Dr. Grow: At war with weeds

The inspiration for this article came while I was sitting on our riding mower. We had not mowed our lawn for the last three weeks because of rain, work and grandkids. Needless to say our lawn looked like a hayfield. The grass and weeds were so tall that quite often I had to put the mower in the lowest gear so that it would not bog down.

Mowing is not the most mentally stimulating activity that I can think of. You must keep your eyes glued to the ground so that you can steer your mower without hitting anything. Therefore you can't let your eyes wander to the birds flying by or to the flowers that you pass. All you can do is watch green grass and weeds move under the mower deck.

Since our lawn had gotten so tall most of the weed grasses had seed heads on them. If you know anything about identifying grasses, you know that the seed head is probably the most reliable structure to use to nail down the name of a grass that you are trying to identify. Of course, since I have been working with lawns for the last 25 years what else would I do but keep track of all of the different weed grasses in my yard. I obviously have not controlled these vegetative pests very well. Just about every one that you can think of was there — crab grass, dallis grass, foxtail, nutsedge and johnson grass to name a few.

Since we have had so much rain during the last few weeks, many of these weed grasses have grown rapidly and have attracted a lot of attention. Many lawn owners have asked me how to control various weed grasses in their yard.

The first step to take when confronted with grassy weeds in your lawn is to identify what pest you are dealing with. It is beyond the scope of this article to explain all of the differences between the various weed grasses that you find in your lawn. If you have some weeds in your lawn that are bothering you, take a sample to your favorite garden center. Ask them to identify the sample. If they can't, find another garden center whose personnel can identify the samples correctly.

I usually separate lawn grass weeds into three categories:annual, perennial and sedges. Each category of grass pests requires different chemicals for control.

Attack before germination

Annual grass weeds such as crab grass, foxtail and silver crab are best controlled by the use of a pre-emergent herbicide around the end of March. There are several on the market such as Team, barricade, or pendamethalin. These herbicides need to be applied before weed seed germinate. Since we live in an area with a long growing season, each of the above herbicides need to be applied a second time about six to eight weeks after the first application. If this booster application is not made, the herbicide will lose its efficacy during the middle of the summer. When making a booster application, make sure that you use the same herbicide that you used on the first application.

Unfortunately, even when a pre-emergent and a booster application are made, weather conditions may be such that some weeds will germinate during the summer. For example if we have a very dry period during the early summer, clay soils will often crack open. Crab grass may germinate in these cracks, below the application zone of the pre-emergent herbicide. Then when we get a lot of moisture and the cracks close, the germinated crab grass will grow very rapidly.

When you have a breakthrough of annual grass weeds, you may want to control them during the summer months with a post-emergent herbicide. The most commonly found ones are DMSA and MSMA. When applying these herbicides, be sure to water the area you are going to spray. The next day apply the herbicide. The following day, again water the sprayed area. Even though DMSA and MSMA are selective herbicides (they kill crabgrass but not desirable lawn grasses) they may burn the desirable grasses. This schedule will reduce the potential for lawn burn.

At this time of the summer, crab grass and foxtail are usually mature plants. Quite often it is necessary to make two or three applications of the herbicides before you get effective control.

If your weed grasses are perennials such as dallisgrass, johnson grass, or bermuda, you should use glyphosate to control them. This non-selective total vegetation killer will also kill the desirable surrounding lawn grasses.Therefore be very careful when making the herbicide application. Weeds sprayed with glyphosate need to be exposed to four or six hours of sunlight after application. This allows enough time for the systemic herbicide to be introduced into the plant through the leaves. It will then translocate down to the root system and kill the entire plant.

Glyphosate will not sterilize the soil. You may reseed the killed areas within a week of application of the herbicide.

If you grass weed is nutsedge, or water grass as many people refer to it, you must use a totally different chemical for control. The most commonly offered product is basagran. As with the crab grass control, basagran may burn your desirable lawn grasses. Therefore water one day, spray the next, and water again the following day. Mature nutsedge may require more than one application for control.

I hope this has made some sense out of controlling grass weeds in your lawn. Remember you must first identify the culprit and then follow label directions on the correct chemical control.