Tips For Controlling Crabgrass

Crabgrass is the worst of the annual weed problems that we have to fight on an annual basis, 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.

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.

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 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 first application Dimension can actually kill the seedlings. 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 55oF 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. We’ll let you know the optimum week to apply your crabgrass fertilizer. This is normally between April 1st and April 15th, but it depends on how early we warm up in April. There is no reason to apply your Dimension too early in the spring--resist the temptation! Ignore the radio personalities, the big-box advertisements and seductive displays at the garden center. Ignore the professional lawn care operators that will be out treating your neighbor’s yard in March! 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 a week or two before the average emergence for your location. If you make your first application before that you’re wasting the product, in that you’re failing to take advantage of its longevity in the soil. By waiting until the late March to early April timing for your first application, you’ll have mowed quite 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 defense 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 Bayer^® Lawn Weed and Crabgrass Killer product. It contains the active ingredient, quinclorac, which will provide postemergence control of small crabgrass seedlings, less than two inches wide. Plus, it controls 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 remember, you’ll be getting the email blasts from THE Turf Plan^® which takes all your worry away.

THE Turf Plan^® strongly promotes a “sequential” or "two shot" program, which ensures that you’ll have crabgrass control into 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 April and then again in June, 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.

Here are a few examples that are very active in the summer months.

Dallisgrass (Paspalum dilatatum) is a perennial grass that grows in clumps. It has wide leaves with a lot of fuzz.

Dallisgrass close-up: Note the fuzzy leaves and the dominant mid-vein.

Topics

The Reseeding Bible
Controlling Nutsedge
Moles & Their Control
Irrigation Tips
Our Newest #1 Turf Weed
Controlling Crabgrass
Galls on Pin Oaks
Controlling Adult Japanese Beetles
Grub Control
Know Your Sward Size
Soil Science Basics

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