How to Spray Lawns for Weeds, Safely and Effectively

Weed killer on lawn

Even for the most fastidious gardener, weeds happen.

Spraying to kill weeds effectively and safely takes time and effort, but done right, this last-resort practice can teach you about your lawn so you don’t have to make it an annual chore. Here’s how.

3 Classes of Weeds: Broadleaf, Grassy, Sedge

First, know your target. Weeds fall into three classes: broadleaf, grassy and sedge.Broadleaf, grassy, sedge - the three classes of weeds

Broadleaf weeds produce two seed leaves as the plant emerges and generally display wider leaves and branched stems. Broadleaf weeds include white clover, dandelion, thistle, pokeweed and even poison ivy.

Grassy weeds produce one seed leaf, like regular turfgrass. Examples include goosegrass, crabgrass and rushes.

Sedges also produce just one seed leaf, but in cross-section, the leaves show a triangular shape, as with yellow nutsedge.

Annual, Perennial Weeds

Weeds have one of two types of growing habits — annual and perennial.

Annual weeds live for just one season, which makes them easier to control.

Perennial weeds such as dandelions or crabgrass recur each year from the same root system.

“A lot of our weeds are annuals that germinate from seed, flower, produce seed, and die within one year,” says Horticulture Agent Mary Helen Ferguson with the Louisiana State University Extension in Livingston Parish. “Once an annual weed has flowered and gone to seed, it’s already done most of its damage and will die soon anyway. Once seed has been produced, that seed will be in the soil for the following years.”

Identify Your Weeds

But before you head to the store to pick up weed killer, you have to know a few things.

First, identify your weeds, says the Weed Science Society of America, a nonprofit organization fostering an awareness of weeds and their impact on the environment.

A herbicide label will list the weeds it kills on the label, according to WSSA member Kai Umeda.

WSSA, like many sites, offers a rogue’s gallery of weed photos that can help homeowners identify their culprit. Ferguson said your local cooperative extension office can generally get you this type of information as well.

“The type of product that’s best will depend on the type of weed you’re trying to kill and the type of turfgrass that you have,” says Ferguson.

Herbicide Formulations: Liquid, Granular, Selective, Nonselective

Now that you know the type of weed you have, you face more choices.

Your particular weed may be best fought by either a liquid or granular herbicide.

Liquid herbicides are usually mixed with water and sprayed from a handheld pressure sprayer.

Granular herbicides are mixed with an inert carrier like clay or lime, or a fertilized carrier and spread from a fertilizer spreader.

Selective herbicides kill specific plants. If the label says “broadleaf herbicide” or something similar, it is selective. If you follow label directions, you can spray it and it will kill the weeds without killing your grass.

Nonselective herbicides kill whatever they touch. For these, you need a spray bottle and careful aim.

There are two types of spraying, broadcast and spot. Don’t mix them up, or you could kill your lawn.

Broadcast is applying the herbicide across the entire lawn, You broadcast spray selective weed killer, since the lawn can tolerate it. 

Spot application uses an applicator, like a pressurized sprayer, to apply the herbicide directly to the weed in the effort to affect no other plants.

Pre-Emergent, Post-Emergent, Weed and Feed

Pre-emergent herbicides can be especially helpful for crabgrass and for places in your lawn where weeds pop up year after year. “Pre-emergence herbicides are only appropriate for weeds that come back from seed, and they generally work better on small-seeded weeds,” Ferguson says. “They’re most often used for annual grasses like crabgrass.” Timing is critical for pre-emergent herbicides. The critical number is 52: When your soil temperature (not air temperature) reaches 52 degrees Fahrenheit, crabgrass and other seeds begin to germinate. You must apply the pre-emergent herbicide before that moment for them to be effective.

Post-emergent herbicides target existing weeds and are especially helpful in places such as driveways or sidewalks. Says Ferguson: “Post-emergence herbicides are best applied early enough in the life of a weed to kill the weed while it’s still small before it flowers and produces seed.”

Herbicides that include a fertilizer are called “weed-and-feed.” This product catches a lot of grief from some gardeners and environmentalists since its application is hard to get right and easy to abuse. The best time to apply fertilizers is often the wrong time to apply weed killer and vice versa.  People often get it wrong, Ferguson notes.

“For example, here in Louisiana, the appropriate time for making an initial application of a pre-emergence herbicide for crabgrass is earlier than the time period when nitrogen-containing fertilizer should be applied,” she said. “If you’re thinking of using a weed and feed product, make sure that it’s the right time to use the type of herbicide that the weed-and-feed contains and the right time of year to apply the nutrients that it contains.”

Weed Spraying Tips

  • Read the labels of herbicides in their entirety and follow the guidelines laid out by the manufacturer.
  • Spray in the right weather conditions. Rain or wind can send herbicide drifting into nearby flower beds or other areas where it will harm desirable plants.
  • Target only the weeds you want, using a foam applicator or precise spraying.
  • Wear personal protective equipment, especially gloves. Hands are the most commonly contaminated area of the body. Reusable gloves should be cleaned before removal and stored in a plastic bag.
  • Wear goggles, a long-sleeved shirt and long pants that reach your shoes, which should be chemical-resistant.
  • Hats, face shields, dust mask, disposable jumpsuit and chemical aprons for chemical mixing also help protect you..
  • Calibrate the sprayer is for application at the recommended rate.
  • Calculate the amount of herbicide to add to the tank and read and follow instructions printed on the manufacturer’s label.
  • Be sure to fill the sprayer with at least half the volume of water or fertilizer you will ultimately need, starting with moderate agitation and keep going as needed. 
  • Soluble liquids and powders, such as the controversial weed-killer Roundup, dissolve in water to form a solution, and once mixed don’t require additional agitation.
  • Granular herbicides, once added to a carrier, can be applied directly to the soil without further dilution. However, they usually require more rainfall activation than comparable sprayable formulations.
  • Fully clean your sprayer before loading in the new herbicide,
  • Use the correct application nozzle, found on the product label.
  • Check and fix any problems such as leaks or busted valves and adjust the pressure regulator to get the right spray pressure.
  • Maintain a spray height of about 2 feet above the plant encourages maximum coverage and reduces the potential for drift.
  • After spraying, clean off all equipment thoroughly.

Raising a weed-free lawn

All the experts agree that the best advice for spraying for weeds is: don’t. In a sense, weeds are your friends, not your enemies, in the sense that they are telling you a story about what your lawn and your soil are lacking. A healthy lawn shouldn’t require weed-killing spray. So the best plan going forward is to change your lawn care habits so you won’t need to.

Good watering, mowing and fertilizing practices, “are the foundation of good weed management in lawns,” Ferguson says. “Make sure that you’re growing a turfgrass that is well-suited to your conditions.”

“Vigorous, healthy turf shades weed seeds so they don’t germinate,” says a guide in weed control from the University of Delaware Cooperative Extension.

Ferguson said to make sure and get a soil test to make sure you have appropriate soil pH and nutrient levels.

If not, apply nitrogen-containing fertilizer based on recommendations from your local Cooperative Extension resources.

The Maryland guide notes that homeowners have been using herbicides for weed killing since the 1950s, “but the potential risks to people, animals and the environment should cause people to reconsider their use as part of routine lawn care.” Many gardeners and lawn care professionals have switched away from chemical herbicides and prefer to practice organic lawn care, which does not use them.

The guide also reiterates that good lawn care practices, in general, are the best way to prevent weeds and that “herbicides should be used as a last resort and not a substitute for good lawn care.”

Derek Lacey

Derek Lacey

Formerly the agriculture writer for the Hendersonville Times-News, Derek Lacey’s articles have appeared in U.S. News & World Report, The Charlotte Observer, News & Observer, and The State. He has won 15 awards from the North Carolina Press Association and GateHouse Media, for pieces ranging from news features and investigative reporting to photography and multimedia projects.