12 September 2012

When to Apply Pre-Emergent Herbicides

Here in coastal South Carolina, we are approaching the time in the fall when pre-emergent 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.  Pre-emergent 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 pre-emergent herbicide in the fall?  Where I live, it is typically during the second half of September, but there is a universal way to get the timing right no matter where you live.  When nighttime temperatures reach 55-60 degrees for four consecutive days, it is time to apply the pre-emergent herbicide.  This will control seed germination of winter annuals - perennial weeds, such as Florida Betony, will not be affected.

What about spring?  Here,  PE is applied around March 1st, but, for everyone else,  it is when high temperatures reach 65-70 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 pre-emergent 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 lawns have completely "greened up", probably in late April.  If it is applied early, it will control weed germination, but will burn 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 and the fertilizer after the lawn has greened up.

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