Crabgrass Fight Can Start Early

By LARRY CAPLAN

I’ve had quite a few calls over the past week, with people being surprised to see lawn companies already out treating lawns. “There’s still snow on the ground,” one person told me. “What could they possibly be spraying for?”

Most likely, the companies are applying pre-emergent herbicides, used to prevent crabgrass and certain other annual weeds from germinating. If you have had a recurring problem with crabgrass, you would want to use one of these products early in the spring, before the crabgrass begins to sprout.

But is it too early? Not really. Research at Purdue University has shown that applying crabgrass herbicides early is not a problem. The various chemicals used do not break down quickly in cold weather, so will still be active when the crabgrass germinates a few weeks later. In fact, Purdue turf specialists got pretty good control when they used pre-emergent crabgrass herbicides in the fall.

Applying the products too late in the spring will fail to control the weeds, since the chemicals must be applied before the weed seeds germinate.

There are a couple of things to be concerned about, though, if you are applying these products.

First, these chemicals can’t tell the difference between crabgrass and bluegrass, so we strongly recommend that you wait four to six weeks after application before spreading the seed. If your lawn is really thin from last year’s drought, it would be better to spread grass seed in April rather than crabgrass preventive now. However, if your lawn is reasonably thick, the pre-emergent products will reduce summer weed competition.

Another concern when applying pre-emergent herbicides is that the chemical must move into the upper inch of the soil to be effective. Usually, a half-inch of rain is sufficient to dissolve the chemical and move it down to where the weed seeds are. We’ve had years, however, when heavy spring rains have diluted the chemical too much and washed it out of the soil.

Stay aware of what the weather is in your neighborhood. You may need to put down a second application.

One final drawback to the pre-emergent herbicides on the market today is that nearly all of them are premixed with fertilizer. This makes it easier to get the chemical spread, and it provides a quick burst of green, which some customers want to see.

However, all the research I’ve seen says that you shouldn’t do a heavy fertilization until May in our area. In fact, the bulk of your lawn’s fertilizer should be applied in the fall. Not much we can do about this, other than not adding additional early spring fertilizer.

For more information on early spring lawn care, contact the Purdue Extension Service at 435-5287.

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