Purdue Extension Service

Vanderburgh County, Indiana

Spring is in the Air

By Larry Caplan, Extension Horticulture Educator, Vanderburgh County, IN

For the Evansville Courier and Press, March 7, 2008

I've had quite a few calls over the past week, with people being pretty 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 lawn care companies are applying pre-emergent herbicides, which are 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 isn't it too early? Not really. Research at Purdue has shown that applying crabgrass herbicides early is not a problem. The various chemicals used do not break down quickly in cold weather, and will still be active when the crabgrass finally 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 cause a failure to control the weeds, since they 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 4 to 6 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 preventative now. However, if your lawn is reasonably thick, then the pre-emergent products will reduce summer weed competition.

Another problem to be concerned with when applying pre-emergent herbicides is that the chemical must move into the upper inch of the soil to be effective. Usually, one-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, where the heavy and frequent 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 pre-mixed with fertilizer. This makes it easier to get the chemical spread, and it also provides a quick burst of green, which some customers want to see. However, all of the research I've ever 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 (812) 435-5287.