Crabgrass is a remarkable plant. It’s tough and adaptable. A single flower can produce 150,000 seeds in a year, and spread them widely. This is great news for farmers; loaded with nutrients, hay made from crabgrass makes great winter feed for animals. In Poland and parts of Germany it feeds people, too; the seeds, known as Polish Millet, are harvested and ground into flour. Native to Europe, related species are also important in India and Africa. In much of the world crabgrass is a valuable and useful plant.
For gardeners, however, it’s a nuisance. Crabgrass’s adaptability makes it a stubborn and destructive weed, particularly on lawns. It’s ugly, with the untidy-looking plants growing up to three feet tall if left unmown. If a small patch of your lawn is in poor health it’s likely to be quickly colonized by crabgrass, and the regular grass will be crowded out of a steadily expanding area. Then, in late fall, crabgrass pulls its next trick. It’s an annual plant, and usually dies off with the first frosts. That leaves unsightly bare patches on your lawn through the winter, and those bare patches will be prime targets for next year’s crabgrass when spring comes. If you give it a chance, in a few years it can replace most of your painstakingly grown turf.
Getting rid of crabgrass is notoriously hard (but not impossible – we’ll cover how to do it further down the page). It’s much better to prevent it establishing itself in the first place. The good news is that all the steps you take to deter crabgrass will also make your lawn and yard look better. It’s all about basic maintenance and promoting healthy turf.
Apart from its annual die-off crabgrass has a couple of other weaknesses that can be exploited. For a start, its seeds don’t germinate well in shade. You can reduce the chances of it getting a start by keeping sunlight away from the ground, and the simplest way to do that is to not mow your grass as short. Set your mower to cut at a height of about three inches. Your turf will still look good at that length, but any crabgrass seeds that land in it will struggle to grow.
Crabgrass has shallow roots, so it thrives best when the ground gets regular shallow watering. Turn that around and give your lawn less frequent but heavier watering that goes at least four, and ideally six, inches deep. In between the top layer of soil will dry out. This encourages your turf to grow deeper roots, which makes it generally healthier. It will help your lawn resist dry spells, because the grass will be pulling water from deeper in the ground. But if crabgrass seed lands there it’s going to struggle to survive in the dry top layer.
Think about when you fertilize. Lots of gardeners like to feed their turf in the spring. That’s a bad idea; you’ll feed the grass, but you’ll also be pouring nutrients on all those crabgrass seeds that have been lying there dormant since the year before. Spring is when they wake up and start to germinate; fertilizer will give them the head start they need to take over huge areas of your lawn. Instead fertilize late in fall, after the first hard frost has killed the crabgrass. The grass is still alive and can take advantage of the fertilizer, so it’ll be in better condition – and more able to resist crabgrass – when spring comes.
What you should do in spring is apply a preemergent herbicide. The clue is in the name; these treatments kill the new crabgrass plants before the seedlings emerge from the ground. The benefit of these is that they won’t affect plants that are already growing – like your grass. There are some very good preemergent herbicides aimed at crabgrass; look for ones whose active ingredients include dithiopyr, prodiamine or pendimethalin. If you don’t fancy pouring chemicals on your yard there are natural options too. Corn gluten meal is a natural corn product you can spread on lawns or anywhere else in your yard. It doesn’t actually kill crabgrass, but it interferes with its growth and prevents it growing a full root system. Then, when the soil dries out, the crabgrass with rapidly die. Whatever preemergent herbicide you use, it can’t be combined with reseeding – it will kill the newly germinated grass, too. Alternate the two – preemergent killer one year, reseeding the next.
You can also use some specific herbicides after crabgrass appears, although that’s tricky when it’s growing on your lawn – many postemergent pesticides will kill the grass too. Try a natural option like Agralawn Crabgrass Killer; it’s made from cinnamon bark, and it selectively kills crabgrass and other common lawn weeds (clover, for example) without harming most of the popular grasses. Test it on a small patch first though, just to make sure your lawn isn’t going to die – results appear in three to five days.
If you find growing crabgrass and can’t use herbicides for some reason you can always remove it by hand. This can be time-consuming, but it is effective. The problem is you can only pull up existing plants; seeds will still be lying in the soil waiting for next spring, so you need to either apply preemergent herbicide before the new growing season begins or get your lawn healthy enough to resist the invading species. Overseeding is a good idea. This is a big job, but should be done anyway every five years or so. If you’ve suffered from crabgrass and managed to get rid of it overseeding will thicken the turf and fill in any bare patches that crabgrass could exploit.
It’s almost impossible to keep crabgrass seeds out of your yard, but the problems you have are going to depend on how you deal with it. Keep your lawn in top condition and the weed will struggle to get a grip; look out for, and destroy, any plants that might take hold elsewhere in the garden. A neglected one in a flowerbed can dump a lot of seeds around the place in a short time, so your problems will quickly magnify. Like most garden pests, some prevention will save you a lot of time-consuming cure!