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Thursday, June 7, 2012

What herbicide should I use to control hoary alyssum?

Q: We have been attempting to control weeds with frequent mowing, but now have seen some hoary alyssum and have decided to use an herbicide. We plan to use Forefront. However, upon reading the label and communications with the manufacturer, it appears that manure from horses grazing on Forefront treated pastures can only be spread in the treated pasture because it will kill broad-leaf plants. We have small paddocks and collect and compost our manure for use on our garden. It is my understanding that Forefront is very persistent and will not necessarily break down during the composting process. We also are located near a lake and designated wetland, so I am concerned about aquatic toxicity as well as impacts to amphibians. Upon review, it appeared that Stinger may be my best bet. Can you advise further?

A: Good for you for reading the label! You are correct that Forefront can persist in compost. However, if you follow the grazing restriction (time period after you apply the herbicide and before the horses can begin to graze again) on the label, you should not have these issues, but it's not guaranteed (chemical breakdown is dependent on weather and other conditions).

Stinger, Milestone, and Curtail (all commonly used herbicides for this situation) have the same potential issues (same family of herbicides). Even though these herbicides have no grazing restrictions (you should still check your specific label), we recommend at least a 7 day grazing restriction.

Hoary alyssum can be very difficult to control. The previously mentioned herbicides will work, but because of your specific issues, I'd recommend 2,4-D or Banvel. They may not give you as good of control, but you will not have the manure carry-over and compost issues to deal with. Since you can mow, I'd mow the pasture, wait about 10 -14 days (hope for some rain) and then spray the pasture. This will help achieve better control since the weeds will be younger/immature. Younger weeds are always easier to control versus mature weeds.

By Krishona Martinson, PhD, University of Minnesota

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