Maybe you’ve heard of pre-emergent weed control, but you’re not sure what it is, why it’s important in your lawn care regimen, or when you need to apply it. Have no fear! Heritage Lawns & Irrigation is here to answer all of your questions. Fasten your seatbelt and we’ll fill you in on everything you need to know about pre-emergent weed control.
WHAT Is Pre-Emergent Weed Control?
Sometimes called crabgrass preventer, a pre-emergent kills the weeds before they emerge from the ground. A common misconception is that they keep the seeds from germinating, but that is not the case! When the product is applied it creates a barrier layer on the surface so that the young seedlings die when they come in contact with the barrier. If it is a granular product, when it gets watered in, the solution creates the barrier. If it is a liquid, the barrier sets up right away.
The length of time the barrier is in place depends on the concentration of the product used and the amount of moisture received. In an ideal world, you want the barrier in place throughout the weed germinating season but to break down just before lawn seeding season. This is the “art” part of lawn care, it is always a balancing act with mother nature.
Related Read: 5 Reasons Your Kansas City Lawn Still Has Weeds
WHY Does Your Lawn Need Pre-Emergent?
Plants and weeds will begin to germinate sometime around April 15th or when the ground temperatures hit 55 degrees Fahrenheit. This makes it the perfect time to break out the products to keep them from sprouting. The benefit of this one-time treatment is that you are ahead of the problem which will make these trouble spots easier to manage. Instead of having to fight your entire lawn, you should be able to spot treat with post-emergents for the rest of the year.
Grassy weeds can be the most difficult to control. Luckily, by using a pre-emergent, you can get a jumpstart on them before they hit maturity. This is important if you want to avoid killing desirable lawn grasses along with the weeds in later treatments. The pre-emergent is used to control grassy weeds such as crabgrass and foxtail. Plus, it can also help with a select few broadleaf weeds, such as dandelions. Unfortunately, there isn’t a surefire way to prevent all weeds.
A quick reminder and a common mistake:
- You can’t seed and apply a pre-emergent at the same time. It can’t tell the difference between a good seedling and a weed seedling. Your grass seed won’t grow!
- There are many other aspects of keeping weeds under control, this is just one tool. The best defense against weeds is still a thick healthy lawn that can choke out weeds.
WHEN Do You Need to Apply Pre-Emergent?
One of the first lawn care treatments of the year is the pre-emergent application that controls annual grassy weeds like crabgrass and foxtail. These weeds appear in late spring or early summer and can be difficult to control.
The key is to time your treatment before those nasty weed seeds germinate and kill them as they emerge. Crabgrass germinates when the soil temperature hits the low 50’s and stays there for a few days. In Kansas City that is usually around April 15th, but Mother Nature doesn’t always use the same calendar we do. Here are options we have for scheduling:
- Use the calendar. Have your pre-emergent treatment down before April 15th. For most homeowners in Kansas City, you will be close and more right than wrong.
- Measure the soil temperature. For the real geek in us. It’s not hard to get a soil thermometer and measure the temperature in the hottest part of your lawn and make your application when you hit the upper 40’s.
- Use web data. You can use the Internet to determine soil temperatures and time your applications accordingly. Websites like GreenCast give good data for your area. You can zoom in and get a decent idea of your part of town.
- Use indicator plants. Now this is the exact opposite of high tech #3. This is the old-school farmer version. Certain plants, like the forsythia bush and the lilac, flower at consistent soil temperatures every year. They naturally compensate for the curves mother nature throws each spring.
 Time your treatments to be completed before the forsythia drop all of their yellow flowers. |
 Time your treatments to be completed before the lilac blossoms open.
Timing your lawn weed control pre-emergent applications can be difficult if you don’t know what to look for, but now you can beat them before they really get started. If you hire a professional like Heritage Lawns & Irrigation, you now know enough to make sure you are getting what you’re paying for. |
There are two ways to apply pre-emergent weed control products. It all depends on which type you get – granular or liquid. HOW Do You Apply the Pre-Emergent?
Liquid – For your pre-emergent to work, it must be properly mixed and applied evenly. Read the instructions on the label closely to make sure you’re applying it properly. Don’t forget to calibrate your sprayer! Thorough coverage is the key to success; spot spraying will just allow the weeds to pop up where there is no pre-emergent.
Granular – Apply when grass is dry. Set your spreader to the appropriate setting as directed on the instructions on the bag. Granules do not need to be watered in, but for best results, the lawn should receive 1/4 to 1/2 inch of water from rainfall or watering within 2-3 days after application. Wait to see if Mother Nature is going to help, and if not, water on the third day. Watering in the product activates the weed killer. It helps it to sink into the soil creating a barrier just below the surface. Most products call for a half inch of water within three weeks of the application.
Bonus Tips: What Can Make Your Pre-Emergent Fail?
If the barrier breaks down, then the weed seedling can escape and grow to maturity. The following factors affect the barrier and affect your control:
Drought: If the ground dries out and cracks then you have created a breach in the barrier. The weed seedling can grow with very little water and can work up through the cracks. We see this during the summer when our soil pulls back from the concrete curbs and sidewalks. While these gaps aren’t exactly cracks, they create an opening in the barrier that the weeds will take advantage of.
Soil Disturbance: If you dig up an area (for instance have a sprinkler line installed) the mechanical process of digging or trenching will break the layer of pre-emergent and allow weeds to grow through.
Heavy moisture: In very wet years, or lawns that are over watered, the barrier breaks down too early and we get late season break through.
Improper Application: As you can imagine thorough, even coverage is critical to set the barrier up in the first place. Areas that are missed, or not covered thoroughly, will not have an appropriate barrier in place.
Improper Timing: This is one that many people get wrong. Once the weed has emerged, control is reduced. Once the weed is past the six-leaf stage, it won’t be controlled at all. You must get your pre-emergent on before the weed seeds germinate so the barrier is in place for the seedling.
As you can see there are many factors that can impact the effectiveness of your pre-emergent and grassy weed control. When used correctly, they are a great tool for keeping our lawns looking good.
We hope these tips will help you, and if you need help with your Kansas City lawn care, call Heritage Lawns and Irrigation at (913) 451-4664.
