Few weeds provoke greater ire for those who tend to their lawns than crabgrass. Arriving in most areas in early to mid summer, crabgrass is acutely invasive and capable of taking over entire yards if left unchecked. Its pale, green color and low growing habit stand out and make it a particularly conspicuous eyesore in home lawns. However, with a good plan, crabgrass can be easily prevented and any that manages to show up can be quickly done away with.
What is Crabgrass?
Native to Africa, crabgrass is a warm-season annual. This means that in most places it lives its entire life in the course of one lawn season. Crabgrass develops from seeds in the soil which germinate when temperatures reach approximately 55°F. It will continue to develop in the ensuing days and become visible in the lawn within weeks. Crabgrass will grow in size until mid to late summer, thriving in the sunshine and heat. It is remarkably drought tolerant and vigorous and can out-compete and ultimately do in many desirable grasses. In late summer it enters its reproductive stage, producing as many as 150,000 seeds. It will drop these in early fall – ensuring next year’s crop – and die after the first frost, decaying and ultimately leaving bare spots where it once grew. In places where frosts do not occur, crabgrass can overwinter and even produce a second crop of seeds, but this is relatively rare.
Crabgrass grows low to the ground with arm-like tillers extending from a central base. Its leaves are thick and fat, larger than almost any desirable grass. Young crabgrass has a bright, lime green color that grows duller as the plant matures. In most cases, it is quite easy to identify. However, due to its ubiquity, many who are new to lawncare will call any odd looking grass they find in their lawn “crabgrass.” This is a mistake. There are at least a dozen grassy weeds found in home lawns ranging from quackgrass to rescuegrass to poa annua and trivialis. Most of these weeds are tall-growing but several share crabgrass’ diminished height. Dallisgrass, goosegrass and stiltgrass can all resemble crabgrass and care should be taken to ensure a correct identification.
Getting Rid of Crabgrass
Combating crabgrass begins with maintaining as thick and healthy a lawn as possible. Proper fertilization and deep and infrequent watering will go a long way in accomplishing this as will regular mowing. While different grasses should be mowed at different heights – ranging from roughly 1 inch for bermudagrass to up to 4 inches for St. Augustine and tall fescue – it is always best to mow at the higher end of any recommended range. The taller the grass the more shade it will provide and shade is truly crabgrass’ kryptonite; it will inhibit both its germination and development. However, because crabgrass can emerge even in a well-tended yard it is best to take action each year to prevent its emergence.
Step 1: Spread Pre-Emergent Herbicide
Some who prefer natural or organic lawncare will employ corn gluten meal in the hope of doing so. This is not something that I would recommend. Results from corn gluten meal are far from guaranteed; both academic research and anecdotal reports have shown decidedly inconsistent outcomes. Corn gluten meal also contains roughly 10% nitrogen, the principle ingredient in any fertilizer. As application rates can be as high as 20 pounds of corn gluten per one thousand feet of grass, this can result in truly massive applications of nitrogen. Not only can these applications wreck havoc in the lawn, but the excess nitrogen not absorbed the grass can easily leach into the water table and make its way to lakes, streams, rivers and larger bodies of water. Paradoxically, many homeowners who choose to use corn gluten meal out of concern for the environment will actually end up contributing to environmental degradation via this leached nitrogen.
The best option for preventing crabgrass is to apply a pre-emergent herbicide. These products are exactly what they sound like: herbicides applied to the lawn before crabgrass emerges that stop it from doing so. When applied correctly they are remarkably effective and safe for the applicator as well as kids, pets and the environment. The three most common pre-emergents are:
- Prodiamine (brand name Barricade)
- Dithiopyr (brand name Dimension)
- Pendimethalin (brand name Pendulum; Scott’s Lawn Care uses this)
They come in a dizzying array of formulations, both brand name and generic, liquid and granular, and mixed with fertilizer or without it. Those who are new to lawncare are best advised to stick with granular products as they are easy to apply and require only a simple broadcast spreader. It is also best to avoid products that contain nitrogen fertilizers as pre-emergent herbicides are in most circumstances applied early in the lawn season before grass requires any fertilization.
- Stops crabgrass all season and can be applied up to 4-weeks after crabgrass emerges
- Prevents crabgrass and 40+ common lawn weeds
- Prevents crabgrass all season
Best Time to Apply Pre-Emergent Herbicide
The ideal time to put down a pre-emergent is just prior to crabgrass germinating. As discussed earlier, this happens when soil temperatures reach roughly 55°F and stay that way for several days in a row. As a matter of best practice, applications should begin when soils are hovering in the the 51-54°F range. Note: Syngenta has an awesome online tool to see exactly what your areas soil temperature is.
I like to employ a technique commonly called the “split app.” This involves splitting the pre-emergent application into two doses, the first applied when soil temperatures are approaching 55°F and the second when they are approaching 70°F, about forty-five days later. Splitting up the applications extends the pre-emergent’s duration and also provides some coverage should torrential rains wash away the first dose. While these sorts of rains can indeed be problematic, it is worth noting that pre-emergents do require about a half inch of water to push them from the soil’s surface down to where crabgrass seeds may be hiding in the soil. This water can come from in-ground or above-ground sprinklers, but it is often easier to time applications to the day before a mild but steady rainstorm is predicted.
Am I Too Late?
Some readers may be encountering this post at a point when their soil temperatures are well past 55°F. The question arises: should one apply a late dose of pre-emergent? The unsatisfying answer: it depends. While crabgrass begins to germinate when soil temperatures reach 55°F, its seeds will continue to do so for about fifty or sixty more days. Within that time frame it certainly makes sense to put down a late application, though it must be accompanied by reasonable expectations: some crabgrass will have already developed beyond the point that a pre-emergent can prevent it. Any late applications should also involve a review of the product’s label and careful dosing so as to ensure a length of pre-emergent coverage that does not intrude on any plans for seeding or renovation later in the year. This is especially important for those growing cool season grasses – fescue, rye or bluegrass – who regularly seed in the fall. Active pre-emergents will not only prevent crabgrass seeds from developing, they will also prevent desirable grass seed from doing the same.
For those with warm season grass (bermuda*, centipede, zoysia, St. Augustine and bahiagrass), or those with cool season grass who are not planning to seed in the fall, the best option for a late pre-emergent application is to apply dithiopyr at the maximum rate allowed by the label’s instructions. Unlike prodiamine and pendimethalin, dithiopyr, when applied at this rate, has the ability to kill crabgrass that has already emerged, so long as it is relatively immature. (*Note: dithiopyr should not be applied to TifGreen bermuda.)
Step 2: Spray Post-Emergent Herbicide OR Pull
There are several post-emergent herbicides that can kill crabgrass that has already appeared in the lawn. However, the advice dispensed earlier is worth reiterating: whenever possible crabgrass should be treated in its youth. The more tillers (or arms) a crabgrass plant has, the more mature it is, and the harder it will be to treat with herbicides. When choosing any post-emergent to kill crabgrass, care should be taken to ensure that the product is safe to apply to the desirable grasses in the lawn. Things can get complicated here as some herbicides are safe to apply to certain grasses but not others. In an effort to keep things simple, consult the following list:
Cool Season Grasses:
– Kentucky bluegrass: Quinclorac (Drive) or Mesotrione (Tenacity)
– Tall fescue: Quinclorac (Drive), Mesotrione (Tenacity) or Fluazifop (Fusilade II)
– Rye: Quinclorac (Drive) or Mesotrione (Tenacity)
Warm Season Grasses:
– Bermuda: Quinclorac (Drive)
– Centipede: Mesotrione (Tenacity) or Atrazine – suppression only*
– Zoysia: Quinclorac (Drive) or Fluazifop (Fusilade II)
– St. Augustine: Atrazine – suppression only*
– Bahiagrass: Only the aforementioned dithiopyr application is safe
*In this context “suppression” implies that Atrazine will damage crabgrass but not completely kill it.
Nearly all of the above herbicides should be applied when air temperatures are below 85°F. This is easier said than done when treating crabgrass, a weed that flourishes in the summer. The best method for application is to apply in the late afternoon or early evening, as soon as temperatures get below 85°F. This will give the herbicide the maximum amount of time to work its way into the crabgrass plants before temperatures warm on the following day. All of the listed herbicides should all be accompanied by a surfactant when being sprayed to ensure the herbicide sticks to the weed and does not run off into the soil. A non-ionic surfactant intended for herbicides would be best but a small dash of baby shampoo could do in a pinch.
Step 3: Remove Dead Crabgrass & Plant Seed
Getting rid of dead crabgrass is important as well because the dense growth smothers nearby grass and prevents it from growing in thick. Pulling up the dead plant gives your grass the opportunity to recover. Once they’re all pulled out, kick-start the recovery process in the fall by replanting grass seed in the bare spots.
And always remember: crabgrass should be eliminated prior to it reaching its reproductive stage. Seeds are how the crabgrass plant ensures future generations. The more this cycle can be interrupted, the easier next year’s fight will be.