When to Apply Crabgrass Preventer – The Best Time

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 to germinate.

Spring is the best time to apply a pre-emergent crabgrass preventer to your lawn. Spray when the soil temperature reaches 55 to 60 degrees Fahrenheit for 3 to 4 consecutive days. Apply before rains preferably in the morning when the weather is calm and sunny.

When to apply crabgrass preventer

Understanding when crabgrass germinates is an essential step in controlling the weed for good. 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 emerges through the soil. The best time to apply the preventer is when the crabgrass seed has germinated. Crabgrass seeds usually germinate from early spring to late summer when the soil temperature ranges reach 55-60 degrees Fahrenheit.

This means that, before you apply your crabgrass preventer, take a soil thermometer and track the soil temperature for several days or one week.

Alternatively, you can tell when soil temperatures are rising by simply observing the blooming of plants such as forsythia shrubs, lilac bushes, and pears. These plants reach full bloom at soil temperatures that also activate crabgrass germination.

How to measure soil temperature

Checking soil temperature is very essential in planting seeds 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 an accurate reading, you may use your hand or any item
  • Take multiple readings at different times and for several days to get a pattern

How does a crabgrass preventer work?

A crabgrass preventer is a pre-emergent herbicide formulated to target and kill the germinating seed of weeds before the shoot emerges from the soil. The chemical also works 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 spot. This is where the weed can find a chance to sprout.

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

How to kill the existing crabgrass

Once crabgrass has germinated and sprouts through the soil, the 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 targets a plant and not a 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.

It is also important to read product the 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.

A post-emergent herbicide for crabgrass should be applied in early summer. Spray the crabgrass killer on visible crabgrass plants on a calm, sunny day when temperatures do not exceed 90 degrees Fahrenheit.

You may need to reapply the herbicide to completely kill mature crabgrass but do it 4 to 10 days apart.

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 naturally. Corn Gluten meal is an organic pre-emergent that is effective in controlling crabgrass.

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


  1. Timing crabgrass preemergence applications in an early spring – Kevin Frank, Michigan State University, Department of Plant, Soil and Microbial Sciences
  2. Managing Crabgrass in Home Lawns – University of Illinois Extension
  3. Crabgrass and its Control – David Bayer, UW-Extension, Outagamie County
  4. Proper Timing for Crabgrass Preventers – Paul C Hay, Extension Educator University of Nebraska-Lincoln Extension in Gage County

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