I have something to tell you, but you have to promise you won't get angry. Okay? Here it is: I've helped spread crabgrass seeds. Not on purpose, of course, but I had fun doing it. Let me explain. When crabgrass grows tall enough, it forms finger-like shoots covered with seeds. As a kid, I liked to run my fingers along those shoots, stripping the seeds from them and letting the clusters of seeds fly off my fingers into the breeze. Oops.
I know now that what I did was unkind to the neighbors, because whether you're a gardener or a homeowner, crabgrass is an annoying weed. There's nothing attractive about it, and because it's an opportunistic plant, it will readily jump into any spaces left in your lawn or ornamental garden. In fact, the best defense against crabgrass in your lawn is to make sure there aren't any spaces available for it to take over. Keeping crabgrass out of your garden is trickier, but there are ways to do it, and I've included some suggestions for that at the bottom of the page, in the section on herbicides. In the meantime, here's how to discourage crabgrass from sprouting up amidst your turf grass.
Set your lawn mower to cut as high as possible, taking into consideration healthy heights for your variety of turf grass. 2-3 inches is ideal for eliminating crabgrass, and most common species of turf grass will tolerate being kept at that height. Crabgrass can't germinate in the shade, so keeping your lawn a little longer will ensure that the turf grass hoards the sunlight and stays healthy, while crabgrass languishes beneath it.
Limit your lawn to infrequent watering that goes 4-6 inches deep. If the ground is allowed to dry between waterings, shallow-rooted crabgrass will have trouble germinating. Deep watering also improves the health of turf grass by encouraging it to grow deeper root systems, which make it more resilient in less-than-perfect environmental conditions. And that, in turn, makes it better equipped to hold its own in a crabgrass invasion.
Overseed your lawn to thicken the turf grass and fill in any bare areas that could be taken over by opportunistic crabgrass. The best time to do this is early fall, because grass germinates more easily in warmish soil and, after the first frost, it won't have to compete with crabgrass or a preemergent herbicide (see below) like it would in spring. Overseeding is a big job, involving a lot of mowing, raking, and watering, but it should be done every five years or so even if you don't have a problem with crab grass. And if you do, overseeding is crucial to crabgrass prevention.
Fertilize your lawn in the fall rather than in spring. Crabgrass, an annual plant, is killed off every year by the first hard frost. If you fertilize after this happens, only the desirable turf grass will reap the benefits of the fertilizer. On the other hand, if you apply fertilizer in the spring, when crabgrass is germinating, you'll be practically giving it permission to take over your lawn.
You can always pull crabgrass up by hand—it isn't the best, easiest, or most long-term solution, but if it's midsummer and you already have a lawn full of crabgrass, it's pretty much your only option. The key to crabgrass control really is prevention, and you can get a jump on next year's infestation by following the steps above, but there really isn't anything you can do about crabgrass once it gets a foothold in your yard. On the bright side, if you hand-pull crabgrass before it goes to seed, it will prevent some of next year's growth.
Chemical Crabgrass Control
Aside from the cultural controls discussed above, the best way to kill crabgrass is with a preemergent herbicide, which does just what it sounds like: kills seedlings before they emerge from the ground. Crabgrass is an annual plant, so the first frost in autumn kills it, but not its seeds. When soil temperatures reach 55-60˚ F in the spring, the seeds begin to germinate. If you've applied a preemergent herbicide, such as Dimension or Tupersan, within a couple of weeks before this happens, the plants don't live long after germination. The best time of year to apply a preemergent herbicide varies from climate to climate, but since crabgrass starts to germinate about the same time lilacs begin to bloom, you can use that fragrant event as a guide. Postemergent herbicides aren't of much use in lawns. They only kill very young crabgrass, which is usually too small to be visible amidst turf grass. But postemergent herbicides can be useful in gardens, where sprouting weeds are noticeable at a much smaller size. Postemergent herbicides, such as Acclaim Extra, can be sprayed directly on crabgrass plants as they break through garden soil. If you prefer to avoid chemicals, but are finding crabgrass in your ornamental or vegetable garden, your only other alternative is to pull up crabgrass plants by hand as they appear. Unfortunately, cultural crabgrass controls are only applicable to lawns.