When to Apply Crabgrass Preventer (Pre-Emergent Herbicide)

Crabgrass is an opportunistic weed that is likely to take over your lawn if you don’t implement proper lawn care and weed control mechanisms. Thousands of crabgrass seeds are potentially lying dormant in your lawn waiting for the right temperature conditions to germinate.

Understanding when to apply crabgrass preventer can help you get rid of crabgrass in your lawn permanently. In this guide, get more about crabgrass pre-emergent control and how to get rid of any existing crabgrass weeds in your lawn or garden.

How does crabgrass preventer work?

A crabgrass preventer is a pre-emergent herbicide formulated to target and kill the germinating seed of weeds before the shoot emerge from the soil. The chemical also work by inhibiting seed germination and root development of grass seed.

Any overseeding can only be done after two to four months following the spraying of the pre-emergent herbicide. For newly seeded lawns or sod, apply the herbicide after you’ve mowed your lawn three times to avoid killing the new grass seedlings.

Don’t dethatch or aerate the lawn after applying a crabgrass pre-emergent herbicide, as this can break the chemical barrier of the herbicide rendering it ineffective. Be sure to apply the preventer uniformly across your lawn without missing a spots where the weed can find a chance to sprout.

Consider re-applying your pre-emergent herbicides for effective outcome. Most weed seed do not all germinate at once. Therefore, re-application can help kill some of the later-germinating seedlings.

When to apply crabgrass preventer

Understanding the crabgrass cycle is essential part in controlling the weed. Crabgrass is a warm-season annual plant with a lifespan of one year. While alive, a single crabgrass can produce over a hundred thousand seeds which are left behind to germinate.

Crabgrass preventers target the weed before it emerge through the soil. Timing when the crabgrass seed has germinated is thus the best time to apply the preventer. Crabgrass seeds usually germinate from early spring to late summer when the soil temperature ranges reaches 55-60 degrees Fahrenheit.

This means that, before you apply your crabgrass preventer, take a soil thermometer and track the soil temperature for several day or one week. Alternatively you can tell when soil temperatures are rising by simply observing blooming of plants such as forsythia shrubs, lilac bushes and pears. These plants reach full bloom at a soil temperatures that also activate crabgrass germination.

How to measure soil temperature

Checking soil temperature is very essential in planting seed and controlling weeds. This is usually done using a soil thermometer. Purchase a glass bulb with a metal point at your local garden store. To correctly measure soil temperature:

  • Follow the label directions on the package to learn how to use the soil thermometer
  • Make a pilot hole using a screwdriver to avoid damaging the device while pushing in the soil
  • Measure the right depth of the soil, go slightly deeper beyond the top layer of the soil
  • Provide shade in sun for accurate reading, you may use your hand or any item
  • Take multiple readings at different times and for several days to get a pattern

What does crabgrass look like?

Crabgrass is a drought tolerant annual weed that often emerge in early summer, and it thrives during the hot weather. It remains active in lawn throughout the year until frost sets in. As the name suggest, the crab-like grass has creeping stems which root freely at the nodes.

The invasive habit of crabgrass makes it a pest in lawns and cultivated fields. If left uncontrolled the weed can overtake your lawn grass after overpowering your good grass.

At a first glance, crabgrass will appear like your other lawn grass and further identification can help you understand it. When newly sprout, the plant appears lighter green in color which may eventually change to dark green as it continue to establish.

The leaves usually appear broader than the blades of lawn grasses. Stems grow laterally low on ground extending outward making it to resemble the shape of a crab. As the stem and branches grows, the center start shaping into a star pattern.

Crabgrass continues to put up new shoots from the center, as the very first shoots also keep branching out to the side. This growth pattern continues until the weed forms a dense thick foliage. In a competitive environment where there are other plants, crabgrass will also grow straight upright before the branches begin to falling to the ground as they spread.

Under proper sunlight and water conditions, crabgrass will blend well with the rest of lawn grasses and you may notice it at maturity or when in large clamps. Due to its low height, lawn mowing may not affect it unless you set the blade to a lower which may also harm your grass.

How to kill existing crabgrass weeds

Once crabgrass has germinated and sprouts through the soil, application of a preventer will no longer be viable. The quickest way to kill existing crabgrass plant is to use a post-emergent herbicide that target a plant and not seed.

Before using your crabgrass killer, understand whether it’s selective or non-selective. Selective herbicides target a specific category of weeds or plants while non-selective herbicide kills all types of plants including lawn grasses.

Always read product label instructions carefully and understand. Every weed killer comes with a label listing the type of grasses to use on. Usually St. Augustine and centipede grass are susceptible to most herbicides that don’t harm other lawn grasses. Further strictly follow safety precautions to protect pets, kids and adults.

Using chemical herbicides to control weeds is usually not the best of the options. Herbicides contain toxic ingredients that also affect other lives and destabilize soil conditions. It is still possible to get rid of crabgrass without chemicals. Corn Gluten is an organic pre-emergent that is effective in controlling crabgrass.

Crabgrass like invading bare spots and areas with thinning grass in a lawn. Proper seed planting, watering and mowing can minimize chances of crabgrass weeds. A well-manicured dense lawn will typically keep away weeds.


Timing crabgrass preemergence applications in an early spring – Kevin Frank, Michigan State University, Department of Plant, Soil and Microbial Sciences

Managing Crabgrass in Home Lawns – University of Illinois Extension

Crabgrass and its Control – David Bayer, UW-Extension, Outagamie County

Proper Timing for Crabgrass Preventers – Paul C Hay, Extension Educator University of Nebraska-Lincoln Extension in Gage County

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