Controlling common mullein and Canada thistle

John Spring
Area Extension Agent, Julesburg Colo.

Due to the wet springs we’ve had, common mullein and Canada thistle have been especially prolific the last several years. With seed-heads and flowers on the plants now, both weeds are at their most noticeable growth stage in infested areas.

While relatively easy to find at this time of year, it is not the best time to try to control them. Once they enter their reproductive stage (begin growing flowering stalks) both plants become much less vulnerable to herbicides. In mullein, all available resources are devoted towards the developing seed, and at this point it is simply too late to get an effective kill with chemical. In thistle, the flow of resources is directed towards seed development as well. When herbicides are applied, they follow this flow and will not reach the roots. Top growth may be burned off to some extent, but the plant quickly regrows from below. It is a good idea to mark or map out patches of these weeds at this time of year while they are easy to see, but spraying or other control measures are usually not effective enough to be worth the cost. For both weeds, waiting until fall (for thistle) or even next spring (for mullein) is usually the best course of action.

As a stopgap measure, mowing mullein as soon as possible to remove the flowering stalks can meaningfully reduce the number of viable seed produced. Herbicides are not effective at this time of year. As mullein is a biennial, the currently flowering plants will die this fall and re-infestation comes exclusively from seeds. The less seed produced the better. With thistle, most reproduction is from root growth and not seed, but mowing, tilling, or burning down the top growth with a contact-type herbicide can set the plants back a bit and make them produce new regrowth that is more susceptible to fall-applied herbicides.

To most effectively control mullein with herbicides, it should be sprayed next spring after green-up, but before plants have begun to bolt (grow seedhead stalks). At this time, all of the plants that will germinate for the year have emerged, but are still in a growth stage susceptible to herbicides. A growth regulator/synthetic auxin type herbicide is usually the best choice, and make sure to use a good surfactant and plenty of carrier water to fully cover the hairy leaves and get good chemical uptake.

For Canada thistle, fall is the most effective time to spray. After the first frost but before it freezes hard enough to kill above-ground leaves is optimal. At this time, plants begin to send their resources down to the roots to prepare for winter. When herbicides are sprayed, they are also transported to the roots and are much more effective at controlling the plants. If this fall application window is missed (or as a follow-up to control survivors), waiting until the early bud stage next spring is the next best option. A number of effective herbicides are available, but many have long residual activity or other use restrictions, so choosing the right product depends on the site to be treated.

For both weeds, a single treatment is unlikely to entirely control the problem, and several years of follow-up will be needed. Mullein seed is numerous and long-lived in the soil, so multiple years of successful control will be needed to deplete seed stocks. Canada thistle is an extremely vigorous plant with extensive root systems, so it generally takes multiple treatments to fully eliminate a patch. If follow-up is not done, it doesn’t take long for the infestation to reach pre-treatment levels and the money and time invested in the initial control is entirely lost.

If you would like to discuss control options or herbicide recommendations in more detail for a specific site, please contact me directly: John Spring, (970) 474-3470, or ❖


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