When To Apply Preemergence Herbicides - Spring 2013

Chamberbitter

Here in coastal South Carolina, we are approaching the time in the spring when preemergence herbicides need to be applied.  What are they, you ask? Herbicides are chemicals that kill plants.  Most people are familiar with post-emergent herbicides that kill actively-growing weeds.  Preemergence herbicides do their job as the weed seed is germinating - that's why the timing of their application is critical.  Too early and it is washed away.  Too late and the weed is past the point of being affected by the herbicide.

So, when should you apply preemergence herbicide in the spring?  Where I live, it is typically during early March, but there is a universal way to get the timing right no matter where you live.  When dayime temperatures reach 65-70 degrees for four consecutive days, it is time to apply the preemergence herbicide.  This will control seed germination of summer annuals.

Here in coastal South Carolina, we are about at the right time for a spring application.  With the exception of Thursday, we are in a week-long-plus warm spell that meets the criteria set out above.  High temperatures most days will exceed 65 degrees.

What about fall?  Here,  PE is applied in late September, but, for everyone else,  it is when nighttime temperatures reach 55-60 degrees for four consecutive days.

Florida Betony

Since we're talking about applying chemicals to lawns, I thought I would talk about something that some of the master gardeners feel very strongly about:  Don't use "weed and feed" products.  These products have both a preemergence herbicide and fertilizer in a single product.  They should not be used on lawns in our area, and probably throughout the south.  As discussed before, PE typically will be applied around March 1st, when weed seeds are germinating.  Fertilizer should not be applied to lawns until the grass has completely "greened up", probably in late April.  If it is applied early, it will control weed germination, but will burn or stress the still-dormant grass.  If applied later, the effect of the PE will be significantly diminished.  The best course of action is to buy separate products, applying the PE earlier, when temperatures warrant it, and the fertilizer after the lawn has greened up.