How to Kill Crabgrass

Getting Rid of the Weed With Preemergent Herbicides

Crabgrass herbicide
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Do you know what crabgrass looks like? Digitaria (the plant's botanical name) is a warm-season annual; it reproduces by seed. People often ask me how to kill it. The short answer is: applying preemergent herbicides at the right time is the best way to get rid of this weed.

Preemergent Herbicides for Killing Crabgrass: When to Apply

To get rid of crabgrass, it helps to know its life cycle. When spring soil temperatures (at a depth of 2-3 inches) reach 55-60 degrees Fahrenheit, the first crabgrass seed will germinate (unless you stop them at this time by applying a preemergent herbicide!).

From mid-summer to fall, seeds are produced. The plants, themselves (but not the seeds) are killed by frosts in autumn.

Preemergent herbicides (you will also hear the term, "crabgrass preventer") come in either granular or liquid form and kill crabgrass seedlings as they germinate. Think of preemergent herbicides as forming an invisible shield across the soil surface that stops emerging crabgrass dead in its tracks. This shield image will serve as a reminder not to practice core aeration on lawns after applying preemergent herbicides, since doing so would only "puncture" the shield. Aerate lawns beforehand, instead.

As their name suggests, preemergent herbicides kill crabgrass at a specific time: before its seedlings emerge. For success in getting rid of crabgrass in this manner, timing is of the essence. Apply preemergent herbicides before germination, but not too far ahead. Crabgrass germination coincides approximately with the blooming of the lilac bushes.

Thus the old saying (an example of the use of "phenology" to guide one's landscaping work) that preemergent herbicides should be applied sometime between the time the forsythia bushes (which precede the lilacs by a few weeks) stop blooming and the lilac bushes begin blooming.

Types of Preemergent Herbicides

There are many different types of preemergent herbicides for killing crabgrass.

"Weed and feed" products often contain preemergent herbicides, although some question whether their concentration is strong enough to be effective. I will focus on two preemergent herbicides: Dimension and Tupersan.

Dimension (active ingredient, dithiopyr) is safe to use on most lawn grasses (check label first) and provides long-lasting coverage -- an important consideration since not all crabgrass seed germinates at once. Thus Dimension will kill later-germinating crabgrass, too. Dimension also displays some effectiveness as a post emergent herbicide.

Tupersan (active ingredient, siduron) is worth mentioning because, unlike other preemergent herbicides, it will not damage germinating lawn grass seed (in fact, its active ingredient is often combined with starter fertilizers for new lawns). This quality about Tupersan makes it convenient for those starting new lawns from seed. But what if you are using a crabgrass preventer other than Tupersan and also starting a new lawn? You have two alternatives:

  1. For newly-seeded lawns, wait until after three mowings (or three months, to be on the safe side) before applying preemergent herbicides.
  2. Or reverse the order of your projects: that is, use the preemergent herbicide first, wait three months, then sow your grass seed.

    Using Preemergent Herbicides for Killing Crabgrass: DOs and DON'Ts

    • Do:
    1. Irrigate afterward: water activates preemergent herbicides. For the same reason, a good time to undertake this project is when steady rain is forecasted for the next day.
    2. Re-apply preemergent herbicides, if you question your product's coverage. Because crabgrass seedlings do not all germinate at once, re-application can kill some of the later-germinating seedlings.
    3. Follow label directions and apply the proper rate. Measure the lawn area and calibrate your spreader carefully.
    • Don't:
    1. Dethatch or aerate the lawn after applying preemergent herbicides.
    2. Apply preemergent herbicides on new sod.

    What If My Lawn Is Full of Crabgrass in Summer?

    The instructions above are geared to readers with the forethought to inquire about prevention, which must be undertaken when seeds are about to germinate in spring.

    But what if you're reading this article in summer, after becoming aware of a crabgrass invasion in your lawn? For you, it's too late to use a crabgrass preventer.  

    Luckily, you have more than one arrow in your quiver in your attempt to kill crabgrass. For example, there are also post emergent herbicides for killing crabgrass well after it has germinated (e.g., Acclaim Extra). These are the products to look into if you're fighting this tenacious weed in summer.

    However, post emergent herbicides tend to be effective only at killing young plants. Because these young plants are small, they are very difficult to spot in a lawn. You must be vigilant in detecting them and prompt in applying the post emergent herbicide to kill them. That is why such a herbicide is far less easy to use than preemergent herbicides for killing crabgrass. I present some examples of post emergent herbicides, along with tips on how to use them, in my article on along with tips on how to use them, in my article on the best crabgrass killers.

    Organic Control: 

    "But do I really need chemicals for crabgrass control?" you ask. The answer is, No. While the best way to kill crabgrass is with preemergent herbicides, the best way to control it is by having healthy green grass. Here are some ways to promote lawn health -- at the expense of crabgrass:

    • Fertilize (compost is fine) more heavily in autumn than spring. By autumn, frosts will have already killed any crabgrass.
    • Don't let bare spots remain uncovered for long, else opportunistic crabgrass will take root. In the fall, fill in those bare spots by overseeding.
    • When irrigating the lawn, water more deeply and less frequently. Crabgrass is a notoriously shallow-rooted weed; a regimen of frequent, shallow watering plays right into its hands.
    • One of the most important tips on mowing lawns that I can offer as it relates to weed control is to "mow high." This means leaving the lawn grass at a height of 2 1/2"-3". Doing so will allow the lawn grass to "protect its own turf" better, depriving crabgrass seeds of the light they need to germinate.

      Final Notes on Crabgrass Control

      A great organic "weed and feed" product is corn gluten. An organic preemergent herbicide, corn gluten will suppress crabgrass germination, while fertilizing your lawn.

      Don't forget good old weeding as a method of crabgrass control. Hand-pulling small patches of crabgrass before it goes to seed makes eminent sense. An ounce of prevention is worth a pound of cure, and this method of prevention has the added benefit of being organic. To facilitate weeding, water the lawn first (weeds are more easily extricated from wet soil).

      If you do choose to apply an herbicidal product to your lawn, never forget that you will be handling a chemical that is potentially harmful to your health. To help protect yourself, wear a long-sleeved shirt, long pants, gloves, and safety glasses. See my full article for more on yard safety tips.

      Index to Common Lawn Weeds