Click below to listen to my 2 min. Garden Bite radio show: Lawn care – weed killing
Does forsythia bloom foretell the coming of crabgrass? Actually… sorta. This year is a bit wonky with cold weather which means we have some time, but not much. Generally speaking…
Preemergent herbicides affect germinating seeds. That means, to be effective, the herbicide should be applied two to three weeks before weed seeds germinate.
Some researchers say that’s about the time when forsythia starts to drop its blooms. The University of Minnesota Extension, which is in zone 4, says application of PREemergents should be between May 5 and May 20. Here’s a link to their information on Control Options of MN lawn weeds. The University of Wisconsin has THIS list of what to do and when. This is the Lawn Calendar for Minnesota.
But what about POSTemergents?
There are a lot of varying views as to the HOW of this operation. I’m using university based information. Post Emergent herbicides may be applied any time the weeds are actively growing, the air temperature is 60–80 degrees F, there are no winds, and there is no rain in the forecast for 48 hours. Most effective control of perennial broadleaf weeds is when applied in early fall (August 15–October 15) or in spring (May 1–June 1). Some weeds will need repeated applications.