CRABGRASS PREVENTION: Crabgrass can be one of the most difficult weeds to control. There are many factors involved when attempting to control crabgrass by pre-emergent herbicides. These include herbicide selection, application rates, soils, cultural practices, environmental factors and most importantly timing. Improper application timing in many cases is the leading cause of crabgrass breakthrough. If applied too late, crabgrass will germinate and begin growth before the pre-emergent herbicide is incorporated into the soil. Applications made too early will have a depleted residual before all weed seeds have germinated.
When it comes to application timing there are a few theories that are used regarding application. These theories have some relevancy but with ever-changing weather patterns a more precise determination may be necessary. Pre-emergent timing is sometimes based off of ornamental plant activity. These plants may include forsythia as well as lilac. It is often recommended to apply pre-emergent herbicides when these plants begin to bloom. This theory can somewhat be accurate but does the germination of crabgrass really correlate with these plants? The answer to this question is No. A good example of this would be a short period of above normal temperatures. These temperatures may cause some ornamental plants to bloom, but the soil temperatures may remain the same or increase only a degree or so. Also, the ornamental plants may have certain stresses on them which may delay the blooming period. As we have all seen, crabgrass germinates and thrives just as well in thick healthy turf as weak thin turf. For these reasons we must monitor soil temperatures when attempting to time herbicide applications.
Universities base herbicide application timing by the temperatures of the soil. By monitoring the soil temperature our turf mangers can accurately plan for crabgrass germination. By most indications crabgrass will germinate when the soil temperatures reach 52 to 55 degrees F for at least 4-5 days. With recent inconsistency in weather patterns, crabgrass is germinating earlier in certain areas. Unseasonably warm winter temperatures may cause earlier herbicide application resulting in subsequent applications thereafter.