Pre-emergent products prevent weeds from growing in your lawn but there is much confusion about how they do that and how they should be applied. Let’s start with what pre-emergents don’t do. Pre-emergent products do not keep seeds from germinating. In fact, seeds must germinate before pre-emergents can work.
So what are they? Pre-emergent products come in both liquid and granular form. The most common pre-emergent compounds are pendimethalin, prodiamine and dithiopyr (Dimension). Granular products are applied with a rotary or drop style spreader. If it’s a liquid, it can be sprayed on with a backpack sprayer, hose and tank system or a motorized wheeled spray system.
Regardless of whether you apply a liquid or a granular, these products must be watered in to be effective. Most manufacturers recommend at least 1/2 inch of water (normally one complete cycle of your sprinkler system).
Applying a pre-emergent right before a steady rain is also effective. As the product is watered in, it dissolves and binds with the organic materials in your soil. This creates a shield or protective barrier. This “shield” is at full strength right away and slowly deteriorates and loses its effectiveness over the course of a few months.
As weed seeds germinate (crack open), the initial stages of growth come in contact with your herbicide barrier and stop their growth. The product has stopped the plant from “emerging” into a fullplant.
If you are just starting your lawn treatment now, apply a pre-emergent and also kill off existing weeds. Do this before you fertilize. The reason for this is that we are still a few weeks away from really needing to fertilize and in the meantime, you don’t want to feed your existing weeds.
Most recommendations for our area are to apply pre-emergent products before spring, early summer and fall. There are many weeks in the winter where the temperature is warm enough for both winter and spring season weeds. Applying a “shield” every few months ensures that your lawn doesn’t go unprotected for long periods of time.
A head start now means a healthier, greener lawn going into summer.