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Timing Crabgrass Treatment


Timing is everything when controlling crabgrass with pre-emergent herbicides. Here are some solutions to help get the timing right.

SiteOne March 6, 2018

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Crabgrass can be one of the most difficult weeds to control. There are many factors involved when attempting to control crabgrass with pre-emergent herbicides. These include herbicide selection, application rates, soils, cultural practices, environmental factors, and most importantly, timing. In many cases, improper application timing is the leading cause of crabgrass breakthrough. Applied too late, crabgrass will germinate and begin growth before the 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 typically followed. These theories have some relevancy, but with changing weather patterns a more precise determination may be necessary.

Pre-emergent timing is often based on ornamental plant activity, including forsythia and 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? Maybe not. A good example of this would be a short period of above normal temperatures. These temperatures may cause ornamental plants to bloom, but the soil temperatures may remain the same or increase only a little. Also, the ornamental plants could have certain stresses on them, which may delay the blooming period.

Another theory based on the thickness of the lawn may be used. This theory suggests a thin lawn should have herbicides applied a little early and thicker lawns should have herbicides a little later in regard to forsythia blooming. As we have all seen, crabgrass germinates and thrives just as well in thick, healthy turf as it does in weak, thin turf.

For these reasons, the most beneficial time to apply pre-emergent herbicides can be determined by monitoring soil temperatures. This is the method most universities rely on. By monitoring the soil temperature, turf mangers can accurately plan for crabgrass germination. By most indications, crabgrass will germinate when the soil temperatures reach 50 to 55 degrees Fahrenheit for at least three days.

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