When to Apply Crabgrass Preventer [Pre-emergent Timing]

Crabgrass-plant

One of the first steps in any lawn care plan is applying a crabgrass preventer. Crabgrass is a very common lawn weed. In fact, it’s one of the most invasive lawn weeds, too. And without a pre emergent herbicide and prevention strategy, this grassy annual weed can take over an entire lawn in no time. We’ll discuss the best crabgrass control program for your cool-season lawn and answer the question of “when to apply a crabgrass preventer?”.

When Should I Apply Crabgrass Preventer?

Apply your first round of crabgrass preventer (also called pre-emergent herbicide) in the early spring before the soil temperature reaches 55 degrees Fahrenheit (12 Celcius) to control crabgrass, henbit, chickweed, poa annua, and other common lawn weeds. For best results, apply a second application of pre-emergent in the early summer – or about 8 weeks later – when soil temps hit 65-70 degrees Fahrenheit.

Be sure to read the labels of each herbicide as there will be a maximum annual amount that you can apply in a calendar year. For example, the maximum application rates per year for 0-0-7 with 0.37% Prodiamine Herbicide is between 4.0 – 9.3 pounds per 1,000 square feet, depending on the grass type.

Crabgrass Preventer Schedule for Cool-Season Lawns

CRABGRASS PREVENTERAMOUNT PER 1,000 sq. ft.TIMINGSOIL TEMPS
0-0-7 with 0.37% Prodiamine5 lbs.Early Spring 50-55
21-0-10 with 0.25% Dimension4 lbs.Early Summer65-70
For most cool-season lawns with KGB, PRG, TTTF

The table above is the same program I use on my personal yard and the lawns of my clients.

  • Early spring (early April) is too early to apply Nitrogen, especially if you spread a winter fertilizer the year prior – apply Prodiamine 0-0-7
  • Dimension contains some post-emergent qualites and can act as a post-emergent crabgrass killer, particularly on younger crabgrass in the first 1-2 tiller stages
  • A combo fertilizer (21% nitrogen + pre-emergent) is perfect timing to feed your lawn
  • The 10% Potassium can help with the stresses of the hotter summer months ahead

Crabgrass Preventer Schedule for Cool-Season Lawns (Not Overseeding – Poa Annua control)

CABGRASS PREVENTERAMOUNT PER 1,000 sq. ft.TIMINGSOIL TEMPS
0-0-7 with 0.37% Prodiamine4 lbs.Early Spring 50-55
21-0-10 with 0.25% Dimension4 lbs.Early Summer65-70
0-0-7 with 0.37% Prodiamine5 lbs.Late Summer70-75
For cool-season lawns, not overseeding – for poa annua control (annual bluegrass)

The table above is a good crabgrass preventer schedule for those looking to control Poa Annua (annual bluegrass) and not fall overseeding. These crabgrass pre-emergent herbicides will affect not only your targeted weeds, but also the seed germination of your good grass. So you cannot use any of these weed preventers while seeding or overseeding.

Another option in lieu of Prodimaine in late summer (late August/early September) is Pendulum 2G at 0.5 lb. per 1,000 square feet.

Soil Temperatures

The key to stopping crabgrass seeds from emerging is timing the soil temperatures. The temperature of the soil – not outside air temps) dictate when the crabgrass seeds germinate.

The first soil temperature to check is when the ground approaches 55 degrees Fahrenheit. The second is a soil temp range of 65-70 degrees Fahrenheit. These ground temperature ranges can span over the course of 3-4 months.

A crabgrass seed can be in the soil near a sidewalk or driveway where the ground heats up much quicker, and the seed germinates much sooner. Whereas another crabgrass seed might be in the middle of a shaded area, and not warm up and germinate until much later. This is why we always use a split application of crabgrass preventer to control crabgrass.

Crabgrass Preventers (How Pre-Emergents Work)

The most common and effective crabgrass pre-emergent herbicides are Prodiamine (Barricade), Dithiopyr (Dimension), and Pendimethalin (Halts). For this article, we are going to continue to focus on the most popular Group 3 herbicides: Prodiamine and Dithiopyr.

Both crabgrass preventers are considered Root & Shoot Inhibitors. A vapor barrier is formed in the soil layer that prevents the weeds from emerging while inhibiting the root system from developing. The undeveloped roots cannot take up water or nutrients, and thus die off.

Note these pre-emergent herbicides do not prevent the crabgrass seeds from germinating, but rather inhibits the plant’s cell division at the root zone, and prevents the crabgrass seedlings from growing and emerging. They are coined as root pruning herbicides or root clubbers.

Granular products are spread over the lawn using a fertilizer spreader, and the liquid versions of these pre-emergents (or Wettable Dispersible Granule) are sprayed evenly over the turf with a backpack sprayer or two-behind sprayer.

Prodiamine vs. Dithipoyr

  • Prodiamine is generally less expensive than Dithiopyrn (Dimension)
  • Prodimaine is longer lasting – can withstand excess irrigation and rainfall
  • Prodiamine is better applied early in the season – Dimension later in the season

Another popular AI is Mesotrione. The most well-known Mesotrione product is Tenacity herbicide, and the generic Meso 4SC. Scott’s Starter Fertilizer with weed preventer also contains Mesotrione. If you are seeding in the spring, apply one of these pre-emergent herbicide products as it’s the only chemical that can be used as a pre-emergent without affecting the germination of your good grass. It lasts about 30 days.

If you are not seeding, stick to the previous three options above as you will get much longer efficacy and coverage.

Crabgrass Lifecycle

Crabgrass seeds start to germinate in the early spring as soil temperatures reach 55 degrees Fahrenheit for 2-3 consecutive days. However as we previously mentioned, the germination window can span from early spring and into the early summer months when soil temperatures are optimal.

The crabgrass does not grow in full shade, but rather thrives in the sun and hot summer months. This grassy weed likes open areas – so any bare spots in your lawn will soon be occupied by crabgrass.

Crabgrass sheds thousands of seeds, and without prevention, multiplies rapidly each year. It can quickly outgrow and choke out your good grass – robbing it of sun, water, and nutrients – and completely takeover your lawn.

At the end of the season, the weed naturally dies off while the crabgrass seeds remain in the soil over winter, only to begin the germination and growing cycle again the following spring and summer.

That is why it is very important to apply a crabgrass pre-emergent before the seeds germinate. It’s best to prevent crabgrass before these pesky weeds mature. For post-emergent crabgrass killers, read how to get rid of crabgrass and the best crabgrass killers.

Crabgrass Identification

Crabgrass (large crabgrass and smooth crabgrass) is a grassy weed and annual weed. These are different from broadleaf weeds, and biennial and perennial weeds Younger tiller stages of crabgrass will be a lighter green and turn darker as it gets more mature. Some crabgrass species can also take on a purple color.

The crabgrass grass blades have a pointed spade tip and can have white fuzz. As it matures, it will begin to go to seed. As the seed heads grow, there will be four seed heads in a bunch. – like a fan.

Crabgrass can be mistaken for other grasses and weeds, particularly if you have a patchy lawn – clumping tall fescue, creeping bentgrass, poa trivialis, etc.

Crabgrass grows from a central vine and branches outward from the center. It grows low and parallel to the ground in a star-like shape, and begins to take on the appearance of crab legs as it matures (hence the name, “crabgrass”).

Crabgrass Pictures

Crabgrass Preventer FAQs

Should I water after applying a crabgrass preventer?

Yes, both prodiamine and dithiopyr need to be watered in and moved into the soil to activate and form the vapor barrier.

Can I apply crabgrass preventer twice?

Yes, a split application of crabgrass preventer is the best strategy. Apply crabgrass pre emergent (prodiamine) in the early spring, and another round (dithiopyr) in the early summer for season-long coverage.

Can you apply too much crabgrass preventer?

Yes, there are maximum annual rates on these Group 3 herbicides. Crabgrass preventers are root pruning and inhibit root growth, and over-applying can affect the roots or your good grass.

Can you apply Scott’s crabgrass preventer on wet grass?

It’s best to apply to dry grass blades. However, if the turf is wet or damp, be sure to water the product in immediately after spreading.

Should I apply crabgrass preventer before or after rain?

You can apply crabgrass preventer before the rain so the product is watered into the soil and activated. However, excessive rainfall or torrential downpours could wash the herbicide away. Also see: Fertilize before or after rain?

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