measures.
Although it’s a major nuisance weed, Canada thistle has some unique characteristics and history.
In his book, “Fundamentals of Weed Science,” author Robert Zimdahl relates a story about British General John B. Burgoyne. The British had planned to divide the states along the Hudson River. Burgoyne’s orders were to proceed from Canada by way of Lake Champlain, which forms part of the boundary between New York and Vermont. The unsuccessful campaign began in January 1777 and ended in October with the surrender of Burgoyne and his troops.
The subplot of this story involves the hay that Burgoyne had brought with him from Canada to feed the troop’s horses. As the story is told, it is thought that the foreign hay was contaminated with Canada thistle seed.
Canada thistle was also the first weed declared noxious in the United States. Vermont gave it that dubious status in 1795. These days, Canada thistle is cursed throughout much of United States. It has reached the point where total eradication is an unrealistic goal. The same weed in Australia is called California thistle, no doubt a reflection of where they think it came from.
Interestingly, Canada thistles grow in patches of either male or female plants. Unless gender-different patches are near each other, no seed can be produced through pollination — but that never stops the spread of the plant through underground rhizomes.
Farmers often lump thistles into a single category, but Canada thistle’s perennial nature and ability to shoot its underground rhizomes in a flurry of directions makes the weed difficult to control, especially once it gains an offensive stronghold.
To measure how many roots and rhizomes Canada thistle can produce, researchers in three different states planted either a single root segment 12 inches long or a 6-inch diameter plug of Canada thistle plants in 4x4x8-foot aboveground boxes filled with soil. No tillage or irrigation was applied to the boxes. Within 12 to 16 months, buds on these roots produced an average of 174 shoots and 930 feet of new roots, illustrating this weed's ability to spread unless adequately controlled.
Few nonchemical control options
Mowing Canada thistle is akin to putting lipstick on a pig. The field may look better for a short period of time, but the thistles often grow back and spread with even more vigor and resolve.
It’s often recommended to repeatedly mow Canada thistle in hayfields and pastures to weaken their root reserves and ability to grow back. Although this might be true, not many people are willing to mow with the same frequency as their lawn, which is what it would take to be effective. Grazing a herd of goats might be another nonchemical option but also one not often desired.
If living on your mower or grazing goats aren’t your thing, then herbicides offer the best control option. The cheap list begins with 2,4-D or dicamba, but it will take more than one application. Both a spring and fall application are typically recommended when plants are actively growing. If the patch of thistles isn’t too large, spot spraying is recommended because the herbicide will kill any desirable broadleaf species.
Products containing aminopyralid or clopyralid are effective for the control of Canada thistle, especially in the early fall before temperatures turn too cold. Generally, there is no grazing or haying restrictions after application of these chemicals; however, they do stay in the plant tissue and will be present in the manure. For this reason, using plant mulch or composted manure from animals consuming the treated forage is not recommended.
Picloram is another active ingredient that is effective in controlling Canada thistle, but post-application grazing and haying intervals are longer for these products. The nonselective chemical glyphosate will also kill Canada thistle; its use is generally limited to small patches that need spot sprayed.
For any chemical control method, always read the label for application guidelines and note any grazing or haying restrictions.
The best approach for controlling Canada thistle in pastures and hayfields is to jump on the problem early. Keeping a small outbreak under control is much easier than an outright invasion. Never underestimate the future capabilities of a single Canada thistle plant.
