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Tips for Controlling Crabgrass

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Crabgrass is the worst of the annual weed problems that we have to fight every season, to keep them out of our turfgrass and landscape beds. There are two common species of crabgrass that we battle in our turf here in Missouri, smooth crabgrass Digitaria ischaemum and large crabgrass Digitaria sanguinalis.

The species are very similar in appearance but the simplest way to tell one from another is that large crabgrass has “hairy” leaves and stems. The pictures at the links above show this very clearly. It doesn’t really matter, because they’re both big problems, especially as you’re building your lawn up.

Because these are annual weeds, we have to take preventative control measures every spring. But if you can develop and maintain a thick, luxurious carpet of turf several years in a row, crabgrass becomes less and less of a problem. But when moles dig up the yard, or diseases kill patches of grass, you’ll see that crabgrass suddenly reappears. Those little seeds stay viable in the soil for many years. That’s why it can suddenly reappear after several years of effective control.

Crabgrass is effectively controlled with preemergence herbicides (before it’s up). The herbicide options vary tremendously in their efficacy; there are 8 to 10 different active ingredients available for crabgrass prevention in turf, and at least two products can be used to achieve postemergence control (after it’s up), as long as the seedlings are reasonably small (~2 inches or less). We recommend the preemergence product known as Dimension® (dithiopyr from Dow AgroSciences) because it’s one of the longest-lived (length of control). And best of all, it’ll actually “knock down” small crabgrass seedlings (one inch tall or less). If for some reason you’re a week or so too late for your preemergence treatments, Dimension can actually kill the seedlings, even when spread on our fertilizer granules. None of the other preemergence products can do this. Most of you know that crabgrass preventers are blended along with fertilizer granules. You’ll get good crabgrass control if you spread the granules evenly, at the proper times. If you’re a sloppy spreader, you’ll have skips and streaks where crabgrass will be evident.

Crabgrass is a warm-season grass species, and won’t begin to emerge until we have 3 to 4 consecutive nights above 55F degrees. “THE Turf Guys” will monitor the soil temperatures in late March and April, and scout the southern exposures, looking for early-germinating patches of crabgrass. With our “split application” recommendations for crabgrass control, we’d like you to put down the first dose of crabgrass prevention (STEP 1) between March 7th and March 15th. Crabgrass won’t typically emerge until mid-April. For other locations in the transition zone of the U.S. (Transition Zone Lawns Map) check with your local horticultural extension service and time your application for several weeks before the average emergence for your location. Your second application (STEP 2) will be timed for May 7th to May 15th, and this application will keep your sward free of crabgrass through the entire summer. It is VERY important that your second dose of prevention be applied no later than May 15th if you have turf-type fescue in your sward. We don’t want the nitrogen to stimulate “succulent” tissues in July and August, because that can worsen the brown patch disease.

Before applying STEP 1 you’ve got to make sure you’ve cleaned up the lawn, first. You’ll want to have mowed a few times and you’ll have the opportunity to get all the matted leaves out of the divots and depressions in the yard. These divots are where the crabgrass will be the worst! Applying any preemergent herbicide atop a layer of matted leaves is pure folly…that herbicide ain’t gonna work. Get the leaves up first!

Crabgrass will usually germinate first along the edges of driveways and sidewalks because those spots get warmer earlier in the spring from radiant heating. Your cool season turf species gets stressed along those same spots, and offer little defensive competition against this weed. It’s important that you spread the fertilizer granules effectively along all of your walks and pavement, to ensure good control in these literal “hot spots.” Crabgrass is a distinctive lighter green or grayish color, and its broad leaf makes it fairly easy to discern in your turfgrass. If you do happen to notice some seedlings breaking in spots, use our Broadleaf Weed and Crabgrass Killer hose-end product. It contains the active ingredient, quinclorac, which will provide postemergence control of small crabgrass seedlings, less than two inches wide. Plus, it controls the broadleaf weeds that are probably growing in the same area.

Don’t believe what you hear about the blooming forsythia being Mother Nature’s indicator for applying your crabgrass preventer. It’s not accurate enough. Actually, one of the best indicators for timing your first application is to do it when the local farmers are planting their corn. Corn likes warm soils, pretty much like crabgrass. But you won’t need to worry about all this malarcky, because you’ll be getting the email blasts from THE Turf Plan® which takes all your worry away.

THE Turf Plan® promotes a “sequential” or “two shot” program, which ensures that you’ll have crabgrass control through August. No crabgrass herbicide applied around April 1st will last until mid- to late August! If you apply your first shot in February or March, you’ll be lucky if the ingredient makes it into July. By applying a dose of herbicide in early to mid-March, and then again by May 15th, you’ll get excellent season-long control of crabgrass, and even some control of the nuisance broadleaf weeds like carpet weed, common purslane, and creeping spurge, which grow in the heat of the summer. The hotter and wetter the summer, the quicker the herbicide will dissipate, and the worse the crabgrass will be.

The two shot program is far superior to a one shot program. We’ll remind you when to get that second dose out, too. The best defense against weeds is a lawn so thick that weeds have no place to germinate! However, any mole damage or disease damage will open up the turf for a crabgrass infestation. Our Dimension products are 50 lb bags on a low nitrogen fertilizer granule, and we want you to apply it on to 10,000 sq ft (10M) of turf. If you had crabgrass in your lawn last year, you’ve got crabgrass seeds waiting to grow. Don’t flirt with disaster…use the bag on 10M. Use it twice each season.

Grassy Weeds That Are Not Crabgrass

Novices in turf science are quick to call “other” grassy weeds “crabgrass”, simply due to their lack of knowledge. Most of these mis-diagnoses come in the spring time.

(Note: This blog was originally posted Feb. 20, 2012)