Anyone with a turfgrass lawn has done battle with a particular foe from time to time: that stubborn renegade form of weed grass that disrupts the smooth texture of your lawn and, unless battled, gradually overtakes it.
The culprit is crabgrass, a group of weedy grasses that belong to the species Digitaria. The most common forms of crabgrass that are problematic in North America are large crabgrass (Digitaria sanguinalis) and smooth crabgrass (Digitaria ischaemum). These are vigorous annual weed grasses that can overwhelm lawns that are underwatered, under fertilized, and badly drained—which explains why we come to associate them with neglected lawns. But even well-tended lawns will have occasional problems with crabgrass, especially during periods of drought or difficult weather conditions.
Homeowners across North America do battle with crabgrass every year, and determining the best chemicals or lawn-care practices to kill or prevent crabgrass is a complete industry all on its own.
Categories of Crabgrass Killers
The techniques for controlling crabgrass fall into several categories, which need to be examined individually:
- Organic, non-chemical treatments
- Selective chemical herbicides
- Non-selective chemical herbicides
- Preventive (preemergent) treatments
Also, controlling crabgrass is sometimes not a matter of killing it at all, but establishing practices for maintaining a healthy lawn that will naturally discourage crabgrass from thriving. These methods can include:
- Planting the right turf-grass species
- Watering correctly
- Proper fertilizing
- Regular dethatching and core aeration
Let's look at the best methods and products for controlling crabgrass in each category.
Organic, Non-Chemical Treatments
Crabgrass is a tenacious annual weed that is very tough to control in poorly maintained lawns, partly because it produces thousands of seeds that easily germinate and take root in bare or thin spots in any lawn. While chemical treatments are the methods of choice for most homeowners, there are also ways to deal with crabgrass without chemicals.
Physical removal. It may sound like an impossible task, but crabgrass can be plucked by hand, especially if the weed has not yet completely overwhelmed a lawn. The best way to do this is to water the area around the patch of crabgrass very thoroughly, which will loosen the weed roots. Then, grab the weed close to the ground, and patiently pull up on the weed until it can be plucked free. The best time to do this is early in the season before the weeds have set seeds. Never put crabgrass plants in garden compost, because the seeds may survive and get spread around your landscape the next time you mulch with the compost.
Corn gluten. The use of corn gluten meal to prevent crabgrass was experimental just a few years ago, but it now is an established organic method for controlling crabgrass. Corn gluten is a by-product of the corn-starch manufacturing process, and, when applied to a lawn, it not only prevents weed seeds from germinating but also supplies a healthy dose of nitrogen to the lawn. Be aware, though, that corn gluten may prevent all seeds from germinating—including grass seeds that you may have just applied to fill in bare spots. Proper application is to lay down corn gluten after any new grass seeds have germinated and begun actively growing.
Selective Chemical Herbicides
A selective herbicide is one that targets specific weeds—or specific categories of weeds—while leaving other plants unaffected. Most general lawn weed killers can be considered selective, in that they are intended to kill broad-leaf weeds while leaving narrow-leaved plants, such as turf grass, unaffected. But crabgrass falls into the category of narrow-leaved plants, so a great many all-purpose "weed killers" aren't at all effective on crabgrass—unless they specifically have added selective crab-grass-killing chemicals.
If you are looking for a chemical herbicide that targets crabgrass, look for either a specific herbicide for crabgrass or a multi-purpose weed killer that includes one of the following chemicals, judged to be most effective by recent university-sponsored surveys of industry professionals:
- Quinclorac (Drive 75 DF Herbicide)
- Dithiopyr (Dimension)
- DSMA (Methar 30)
- FEnoxaprop-p-ethyl (Acclaim Extra)
Be aware that many general weed-killers that are marketed as also being crabgrass killers will have one of these chemicals added to them. Check the labels to identify which chemical has been added to target crabgrass. However, if your lawn is not troubled by broadleaf weeds, it is best to use the most selective herbicide you can buy.
Non-Selective Chemical Herbicides
A non-selective herbicide refers to a chemical that is intended to kill all plants, not just selectively target one or more weeds. This might sound like a self-defeating treatment for a lawn, but when a turf lawn is so badly damaged and overrun with weeds, killing it off entirely and reseeding from scratch is sometimes the most effective solution.
These herbicides come in two forms: contact herbicides that kill only the green plant tissues they come into contact with and systemic herbicides that travel through the plant to kill it entirely, roots and all.
Recent university surveys list the following as the best in this class:
Non-selective contact herbicides:
- Diquat (Reward)
- Pelargonic acid (Scythe)
Non-selective systemic herbicides:
- Glyphosate (Round-Up)
- Glufosinate-ammonium (Finale)
Preventive (Pre-emergent) Treatments
Pre-emergent herbicides are chemicals laid down before weeds even sprout. They are designed to prevent new weed seeds from sprouting at all. A recent university study included the following chemicals as effective means of pre-emergent control:
- Benefin (Lebanon Balan 2.5G)
- Benefin and trifluralin (Team)
- Bensulide (Bensumec)
- Bensulide + oxadiazon (Goosegrass/Crabgrass Control)
- Dithiopyr (Dimension)
- Oxadiazon (Ronstar)
- Pendimethalin (Pre-M, Pendulum, Halts)
- Prodiamine (Barricade)
- Siduron (Tupersan)
When to Treat for Crabgrass
The labels on crabgrass-control products give instructions that are based on a considerable amount of research, so whenever possible follow their suggestions as closely as possible. In general, follow these guidelines:
- With pre-emergents, timing is critical. They work best applied early, either just before as young plants are just getting started. In most regions, this means May and early June.
- Before application, water the lawn (if the soil is dry) and mow.
- Apply only if no rain is in the forecast for the next 24 hours.
- Wait at least two days after applying before you mow again.
Maintaining a Healthy Lawn
By far the best way of controlling crabgrass is to maintain a healthy turf lawn where crabgrass cannot get a foothold in the first place. An ongoing lawn care program that includes the following will prevent crabgrass from gaining an advantage:
- Planting the right turf-grass species. Crabgrass tends to get established where bare, thin spots are found in your lawn. Planting the right turfgrass species can help keep your lawn full and healthy. For example, grass seed mixes marketed as "shady lawn" includes species that should take root and grow well in shady areas. Or, plant "high-traffic" seed mixes to create a full lawn where there is the likelihood or heavy foot traffic.
- Watering correctly. Crabgrass is a tenacious plant that does well in parched circumstances. If you keep your lawn well-watered, desirable turf grasses will remain healthy, blocking out crabgrass.
- Proper fertilizing. A healthy lawn is one that keeps crabgrass at bay. Periodic applications of turf-builder fertilizers will keep your turf grasses healthy.
- Regular dethatching and core aeration. Turf lawns need to "breathe" to remain healthy. Removing thatch build-up and having your lawn aerated every couple of years will allow water, air, and nutrients to get down to the root zone of your turfgrass.