The optimal timing for preemergence applications will vary by location, weather trends and the target weed. Some weeds germinate earlier than others based on soil temperature and other factors. Tracking growing degree days (GDD) is a great way to know when it's time to make preemergence applications. You'll want to have your first application done before accumulating 200 GDD in your area. This online GDD tracker is easy to use, simply put in your zip code, select 50 F as the base temperature, make the date range the current year, and it will calculate the GDD for your location. Changes in other plants like spring flowering and budding can also be a good indicator of when to apply weed control products, so take advantage of these cues from Mother Nature. For example, if you have forsythia bushes in your area, they reach full bloom shortly before crabgrass germinates. When you see yellow flowers, it’s time for a preventative crabgrass application. In some areas, one preemergence herbicide application is enough for the year. But, if you have a lot of weeds or live in warmer areas where weeds can germinate for much of the year, you’ll want to make multiple applications well into the growing season. Read product labels to determine how much you are allowed to apply in a given year, there are often limits.

Preemergence herbicides are commonly included with a granular fertilizer that allows homeowners to “weed and feed” with a lawn spreader. Most weed control products have recommended settings for common spreader models that will help you apply the right amount. Splitting the total recommended rate of herbicide into multiple applications is a great way to extend control and get better coverage. Most preemergence weed control products also need to get watered into the soil before they take effect, so you will want to make the application when rain is in the forecast or run sprinklers for several minutes after the product is applied if you have an irrigation system.
 

What To Do Once You See Weeds

Preemergence herbicides are the foundation of successful weed control on your lawn, but they don’t work on all weeds, and factors like excessive rainfall can limit effectiveness. This means that some weeds will inevitably break through your preemergence defenses. To control weeds once they’re visible and growing, you will need to use a postemergence herbicide. These are usually liquid products that come in a hand-held jug with a nozzle attached, ready to apply.

You’ll want to know the type of grass you have in your lawn (warm-season or cool-season) and the kinds of weeds you are trying to control (broadleaf or grassy weeds) before making an application. Most postemergence weed control products are designed to target certain weeds without damaging your grass, but you need to check the label to make sure you have the right product for the job, or you may end up killing grass without hurting the weeds! There are granular postemergence products that can be more efficient to apply over large areas, but they may not be as effective as spraying. Some weeds can be surprisingly stubborn, especially once they’ve had time to mature. If herbicide applications aren’t doing the trick, you can always pull weeds manually or use a small knife to pry them out of the ground. This is time consuming and hard on your back and knees, but it might be the best option if nothing else seems to be working.

Once you kill or remove weeds, you want grass to fill the voids, otherwise weeds are likely to return. Maintaining a healthy lawn with routine watering and fertilizer applications is key to this transition. Also keep in mind that even “selective” herbicides can injure your grass if applied in excess or when the grass is under stress – e.g., a Kentucky bluegrass lawn on a hot summer day. You want to apply postemergence herbicides when the lawn is growing well so that the grass isn’t damaged and can hopefully fill in the areas where weeds used to be. If you need to reseed areas with cool-season grass, fall is the best time to do it because the seedlings can mature in the fall and spring before facing the stress of summer.

Healthy Grass Is a Good Defense

Maintaining a dense and healthy lawn is a critical part of weed control. If the lawn is thin and sparse, that creates opportunities for weeds. Not mowing your yard at the correct height is a common mistake that can promote weeds. Far too many cool-season lawns (e.g., Kentucky bluegrass and tall fescue) are cut too low – often in an attempt to extend the length of time between mowing. For warm-season grasses (e.g., bermudagrass and zoysiagrass) it’s common to see them mowed a bit too high, resulting in a thinner canopy that’s more welcoming to weeds. In general, target a mowing height around 3 inches and above for cool-season grasses and no more than 1.5 to 2.5 inches for warm-season lawns, with some exceptions like St. Augustine grass, which prefers being cut at 3-4 inches.
 

Minimizing shade issues will help maintain healthier grass and prevent weeds. “We don’t really see lush sod in the forest so expecting to have a dense, weed-free lawn on a property with heavy tree shade is unrealistic,” said Brosnan. Homeowners might not want to cut down nice trees to improve their lawn, but pruning low branches to help more light reach the grass can make a big difference. Additionally, most weeds prefer wet conditions for seed germination and growth, so if you have a sprinkler system try to water only as-needed rather than running on a set schedule regardless of the weather. Adding some supplemental fertilizer applications throughout the year, especially in the spring and fall for cool-season grasses and summer months for warm-season grasses, will also boost turf health and give your lawn a competitive edge over weeds. For more general lawn care information, check out the Green Section Record article “USGA Agronomists Share Their Top Lawn Care Tips.” 

Don’t Rest on Your Laurels

Unfortunately, weed control in your lawn requires investment and effort each and every year. “Much like a vegetable garden, a lawn needs constant care,” said Brosnan. “Things such as fertilizer and herbicide applications, proper mowing and timely irrigation will all be needed to keep the grass healthy and minimize weeds year after year.”

If your weed control program is going well – stick to it! If it’s not going so well and the neighbors are starting to make comments, try some of the strategies in this article and take advantage of the many great resources available to help homeowners navigate weed control and other lawn care issues. Turfgrass weed experts like Dr. Brosnan are based around the country at land-grant universities and are usually more than happy to answer questions. Many university turfgrass department websites maintain a list of herbicides recommended for your area with detailed rates and other application information. Individuals can contact their local university extension office for guidance or work with a reputable lawn care business, of which there are many. Homeowners should seek out lawn care businesses that employ certified pesticide applicators licensed in their state. 

Having some weeds in your lawn is to be expected, but you can keep them from getting out of control by using the strategies in this article. You may not reach the standard of the clubhouse lawn at your favorite golf course, but you can certainly have a yard that looks great from your porch.