How to Get Rid of Crabgrass

If you're trying to grow a lawn as eye-catching as the field at a Major League Baseball stadium but you're losing the fight with crabgrass, you can blame the U.S. Patent Office, circa 1849.

That's when the Patent Office was the forerunner to the Department of Agriculture as the agency responsible for approving plant introductions into the United States. It was also the year patent officials authorized the importation of a common crabgrass, called large crabgrass (Digitaria sanguinalis), one of about 60 species of crabgrass. Back then, crabgrass was considered a forage grain — a solution to a growing problem. Livestock numbers were growing rapidly, and good forage was scarce, according to the Weed Science Society of America (WSSA).

Besides that, crabgrasses were among the first grains early humans cultivated as a food source. So, what could possible go wrong with importing crabgrass to feed cows? More than 150 years later, we know the answer: plenty. New waves of immigrants brought additional varieties of crabgrass with them, and, in time, one or more of 13 species of crabgrass have spread to every state in the country, according to the WSSA. The infestation is now so thorough that the WSSA says crabgrass is considered one of the nation's major lawn weeds.

If you're one of the hapless homeowners who consider crabgrass the bane of your lawn care existence, you're not alone. A single plant of large crabgrass can produce as many as 700 tillers, or side shoots, and 150,000 seeds in temperate areas of the U.S. and even greater numbers of seeds in the country's more tropical climates, according to the WSSA.

Controlling crabgrass

Crabgrass is such a problem that "turf and landscape managers often plan their entire spring weed control programs around when crabgrass emerges," said Patrick McCullough, an associate professor of crop and soil sciences at the University of Georgia's (UGA) Griffin campus. For DIY homeowners who don't want to hire a lawn care company, especially homeowners who are environmentally conscious, McCullough doesn't have great news. Unfortunately, there's no natural remedy for crabgrass. There is, however, a solution.

"We don't recommend organic products because the selectivity for crabgrass control is just not to acceptable levels yet where you're not injuring desirable grass species," McCullough said. Instead, he said homeowners' best defense against the pernicious weed is sound cultural practices. Those practices, he said, involve a five-step process: early detection, applying pre-emergents in the spring, mowing at the proper height, a balanced fertilizer program, and sound watering practices. The goal is to have a thick lawn with minimal bare spots, which will reduce the competitive capabilities of crabgrass and other unwanted weeds.

Implementing the steps

Step 1: Early detection

As with personal health, early detection can help head off serious problems later. McCullough advises walking your lawn and scouting the grass for signs of emerging or persistent weeds. "Try to get these controlled as soon as possible, whether that's digging them out by hand or spot treating them with post-emergent herbicides," he said. The idea is to get out in front of weed emergence and encroachment, which can be important to preventing major outbreaks of weeds that could become invasive.

Step 2: Applying pre-emergents

"Most pre-emergent herbicides that are effective against crabgrass must be applied before crabgrass germination begins," McCullough emphasized. Don't use a calendar to try and determine when to apply these, he cautioned. Instead, he said use soil temperatures as your guide for when to make pre-emergent applications. Apply the pre-emergents when soil temperature reaches 50 degrees Fahrenheit, which will be just prior to dormant crabgrass seeds germination. The date when soil temperatures will reach 50 degrees will vary from year to year depending on each year's weather pattern, he added. To track soil temperatures, use a soil temperature gauge that is available at garden centers or from online garden suppliers. If you can't find one, check the websites of local academic or other institutions that track such valuable gardening information as rainfall, day length, and air and soil temperatures. In Georgia, for example, soil temperature and other data are available at UGA's weather website. Soil temperatures on this site can be found under "Maps and Summary" in the menu bar and then on three drop downs: Current Maps, Daily Maps and Daily Summaries.

Step 3: Proper mowing height

Regardless of whether you have a cool season grass such as tall fescue or Kentucky bluegrass or a warm season grass such as zoysia or Bermuda, setting your mower at the proper height is critical to promoting a thick lawn, McCullough said. For tall fescue or Kentucky bluegrass, that height should be three inches. "Raising the height of your lawn mower from the standard two to three inches can help shade out crabgrass seedlings in the spring and reduce its competitive capabilities with the cool season grasses that are not necessarily growing to their full potential during warmer periods," he said. "Zoysia is a little bit different," he added. "You don't want to raise the mowing height too high because that will thin out the grass. The proper mowing height is probably going to be between one-to-two inches depending on the cultivar you are managing."

Your local Extension office can help you identify the cultivar in your lawn if you don't know what type of grass you are growing and can recommend the proper height to mow that grass for your area. Timing is also an important part of good mowing practices for warm season grasses. "Usually, the lower you mow Bermuda or zoysia grass the earlier it is going to green up, and the earlier it greens up the more competitive it is going to be with summer annual weeds that are starting to emerge in the spring," McCullough said.

Step 4: Fertilize

A balanced fertilizer program will ensure your lawn is receiving the right nutrients your variety of grass needs throughout the year. Check with your local gardening center or Extension office for a fertilizer that matches your type of grass and follow the application directions on the bag.

Step 5: Watering

Perhaps the most important thing to remember about watering, McCullough said, is do not overwater your lawn. Applying too much water can promote weed growth. Here, also, timing is important. Water before the heat of the day to reduce loss of moisture to evaporation and don’t water in the evening, which can promote disease.

How to identify crabgrass

Of course, if you're going to walk your yard looking for crabgrass, it helps to know what crabgrass looks like. Three especially prevalent crabgrass species are Southern crabgrass (Digitaria ciliaris), smooth crabgrass (Digitaria ischaemum) and the previously mentioned large crabgrass. Another crabgrass found in Florida and tropical-type areas of the country is blanket crabgrass (Digitaria serotina).

Southern crab grass, as its name implies, is found primarily in the Southern states. As you go towards the transition zone of the United States, such as Tennessee and Kentucky, and head toward the Midwest, you're going to see a lot more smooth and large crabgrass, McCullough said. More northern states may also be dealing with just smooth crabgrass and large crabgrass, not so much the southern crab grass species, said McCullough.

To identify crabgrass, McCullough recommends looking at the hairs on the plants. "Crabgrass often has some distinct hairs around the stems and the leaves depending on the species," he said. "Smooth crabgrass is hairless but it also has some very distinct characteristics like the ligule on the plant, which is a fleshy structure at the base of the leaf. Crabgrass has a noticeable ligule that emerges at the base of the leaf. Another key characteristic is the seedhead or the flowering structure, which has finger-like spikes." Better yet, McCullough suggested looking at pictures of crabgrass and other weeds on georgiaturf.com. When you go to the site, search the list of weeds by the name of the crabgrass, such as large crabgrass, and click on the hyperlink.

How crabgrass grows

Understanding how crabgrass grows will help homeowners understand why following the five-step control program is a good approach to keeping their lawns free of crabgrass and other weeds. "Crabgrass is a true annual in that it completes its life cycle in one year and will return seed for the next generation to the soil as the plant dies out in the fall," said McCullough.

Crabgrass germinates in late winter and spring from seed that has been lying dormant on the ground from the previous year's plants. The seedlings begin vegetative growth in the spring and continue growing into the summer, when they will start to mature. In mid to late summer plants will develop a seedhead that will produce viable, mature seed by fall. As temperatures drop in the fall, the plant will begin to die. As it declines and ends its life cycle, the seed will fall off the seedhead and lie dormant until soil temperatures the following spring reach about 55 degrees. Then, the cycle begins again.

Homeowners shouldn't be lulled into a sense of complacency by the winter dormancy of crabgrass, McCullough advised. "Most of the time the lawns with the heaviest infestation of crabgrass in summer are often correlated with failure to control winter weeds," he said. That's because as the winter weeds die out they leave open areas in lawns that are just perfect conditions for crabgrass to germinate with limited to no competition from desirable grasses.

Topping McCullough's list of winter weeds are annual bluegrass, a winter annual grassy weed that is completing its life cycle as crabgrass begins to germinate. "As one goes out, the others come in," McCullough said. "If you have heavy annual bluegrass infestation it's often a direct correlation with summer annual weed infestation." Other winter weeds homeowners should be sure to control include broadleaf species like henbit, hop clovers, which are annual clovers, and bittercress, all of which complete their lifecycle in late April and May.

"If homeowners can control the winter weeds in late winter, they will significantly improve the capabilities of their desirable lawn species to fill in the void prior to the emergence of summer annual weeds like crabgrass and other warm season weeds," said McCullough. And, voila! The view out the front window will start looking a lot more like a perfectly manicured professional baseball field.