Each type of grass has an ideal cutting height for good health and strong growth. When cut no lower than that height, and when cut before it gets too long, the grass will usually out-compete weeds as long as it’s also fertilized and watered properly. Longer grass helps prevent weeds in a couple of different ways. The taller growth shades the ground, keeping it cooler and retarding weed seed germination. And once weed seeds sprout, they don’t have as much sunlight as they need for hardy growth.
The chart below shows the range of cutting heights depending on the grass type. If you don’t know your grass type, take a plug of turf to a garden center and ask the staff to help with the identification.
It’s also important to mow your grass when it needs it. That’s when the grass is one-third above the ideal cutting height. Depending on the weather conditions and the time of year, that can mean mowing every week or two, or every four or five days. Keeping the height in check also means you’re clipping off weed seed heads before they can mature and seed your lawn.
Ideal Mowing Height Ranges
Cool Climate Grasses
Bent grass - 1/4 to 3/4 in.
Chewing hard or red fescue- 1-1/2 to 2-1/2 in.
Tall fescue - 1-1/2 to 3 in.
Kentucky bluegrass - 1-1/2 to 3 in.
Perennial ryegrass -1-1/2 to 3 in.
Warm Climate Grasses
Bahia grass - 2 to 3 in.
Bermuda grass - 1/2 to 1 in.
Blue grama grass - 2 to 3 in.
Buffalo grass - 2 to 3 in.
Carpetgrass - 1 to 2 in.
Centipedegrass - 1 to 2 in.
St. Augustinegrass - 1 to 3 in.
Zoysia grass - 1/2 to 1 in.
Before you start any weed control program, you need to determine which of the three types of weeds you're controlling. Each requires unique products and application methods. Some treatments are very time sensitive, while others can be done anytime during the growing season.
The key to controlling broadleaf weeds is to use a broadleaf herbicide (see “Getting the Most from Broadleaf Killers,” below) and distribute it with the smallest applicator necessary to do the job. That’ll not only save time and money but also keep you from needlessly introducing chemicals into the environment.
Spot-kill weeds with a small pressure sprayer
No matter how lush and healthy your lawn is, a few isolated weeds will pop up. That doesn’t call for whole-yard treatment. Instead, spot-treat the weeds with a small, trigger- controlled, pump-up pressure sprayer (Photo 1). After pouring in the diluted herbicide, you pump up the pressure with a little plunger and then pull the trigger to release the spray right on the culprits.
Treat weed patches with a 1- or 2-gallon tank sprayer
Patches or clumps of weeds are best treated with a standard 1- or 2-gallon tank sprayer (Photo 2). After spraying, triple-rinse the tank with water. With each rinse, pump up some pressure and flush out the wand,
Use a dial sprayer when weeds are out of control
If your whole lawn is filled with weeds, it calls for draconian measures, and a dial sprayer attached to your garden hose is the answer (Photo 3). It’s fast and efficient. It’s just a matter of adding concentrated broadleaf killer to the pot, and setting the dial at the top to the mixture called for on the herbicide container—for example, 2-1/2 tablespoons per gallon of water. Then hook up the garden hose and apply an even treatment to the weedy areas. Clear the yard of toys, furniture and anything else that can get contaminated by overspray. And be sure to protect your flowers and bushes with plastic sheeting or cardboard. Remember that broadleaf killers will kill or harm anything with leaves—including your flowerbed.
Travis Larson, an editor for The Family Handyman, will show you how to apply concentrated broadleaf herbicide to kill weeds in your yard in our video tutorial.
Quack grass is the widest spread example of a perennial grass that comes back year after year just like your lawn. They spread through seeds and extensive underground root systems and are unaffected by broadleaf killers. Pulling grassy weeds only gets some of the roots, and the remaining ones will quickly sprout new plants. The only effective solution is to use a “nonselective” plant killer like Super Kills-All or Roundup. You can apply non-selective killers with sprayers, but you’ll kill everything in the area, including your lawn and any other nearby plants. The best way to kill these weeds while protecting surrounding plants is by wiping the grass blades with the non-selective herbicide. Wear a cheap cloth glove over a plastic or rubber chemically resistant (they’re labeled as such) glove to protect your skin. Dip your gloved hand into the herbicide and then simply grab the blades near the base and pull the herbicide over the grass blades. Don’t worry about coating every single blade. The chemical will absorb into the plant, make its way down to the roots and kill the entire plant. Most will die in a few days, but survivors may need more treatments.
Crab grass is the best example of an annual weedy grass. It doesn’t over-winter like perennial weeds. Instead, it dies at the end of the growing season and depends on producing thousands of seeds to propagate new clumps in the spring.
The best way to keep crab grass under control is to apply a crab grass preventer between the first and third mowings in the spring. Timing is everything. The treatment prevents the seeds from germinating. If you wait too long, the seeds will sprout. Apply too early and the preventer will dissipate and late germinating seeds
Make notes in the fall about where your crab grass seems to thrive. That, of course, is where the seeds are concentrated, so you don’t need to treat the whole lawn, just the areas that are infested. Crab grass loves areas where the ground warms quickest, especially near driveways or sidewalks where the asphalt or concrete helps warm the soil. That’s the profile of most other annual grassy weeds too. They’re treated much the same way as crab grass, but read the directions on preventer bags to find one that’ll be effective for the annual weeds you want to eliminate.
Once crab grass shows up in your lawn, you have three options:
- Hand-pull the clumps to prevent the plant from reseeding itself. If that leaves bare spots in your lawn, rough up the ground and reseed the patch. Keep it moist and let the grass grow. The grass will grow faster than the underlying crab grass seeds and may become lush enough to shade and prevent them from sprouting next spring.
- Let the crab grass go until the following spring and then use a preventer at the right time. Crab grass dies completely in late fall.
- Treat clumps with a post-emergence crab grass killer. These treatments are specially formulated to not harm your lawn grass. They’re most effective on younger plants and may take two or more treatments in four- to seven-day intervals to completely kill the crab grass. Once seed heads form in the late summer or fall, post-emergence killers won’t work and you’re better off waiting until spring and then treating the seeds with preventer or hand pulling.
Poor light or soil conditions can make it all but impossible to grow grass in some areas. If you’ve tried more than once to nurture grass in an area and failed, it might be time to throw in the towel and treat the area with a landscaping alternative. The obvious choices are stone, mulch and attractive ground cover plants that tolerate the same conditions grass can’t handle. Kill any weeds with a nonselective herbicide (re-treat survivors after 10 days). The herbicide will break down within two weeks and the ground will be safe for new plants. If you’re covering the ground with a decorative material like stone or mulch, consider laying a weed-control fabric on the ground first to keep weeds from getting another foothold.
Getting the Most from Broadleaf Killers
Broadleaf herbicides are extremely effective, provided they’re used according to the directions on the container. Pay particular attention to the moisture, temperature and wind limitations. Most liquid herbicides only work in temperatures between 65 and 85 degrees F. At higher temperatures, the chemicals vaporize before the weeds can absorb them. And at lower temperatures, the weeds aren’t growing fast enough to absorb the chemicals. Here are some general guidelines to help you get the most from the products:
- Buy concentrated formulas. They’re far cheaper than premixed types.
- Mix only what you need, and use it within three days of mixing. Once mixed with water, herbicides lose their effectiveness in a very short time.
- Store herbicides in a cool place and protect them from freezing. They’ll stay effective for a long time.
- Protect decorative plants with plastic or cardboard during lawn treatments. Even just a little overspray can damage or kill your plants.
- Some lawn grasses, especially “warm weather” ones found in the Sun Belt, can be harmed or even killed by broadleaf herbicides. Be sure to read the label to find out which grass types are vulnerable.
- Apply when grass is slightly damp or dry, and avoid applying during hot weather or high humidity conditions. For best results, apply in the morning or early evening.
- Some formulas call for application when there’s no rain in the forecast for 24 to 48 hours.
- Wear socks, shoes, long pants, a long-sleeve shirt, gloves and glasses to keep the liquid off your skin and out of your eyes.
- Keep pets and kids off grass for 48 hours after application, then water the area well to rinse, and let it dry.
- To keep safe, read the label for special warnings about toxicity.