Killing Crabgrass in Summer -
Making Sure It Never Comes Back

Dave Asked: I really need help killing crabgrass. For ten years I had the best fescue lawn in Atlanta, then came our drought. This year we have had a lot of rain and my lawn is FULL of crab grass despite having applied a pre-emergence in February. I have little fescue left. I have a limited budget but want to heavily overseed, but wish not to start from scratch. I will aerate before overseeding, however I'm wondering if the lawn should be slightly plowed for more surface for the seed. Any advice will be deeply appreciated.

(See Answer Below)

Answer: Don't worry, your problem is easily fixable. While killing crabgrass after it has started growing is difficult, it can be done if that is what you decide to do.

The good thing is that crabgrass is an annual plant and dies at the end of each year in the fall. It slows down as temperatures cool and is killed at the first heavy frost. If you can wait, getting rid of crabgrass will be done for you. Crabgrass must start from seed the following year. In your part of the country that would be about mid to late March.

This is important to know: Something fertilizer companies never tell anyone is that frequent, heavy rains will stress preemergence herbicides to the max. Several rains or frequent irrigation won't hurt, but repeated heavy downpours will and decreases the effectiveness of preemergent herbicides. Since they advertise "crabgrass control", they don't want it out that it has several limitations.

Killing Grabgrass with Post-Emergence Herbicides

The best way of killing crabgrass after it has spread is to purchase a product with MSMA as the active ingredient. It is safe for many turfgrasses when used as directed, but is effective in killing crabgrass. You will have to use it soon, before the temperatures dip and the grass hardens off. It is currently prohibited in about 27 states, but is still available for use in GA at the time of this writing. Follow the label directions for mixing and use.

Use a pump chemical sprayer or backpack sprayer to apply the chemical. To eliminate crabgrass, spray your grass so it has a thin film over the grass. Wear rubber boots if you have them. This is the easiest method of killing crabgrass that is growing. It may need a second application. You can apply a "sticker-spreader" into the herbicide solution to help the chemical stick to the grass better. It can be purchased online or other products are also available.

Note: MSMA will harm some desirable grasses and plants, even though most turf species are safe. Read the label carefully to see if you have any varieties that can be harmed and that you have the correct mixing rate for your species.

Facts on Pre-emergent Herbicides

An important thing to know is there are only two types of preemergence herbicides that will last all year with a singe application in early spring. The trade names are "Barricade" and "Dimension". These are most often sold in professional fertilizers, but may be available to homeowners when purchased from turf supply companies. Most homeowner types of preemergent herbicides, like the type used in Scotts fertilizer for example, are hard pressed to last all year. They are good for three to five months depending on the type used. In southern states where the summers are longer, they will often fail too soon and some crabgrass will get through.

Two remedies for the preemergent problem:

  • Apply the first application of fertilizer with preemergence in early spring BEFORE THE CRABGRASS SEEDS GERMINATE. The herbicide won't work if applied after the crabgrass starts growing. Then apply a second application of fertilizer and preemergence again a few months later.

    Your summer application should have a lower nitrogen content than the first one, so you don't apply too much nitrogen when the grass is slowing down because of summer heat. So this means you will have to buy two different bags: one for early spring and a different one for late early summer.

  • The second solution is to have a commercial turf company apply the first application for you using one of the above listed preemergence herbicides guaranteed to last all year.

Now for Overseeding

After killing crabgrass, if it was very thick, it may need to be thinned before seeding. I use an inexpensive non-motorized, pull-behind dethacher. It connects to the back of a riding mower. The dethatcher is about 36 inches wide with tines that drag over the soil and has a steel plate on top for weights. For added weight I use sand bags made from an old car tire tube. (Over the years I have learned that cheap weights work just as well or better as expensive ones) Cut a section of the rubber tube 18 inches long and wire one end closed. Fill the tube with sand or gravel and then wire the other end closed. It can be quite heavy. Two of them will be needed. If they bounce off then tie them on with heavy string or cord. The dethatcher costs about a hundred dollars or so and is sold in many large lawn and garden stores.

In early fall, when the high temperatures are in the low to mid eighties, pull the dethatcher over the lawn several times in different directions. It will pull up a lot of old grass and make a big mess. The purpose is to remove as much dead crabgrass as possible. Remove the debris before spreading seed.

Buy a quality "turf-type" tall fescue seed. Try to purchase seed from a good source, such as a landscaper or turf supply company. They will have the best seed used by commercial companies. Don't buy budget seed or you will probably not be happy with it.

You can apply a starter fertilizer in the fall at the time of overseeding or up to a few weeks after seeding. However, DO NOT apply a preemergent in the summer or fall if you plan to overseed that year or it will keep you seed from germinating. Use proper watering techniques and you should be fine.

I hope this helps. Remember, killing crabgrass can be challenging after it has started growing. "Killing Crabgrass" is not necessary if you use a good preemergent in early spring. Good luck and let me know how it goes.

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