FROM THE FIELD: Maximize Early Weed Control

January is a time for lawn care companies to rest and ready themselves. Spring is coming, and with it comes early mornings, late nights and frantic schedules. All along, the goal for many is to keep up with the work without falling too far behind.

With that goal in mind, the temptation to start round one applications as soon as possible is always strong. Why not take advantage of an early thaw to get those preemergent herbicides on the ground, right? Maybe. And maybe not.

TAKE THE TEMPERATURE. There are no lack of suggestions as to how to time preemergent herbicide applications. While old axioms, like watching for the end of the forsythia blooming period or following certain calendar dates, are popular, the best strategy for timing applications is monitoring the soil temperature. Weed seeds, particularly crabgrass, will not begin germinating until soil temperatures at a 2-inch depth stabilize above 55 degrees Fahrenheit for five consecutive days. 

Why wait for the temperatures to reach that germination level? Why not apply products sooner to wipe out weeds before they start growing? First, the term preemergent herbicide is actually somewhat inaccurate. These products don’t actually prevent the seed from germinating. They control the weed by inhibiting cell division in the developing plant’s young root system, which is usually long before the plant is large enough to be seen. The failure of the root system causes the entire plant to die.

Secondly, you want to maximize the length of control you enjoy from every application you make. Most preemergent herbicides last up to 100 to 120 days, although the best control lasts 60 to 90 days. The control diminishes as the product breaks down from exposure to heat or ultraviolet (sun) light. So, you obviously do not want to waste any of those valuable days by applying product too early.

Monitoring soil temperatures calls for a soil thermometer. Be sure to place your thermometer in an area with full sun since crabgrass generally only germinates in sunny areas, but avoid taking the temperature in areas near concrete or structural walls since they can artificially warm nearby soil through heat reflection.

APPLICATION SUCCESS. Regardless of the product you choose, certain practices will help ensure your preemergent herbicide application’s effectiveness.

The first step is always reading the label, following the directions and applying the product at the recommended rate. Ideally, the product should be watered in with ½-inch of irrigation within 48 hours of application. Without irrigation, the product will remain inactive on top of the soil until rain falls, which increases exposure to sunlight and reduces long-term effectiveness.

Preemergent herbicides control weeds by creating a chemical barrier across the top of the soil. Once the weed grows enough to come in contact with the barrier, the product goes to work. Anything that disrupts this barrier will hurt performance, including mowing too low, aerating, dethatching or any other form of mechanical cultivation.

WHAT WENT WRONG? The frustrating reality with preemergent products is that they are never perfect. In fact, independent research studies have shown that any product delivering 80 percent control or better is performing very well. Customers need to be advised that 100 percent weed control is unlikely.

If you get breakthrough, or weed growth within the first 60 to 90 days after making an application properly, then the odds are good that the breakthrough is in an area of thin turf because the thin turf lets more light through to the soil and improves crabgrass germination.