By Markis Hill

As soil temperatures rise in the coming weeks, our flowerbeds and lawns will begin to green again –an exciting sight to behold after a long, dreary winter. But upon closer look, it may not actually be fescue or bluegrass seen growing in your lawn, nor crocus in your flower beds. More likely, what you’re seeing is cool season weed emergence. Enter the usefulness of pre-emergent herbicides.

When pre-emergents are applied, they create a chemical layer in the soil that inhibits or damages cell growth. These chemicals do not kill the weed seed itself. Instead, the weeds are impacted when germination occurs, and as those seedlings grow through that chemical layer, they are affected. For this type of treatment to be effective, how - ever, you must get the timing right. And, as their name suggests, they must be applied before the weeds start growing.

Pre-emergent herbicides control broadleaf weeds like dandelions and henbit; grassy weeds like cheatgrass and crabgrass; and even woody weeds like honeysuckle or lacebark elm. Targeted selection of certain weeds is crucial for choosing the most effective product.

Many pre-emergents target specific weeds, while others are broad-spectrum. Application context matters. For instance, grassy pre-emergents shouldn’t be used on reseeded lawns, but broadleaf-specific ones might work fine for the scenario. This is why always reading labels carefully before purchasing is crucial.

Many pre-emergents on the market are highly effective at controlling one specific type of weed while others can target several different kinds. Understanding the context of the application is equally important. For example, you wouldn’t want to apply a pre-emergent designed for grasses on a recently reseeded lawn. On the other hand, applying broadleaf-targeting pre-emergent in that same situation is a more appropriate choice. That’s why it’s essential to carefully read the label before purchasing and remember that intentionally seeded areas should be avoided.

Pre-emergent herbicides can break down quickly in sunlight, so they must be “worked in” by lightly raking or watering to ensure they seep into the soil. Once in the soil, they are broken down by microbes over a longer “residual period.” Avoid working the chemical too deeply to prevent it from affecting roots of desired plants.

Although the name implies it won’t target well-established plants, pre-emergents can still do noticeable harm. A list of non-target plants that have a chance of being affected can be found on the label. If you find some of your trees, shrubs or perennials on that list, you should find a different product. Effects can range from yellowing of leaves to damage to growing tissue.

Also note, pre-emergents come as dry granules or concentrated liquids that need to be diluted. In either form, your most important task is to read the label front to back, then following application rates. Under-applying will lower overall effectiveness, while over-applying can harm your property, or even cause injury. For this reason, proper personal protective equipment should always be worn when handling these chemicals. The top two exposure routes of pre-emergents are breathing them in and touching them.

Directions followed, pre-emergent herbicides can significantly reduce the amount of chemicals applied to your landscape, while also reducing the weeds needing to be tackled through the growing season.

Markis Hill is horticulture agent at the Johnson County K-State Research and Extension Office.