Crabgrass is the bane of many people seeking a ‘nice’ lawn. It is a weedy grass which will out-compete desirable grass species and take over in a short time. Crabgrass has a wider blade, is lighter in color and grows faster than the lawn making it obviously stand out as a weed. Its seed germinates earlier and at lower ground temperatures than other desirable turfgrasses giving it a jump in growing time.
The best defense against all weeds of lawns is to maintain a healthy stand of turfgrass by having soil pH and nutrients at the correct levels. Healthy soil supports healthy grass. Lawns mowed at a height of three inches or taller has less crabgrass and other weeds as the soil it shaded, excluding light from reaching the seeds which initiates germination. Crabgrass is an annual growing new plants from seed each year. None of last year’s crabgrass lived through the winter.
Chemical control against crabgrass is applying a pre-emergent herbicide. They attack the newly produced tissue from the germinating seed up to young plants with a couple of leaves. Pre-emergent herbicides have no effect on seeds in the soil that do not break dormancy and start to grow, only the seeds which start to grow. Timing of application is before the crabgrass seeds start to germinate when to the soil temperatures are 50 to 55 degrees Fahrenheit. This is the same time the forsythia is just past its full bloom stage and starting to but out some green leaves. The germination period ends when the lilacs begin to bloom. Just remember to apply after forsythia and before lilac flowers. Forsythia and lilac make a great plant indicator for applying the pre-emergent herbicide against crabgrass.
There are several different pre-emergent herbicide active ingredients with varying rates of how long they last in the soil. Products containing pendimethalin will last about four months out in the environment. Other active ingredients, dithiopyr, benefin+trifluralin, prodiamine, will last a shorter period of time. Read the labels for the residual rate for each formulation’s time it will last. Most pre-emergent herbicides will stop all seeds from continuing to grow after germinating. This means you will not be able to plant desirable grass seed after applying it. Products containing Siduron are the only pre-emergent herbicide that will allow cool season grass seeds to grow while eliminating crabgrass.