Like an ocean crab, 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 annual and perennial types of grass. The species generally have broad, flat blades and produce long flower clusters and thousands of seeds per growing season.
Crabgrass keeps a low profile in the lawn, eluding the mower blade when you 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.
What Does Crabgrass Look Like?
Crabgrass looks like a coarse, light green clump of grass. Its sprawling stems resemble the legs of a crab. It is commonly mistaken for fescue grass, but color and size are good differentiators to recognize it accurately. Tall fescue grass is usually darker green and mostly thick and grows very fast.
The blades of young crabgrass are about the thickness of a pencil, but then the stems get heavy, fall, get scraggly, and you will notice a star-shaped pattern in the middle.
Types of Crabgrass
The two main types of Digitaria are smooth crabgrass and hairy crabgrass. Smooth crabgrass is more common. Hairy and smooth crabgrass are very similar. They are both annual grasses peaking in summer. They thrive in nutritiously rich, sandy, or clay soils. At maximum maturity, hairy crabgrass can grow up to 3.5 feet long, while smooth crabgrass grows to an average full length of 2.5 feet.
Hairy crabgrass is also called long crabgrass. Their leaf blades both get to about the same length, up to 6 inches long, although hairy crabgrass tends to grow longer than smooth crabgrass. The hairy type (Digitaria sanguinalis) is so-called because, if you inspect its leaves and stems, you will find many tiny hairs all over the plant. It is more coarse and has broader blades than smooth crabgrass. A leaf of hairy crabgrass can be up to 1/2-inch wide, while smooth crabgrass is about 1/4-inch wide.
Smooth crabgrass (Digitaria ischaemum) has no hairs around the sheath or the leaves. 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.
Smooth crabgrass is also called "small crabgrass" because it grows shorter. While it can grow to 6 inches, it is frequently shorter than its hairier cousin. Its leaf blades taper to a point. The plant's stems bend at its nodes, and the stems sometimes turn red.
It's an annual plant, so it lives for one year. At the end of the growing season, its primary mission is to produce seed. 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 dead plants completed its mission.
Seeds begin to sprout during the mid-spring when the soil temperature reaches 55 degrees Fahrenheit for a stretch of at least four or five consecutive days. By the time the daily average soil temperature is 73 degrees Fahrenheit, most of the crabgrass seeds should have germinated. In shadier spots, it will take longer. It will sprout faster in areas near concrete and rock since those materials conduct heat.
In the beginning stage, the crabgrass seedlings resemble a miniature corn stalk. Then, the leaves start to branch out.
Crabgrass becomes more problematic when summer approaches because these weeds thrive in hot and dry weather conditions. It is drought tolerant and is often the last green thing on the lawn before a late fall frost finally kills its stems.
How Crabgrass Spreads
The good news about crabgrass is that it is limited in its growth; it only spreads by seed and not rhizomes. If you stop the seeds from germinating, you've prevented the plant from emerging. The tricky part is that you have to get your timing perfectly to stop crabgrass from germinating. If your timing is wrong, you've got crabgrass.
However, sprouted crabgrass can grow more vigorously if not kept in check. If its long stems are allowed to flop over and touch the ground, it can root at those nodes, and secondary stems can sprout off the main stem, a foot or two away from the main crown.
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How to Get Rid of Crabgrass
To get rid of crabgrass, you need to break its life cycle. You can use several methods, including herbicides or keeping your lawn grass healthy. Two types of herbicides are your best line of defense: post-emergent and pre-emergent.
- Post-emergent herbicide: If you have crabgrass, kill it and stop it from setting seed by spraying it with post-emergent herbicides. Post-emergent herbicides come in two types: selective and non-selective. Selective herbicides target specific weeds or plant categories, such as grassy versus broadleaf plants. Non-selective herbicides kill all plants, including those you might want to keep. If you have other plants in the area you want to keep, ensure you get a selective post-emergent herbicide that targets crabgrass.
- Pre-emergent herbicides (crabgrass preventers): Crabgrass preventers go after crabgrass seeds set the previous season. This type of herbicide prevents crabgrass seeds from germinating, stopping them from developing roots and emerging. It is used in the spring. The timing of its application is critical for its success. Crabgrass seeds can germinate from early spring until late summer when soil temperatures reach 90 F. You can get an inexpensive soil thermometer to keep track of soil temperatures. An application of this herbicide can usually last about five months.
- A healthy lawn blocks weeds from taking over. You can maintain a dense turf stand by mowing high and fertilizing. Keeping the lawn cut to a height of 3 inches or more shades and cool the soil, preventing weeds from germinating. To better serve desirable grasses, attempt to water deeper instead of more frequently. Keep fertilizing your healthy grass but don't overdo it. Balancing your fertilizer application will give your lawn the right energy to help it block out the weeds.
- Hand-pull large seedlings: You don't have to use herbicides if you get rid of all the crabgrass by hand and replace it with healthy new grass. To pull it effectively, wait until the seedlings are big enough to hand pull, roots and all. Remove the whole plant in the spring before amending the soil and reseeding. Use a garden weeder tool made to remove the roots of crabgrass, featuring a claw or plunger you can drive deep into the ground, under the roots, to remove the plant. Place the clumps in a trash bag and seal tight, preventing the spread of crabgrass seeds.
- Remove dead crabgrass plants: A dense mat of dead crabgrass can smother nearby grass plants, suppressing healthy lawn growth.
- Reseed bare lawn spots with healthy lawn grass: An exposed patch of soil is an open invitation for any weed seeds to take root. You can start new grass in early fall, getting it established before winter. Do an aeration treatment before putting down some fresh grass seed to fill in the bare spots.
- Mulch: Cover non-lawn areas with a 2-inch thick layer of mulch to reduce weed growth of any kind. Any weeds that sprout through the mulch should be easy to eliminate by pulling or spraying.
Keep in mind that even if you're successful in killing it with a post-emergent herbicide, you can't prevent seeds from a neighboring property from blowing over and landing in your yard. Those seeds will sprout next spring in your lawn unless you prevent them from germinating with a pre-emergent herbicide.
So, while post-emergent herbicide will kill crabgrass permanently that season, it does not guarantee that crabgrass seeds find their way to your grass again.