Why is crabgrass preemergent timing so important?

Every year we receive phone calls late into spring of hopeful homeowners asking if there is still time to apply the crabgrass preemergent. Sometimes, we are well past the appropriate timeframe with to successfully apply a preemergent.

While we do change our products to meet the growing conditions, sometimes it is just too late. Below are some basic facts about crabgrass preemergent applications.

Timing

For most successful lawn care companies, we usually begin our first application well before when you will be thinking about mowing your lawn. While the turf may be slowly coming out of dormancy in the early part of March, we are already well into applying a heavy rate of crabgrass preemergent to our lawns. Timing is crucial to the success of your preemergent application but there are also some other factors that can play into the long term success of a treatment.

There are several kinds of products on the market and most of the time, professionals choose a low nitrogen product that is 100% slow release. This allows us to make a very high rate application of crabgrass preemergent while not loading the lawn down with nitrogen. The slow release component ensures that the nutrients that we apply will be available over 6-8 weeks as the lawn moves out of dormancy.

When applied properly, a high quality crabgrass preemergent can last in the soil for 180 days. For those keeping score at home, If we apply the first application on February 26th, that means the active ingredient should still be present in the soil into August. Throughout the early months of the application season, we are closely watching the average soil temperature to a depth of 2 inches. Once soil temperatures near 55 degrees, we make a change in active ingredient selection to target crabgrass as a preemergent but also with a little post emergent activity to catch any very small crabgrass plants that might be growing.

How does it work?

Not all crabgrass preemergent is the same so before we make a selection of products, we need to understand the chemistry of what we choose. For our company we select a product and apply it at the highest rate allowed to maximum coverage and efficiency. This preemergent is adhered to a fertilizer prill. Once the application has been applied, moisture from the soil and eventual rainfall help to work the active ingredient down into the soil to create a chemical barrier. As crabgrass seeds in soil have an opportunity to germinate, the chemical barrier prevents root development and stops germination.

Different things can affect the success of a crabgrass preemergent application. The most common area we see crabgrass breakthrough is along edges and high traffic areas. Heat from sidewalks, roads, and buildings can degrade the chemical barrier but also if these areas are trimmed using a string trimmer, the chemical barrier will be broken and allow for crabgrass to breakthrough.

Thin areas and parts of the lawn that are bare also tend to have crabgrass breakthrough. These areas provide less competition for weeds and are more susceptible to chemical breakdown.

How to get the most out of a preemergent.

One of the simplest things to do for your lawn is ensure you are mowing at no shorter than 3.5 inches. Tall turf gives your lawn a healthy root structure and an overall healthier plant. Taller turf also helps to shade the soil which in turn protects the chemical barrier from heat degradation. If you think about it, when have you been walking through the woods and seen a healthy crop of crabgrass? Crabgrass hates shade and tall turf provides enough shade to help prevent crabgrass with a good preemergent.

Reseed in the late summer to fill in the lawn a grass seed blend that is strong for your area, soil conditions, and area use. Take into account if it is a shady or sunny area when making your seed selection as well. Not all turf grass seed blends are equal but that will be discussed in a different post.