Control common mullein, Canada thistle

Due to the wet springs the area has 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 toward 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 toward 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.

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