When Should I Apply Crabgrass Preventer in Spring?

Timing Is Everything When Using a Pre-Emergent Herbicide

Picture: crabgrass plant before it goes to seed. Preventing seed is the best strategy.
Picture: this crabgrass plant has not yet flowered and produced seed. David Beaulieu

If you are a homeowner with a lawn to tend to, at some point you have probably wondered, "When should you apply crabgrass preventers in spring?"

This common lawn weed generally appears in late May in the North (earlier in the South), so earlier in the spring is the time for applying crabgrass killers -- if you want to get the jump on your nemesis. You will want to use pre-emergent herbicides, specifically.

These products are also termed "crabgrass preventers," because they do just that (if used properly). Corn gluten is considered an organic alternative to using chemical pre-emergent herbicides.

But since getting the timing right is so critical to success in controlling this weed, we will have to be more precise about when to apply a crabgrass preventer.

Exactly When Do You Put Down a Crabgrass Preventer?

May 1 is the average time for application of crabgrass preventers in the northern U.S. and Canada, but the ideal time will vary depending on where you live and how warm a spring you are having. In the South, you will need to apply a crabgrass preventer earlier than in the North. This is still not precise enough, though, so below I will give you two different guidelines to use so that your timing will be just right. But first, let me explain how the product works in greater detail.

"Pre-emergent herbicides" are not called that for nothing.

The idea behind them is that they nip the weed in the bud, so to speak. Actually, they "nip" it even before that, preventing the plants from germinating from seed in the first place. So their name says it all: they help you control crabgrass before ("pre-") it even emerges out of the earth.

Sounds like magic, does it not?

Well, not quite. You still have to pitch in by applying these crabgrass preventers at the right time -- not too early (because their potency would peter out), but not too late (because once the weed seed germinates, "that ship has already sailed," as they say).

So let's get right down to brass tacks. There are two sure-fire ways to determine when to apply crabgrass preventers:

  1. An old rule of thumb is to put down the crabgrass killer shortly after the flowering period of the forsythia bushes, on the one hand, and shortly before that of the lilac bushes, on the other.
  2. But here is an even better guideline to use. Armed with a soil thermometer, take a reading of the soil temperature a couple of inches down into the ground where your grass is growing. The best time to apply a crabgrass preventer will be when the soil temperature reaches 55-60 degrees Fahrenheit and holds that reading for a few days in a row.

Buy a soil thermometer from Amazon.com. Here is how to use a soil thermometer correctly to take this reading (you will be using a screwdriver first to make a pilot hole):

  1. Run a tape measure up two inches from the tip of a screwdriver toward the handle.
  2. Mark this measurement with a magic marker (or use duct tape). 
  1. Plunge the screwdriver into the ground until this two-inch mark is at ground level. This gives you your pilot hole, so that you do not risk breaking your soil thermometer.
  2. Now insert the soil thermometer into the ground, guided by your pilot hole, to get your reading (follow instructions on the label regarding how long you have to keep it in the soil before withdrawing it).

But what if the right time for using a pre-emergent passes and you end up with the weed in your lawn during the summer? One option is to use a post-emergent crabgrass killer. Make sure you know what you are fighting, though. To that end, my pictures show you exactly what crabgrass looks like.