Much like the crab of the ocean, crabgrass (Digitaria) is tough, has many legs (or stems), and is built low to the ground. Its procumbent growth habit is one reason why this annual weed is so successful, whether it be in the lawn, growing along a driveway, or popping up through a crack in the sidewalk.
What Is Crabgrass?
Crabgrass is the common name for a genus of plants (Digitaria) that includes both annual and perennial grasses. The species generally have wide, flat blades and produce long flower clusters and thousands of seeds per growing season.
Keeping a low profile in the lawn, crabgrass eludes the mower blade when we mow the lawn. And unlike taller weeds, there's not much that can "break" on it, so it holds up well to foot traffic, even in high-traffic areas. The one plant part on the weed that sticks up—the stalk that bears its flowers and, later, its seeds—is very tough and does not mind being tread upon.
It may surprise some to learn that there are different types of Digitaria: smooth crabgrass and hairy crabgrass. You're more likely to find the former in your lawn.
As you might imagine, the hairy type (Digitaria sanguinalis) is so-called because, if you inspect its leaves and stems, you will find many small hairs all over the plant. Likewise, the common name of the smooth type derives from the fact that its leaves and stems are relatively hairless.
But that does not mean that smooth crabgrass (Digitaria ischaemum) has no hairs at all. It does have a few, but it's a matter of where those few hairs are located. The hairs on smooth crabgrass are located only at the plant's auricles—small, ear-like projections on the interior side at the base of the leaves.
At maturity, the leaves of smooth crabgrass can measure as much as 5 inches long, although many factors influence the length of a particular blade. The leaf blades taper to a point. The plant's stems bend at the nodes, and the stems sometimes turn red in color.
Smooth crabgrass is also called "small crabgrass" because it tends to be smaller (6 inches maximum, but frequently shorter) than its hairier cousin, Digitaria sanguinalis. The latter, in fact, bears the alternate common name of "large crabgrass."
In addition to recognizing mature crabgrass, it helps to know what it looks like when it's a younger plant as well as its life cycle. This knowledge comes in handy when you're trying to eradicate the weed because the best way to control crabgrass is to keep it from emerging in the first place rather than trying to kill mature plants.
Crabgrass is an annual weed. It emerges in early summer, and it thrives during the hot weather because it is drought-tolerant. In fact, it's often the last green thing in the lawn in August, before its stems are finally killed by the frosts of fall.
An annual plant lives for only one year, and at the end of the growing season, it's one and only mission is to produce seed. That is its life cycle, in a nutshell. The only way it can continue to be a problem for you in the following year is if it sets seed. So if you're fighting crabgrass in your lawn currently, it's because last year's plants (now dead) were successful in producing seed.
So, how do you break this cycle? You could try to stop the crabgrass in your current summer lawn from setting seed by spraying it with post-emergent herbicides. But you get more bang for your buck by preventing it from bursting upon the scene in the first place by using pre-emergent herbicides in spring.
Even if you're successful in killing it with a post-emergent herbicide, this doesn't prevent seeds from your neighbors' properties from landing in your yard. These seeds will sprout next spring in your lawn unless you prevent them from doing so with a pre-emergent herbicide.
How Crabgrass Spreads
The fact that crabgrass spreads by seed is both good and bad. First the good news: crabgrass is limited in the way it can spread, compared to a perennial weed such as, say, Oriental bittersweet. The latter can spread by seed, but it also spreads via rhizomes. So stopping the seeds of Oriental bittersweet from germinating in your yard won't be enough to eliminate it: the plant lives to fight another day (year), even if it's not successful in spreading itself via seeds.
With crabgrass, if you stop the seeds from germinating, you've stopped the plant, period. The bad news is that you have to get rid of crabgrass at the right time to prevent germination. Time your prevention efforts wrong, and you're stuck with crabgrass in your lawn for another summer.