As the temperatures begin to warm up in the spring or cool off in the fall, it becomes imperative to apply pre-emergent herbicides to your lawn to prevent weed growth. A pre-emergent is a herbicide that is intended to prevent weeds before they have the chance to start developing, so timing their application correctly is critical. Warm-season (summer) weeds begin to germinate when the soil temperature consistently reaches 55 degrees F. Cool-season (winter) weeds germinate in the fall. One option to estimate your location’s soil temperature is to use this online map tool.
Pre-emergent herbicides can harm new grass seedlings and prevent grass seed from germinating, so avoid their use in recently planted areas or areas where you plan to seed.
According to Mr. Graham Simmons, former director of Turfgrass Operations at Woerner Farms in Foley, AL, timing is critical because “once the temperature is right for weed seed germination you are “fighting from behind.” Pre-emergents work by preventing weed seeds from germinating or killing the seed shortly after it germinates, subsequently preventing it from becoming a full-grown plant.
If you are new to lawncare and are not sure what product to use, we can offer suggestions. The best pre-emergents include Hi-Yield Weed & Grass Stopper, Sta-Green Crab-EX, and TURFGRO Professional (the latter two of which can be purchased at your local Lowe’s). Many pre-emergents are mixed with a nitrogen fertilizer (aka "weed & feed" products), which can be problematic for your plants when nitrogen works with spring frost to cause harm. The aforementioned pre-emergents do not contain nitrogen and are therefore our favorite picks. We recommend a 0-base nitrogen product.
Furthermore, Mr. Simmons recommends applying a post-emergent separate from the pre-emergent for winter weeds. Some chemicals like Atrazine have both pre and post emergent control.
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