Freshly germinated crabgrass. Note the broader leaves and paler color than typical lawn grasses. Source: csuhort.blogspot.com
If you don’t have a lawn, you might wonder what all the fuss about crabgrass is all about. So what if crabgrass sets up shop in a lawn? It’s a grass, isn’t it?
Yes, but it’s not like other grasses. The leaves are coarser than those of fine-leaved lawn grasses and are a yellowish green that really sticks out in a dark green lawn. Plus, most of its leaves spread out sideways, creating a starlike pattern, unlike upward-growing lawn grasses. Also (more on this later), it’s an annual grass, starting all over from seed each year.
There are actually many species of crabgrass, but the two main lawn invaders in temperate climate lawns are large crabgrass (Digitaria sanguinalis) and smooth crabgrass (Digitaria ischaemum), both now distributed worldwide. They’re similar, but mature smooth crabgrass leaves, as the name suggests, are hairless while those of large crabgrass are fuzzy. Both bear long, fingerlike inflorescences at the stem tips, which is where they get the botanical name: digitus is Latin for finger.
Healthy Lawns Resist Crabgrass
Healthy lawns rarely suffer from crabgrass infestations. Since the plant is annual, it has to resow itself each year and it won’t do so in a dense lawn. It needs bare patches exposed to sun. So densifying a lawn is the best way to prevent and control crabgrass.
Instead, crabgrass tends to show up in thin or damaged lawns, often ones weakened by drought or poor soil.
- To help keep crabgrass out, higher mowing (at 3 inches/8 cm) is a prime method. This helps keeps the soil shaded and cool, an anathema to crabgrass.
- To counteract poor soil, top dressing with compost or quality soil can be very effective.
- If you water, do so slowly and deeply, then let the lawn dry out, as that stimulates deep rooting and healthy, dense growth from lawn grasses, yet tends to kill off short-rooted crabgrass.
- Adding clover to lawns can be helpful, as it naturally tends to fill in any bare patches.
For minor infestations, hand-pulling (crabgrass is easier to pull when it is young and not yet well rooted) followed by good lawn care may be all you need to allow your lawn to fill in and outcompete the crabgrass. Early pulling also prevents seed production (each crabgrass plant can produce up to 150,000 seeds!). Don’t add pulled crabgrass plants bearing flowers or seeds to the compost unless you know the pile heats up enough to kill weed seeds.
Slowly but Surely
Unfortunately, even if you do prevent crabgrass from going to seed, seed from previous years can still be alive and ready to sprout. (Crabgrass seed is usually viable for about three years.) That’s why many gardeners dealing with serious infestations find it takes about that—three years—to get crabgrass totally under control.
Crabgrass, being an annual grass, dies in the fall, so overseeding thin spots in the fall with top-quality grass seed (you may need to combine overseeding with top dressing so the fresh seed germinates in good soil) can help get your lawn crabgrass free. Any crabgrass seed that does germinate along with fall-sown lawn grass will quickly be killed by frost, so when the revigorated lawn greens up again in the spring, the new grass will be dense enough to keep crabgrass at bay.
Besides, fall (late August to November, depending on the local climate), not spring, is simply the best period for establishing healthy lawns in temperate climates.
Corn Gluten as a Treatment
Oddly enough, you can actually help prevent crabgrass from sprouting in the spring by applying corn gluten meal. It’s a preemergence treatment. Corn meal keeps most seeds, not only those of crabgrass, but also seeds of dandelion, plantain and other weeds, from successfully germinating, since it inhibits root formation in germinating plants, leading to their death. (It will therefore have no effect on already established weeds that have a robust root system.) Do not apply corn gluten if you have sown or will be sowing lawn grass in the spring: it will kill lawn grass seedlings as easily as any germinating weed.
You have to time the treatment carefully. Apply it when forsythias are in bloom, at double the rate recommended for other weed seeds. Corn gluten should be applied when the soil is moist (but not wet) and when rain is forecast within the next two days. If it doesn’t rain within 48 hours of application, water the lawn. The product acts over a period of approximately 5 to 8 weeks
Corn meal has the added advantage of being a nitrogen-rich fertilizer that most lawn grasses will appreciate.
That said, I find that overseeding the infested lawn in the fall gives better results than corn gluten treatment in the spring.
I’m not a proponent of chemical lawn herbicides. They’re banned where I live and that is also the case in many other areas these days. Besides, most “lawn herbicides” are designed to kill broadleaf weeds and won’t do much damage to crabgrass, as it has narrow leaves. If you do want to try to control crabgrass with a lawn herbicide (and if they are legal where you live), look for one that specifically states it’s for use controlling crabgrass in lawns.
Non-selective herbicides will kill any plant, including crabgrass. There are both chemical and organic ones (the latter group includes double-strength vinegar, oil, soap, citric acid, etc.). They can be used in spot treatments, that is, to kill individual crabgrass plants in lawns, but be careful not to kill the surrounding grasses at the same time.
Crabgrass can also show up elsewhere—in gardens, containers, cracks in pavement, etc.—but its control is much easier outside of lawns.
In gardens and containers, for example, just mulch. Crabgrass needs exposed soil in which to germinate. Mulch covers the soil, preventing germination. Problem solved.
For crabgrass sprouting in cracks in sidewalks, paths, driveways, etc., the use of an organic non-selective herbicide may be the simplest choice. Crabgrass is most susceptible to such treatments when it is young. Also, make sure you kill it before it goes to seed or the problem will likely reoccur.
I still believe the best way of controlling crabgrass is to simply give your lawn good basic care: a healthy lawn will fight off crabgrass all on its own.