Pre-emergent herbicides, often called Crabgrass preventers, are applied to prevent crabgrass and other grassy weeds in turf. Crabgrass is by far the most common grassy weed in our area. Other problematic grasses in our area include Goosegrass and Johnsongrass. Crabgrass germinates at a lower temperature than other grassy weeds, meaning it will appear soonest in spring, germinating when the soil temperatures reach around 58 degrees Fahrenheit. Note that the presence of Crabgrass is a symptom of problems with fertility, soil issues, and proper watering. Crabgrass will first appear along sidewalks, driveways, and parking lots – areas that warm up more quickly.
In order for a pre-emergent herbicide to be effective, it must be applied before the weed seed germinates. Pre-emergent herbicides do not kill the plants, they only stop them from germinating for a period of time. Pre-emergent herbicides are available on a granular fertilizer carrier and spread with a broadcast spreader, or in a sprayable form. Prodiamine is the normal pre-emergent we use at L.C.S. Lawn and Tree Service, Inc. It is coated on our first round of granular fertilizer. It can last up to 16 weeks at our usage rate. Moisture, whether in the soil, via rain, or watering, is needed to activate the product.
After application of a pre-emergent herbicide, the herbicide barrier will begin to degrade. Quickly at first, then more slowly over time. Once the herbicide has broken down to a minimum level in the soil, Crabgrass and other weeds will begin to break through. Accurate, uniform application is critical for optimum control. Seeding is not suggested after pre-emergent use until the early fall.