Dr. John Stier, Environmental Turfgrass Extension Specialist, Professor and Chair of the Dept. of Horticulture-University of Wisconsin, Madison.
Winter weather alone doesn’t solve all your weed problems. Most winter weeds are known as winter annuals, which germinate from seeds in the fall, stop growing in the spring and flower shortly before they die. (In the South winter annuals may flower as early as February, but in the North they may not flower until May.) Common winter annuals in turf include annual bluegrass, henbit (a member of the mint family), common chickweed, and some types of speedwell.
Here are some helpful tips for having the best lawn no matter what the weather outside your window. The best way to control winter weeds is stop them before they start. This is most easily done by keeping a healthy lawn that doesn’t allow weed seeds to germinate. All you have to do is take good care of your turf during the summer with proper mowing, fertilization and you’re on your way to preventing winter weeds. If you’ve had problems in the past, and you couldn’t keep a healthy lawn when it was warm, using a pre-emergence herbicide can be very effective at controlling winter weeds. A pre-emergent herbicide is simply a solution you apply before the weeds have a chance to grow (as opposed to post-emergence which you apply when there are already visible weeds). In most cases, it will be important to apply the herbicide before the weed seeds germinate and before any seedlings emerge above the ground.
Wondering how these herbicides work? Pre-emergent herbicides usually aren’t very watery and don’t get absorbed so they tend to form a barrier between the seeds and the soil surface. As the seeds germinate, the roots and shoots hit that barrier, which stops their growth. The trick is getting the herbicides on before the weeds germinate, generally before soil temperatures cool to less than 70 degrees. A good way to gauge the time for a pre-emergent application is when warm-season grasses stop growing for the year. One important thing to know is that most pre-emergent herbicides will also prevent turf grass seed from germinating, so if you’re thinking about re-seeding your lawn when it’s cooler, make sure you use an herbicide that won’t prevent grass seed germination.