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Extension: Pre-emergent herbicides now reduce lawn weeds later
Grass

This past week, we’ve received several calls and emails from people with questions about applying pre-emergent herbicides to their lawns. What kind of pre-emergent herbicide should I use? Should I apply it now? What about fertilizer?

As always when dealing with ecosystem matters, the answer is, “It depends.” Things to consider include the species of grass, soil temperatures, your equipment, and lawn goals.


When to apply pre-emergent herbicide

A pre-emergent herbicide acts on plant seeds to halt the germination process. That’s why it’s important to apply the product to the lawn area before the season of growth begins. Simply stated, we have two seasons for annual weeds: cool season and warm season. Cool season (winter) weeds grow from late fall to early spring, and warm season (summer) weeds grow from late spring to early fall. To control both, established lawns need a pre-emergent herbicide application twice each year.

Because warm season weeds can begin germinating when soil temperatures warm to 55 degrees Fahrenheit, the window for applying a pre-emergent herbicide to prevent summer weeds is late-February to mid-March. To control winter weeds in north Georgia, apply a pre-emergent as early as late August but before the end of September. Pre-emergent herbicides continue working in the soil for several weeks.


When not to apply pre-emergent herbicide

Since pre-emergent herbicides have long residual activity that effectively halts germination of all types of seeds, you’ll need to carefully consider the timing of application — or forego it altogether — if you’re planning to reseed or over-seed your lawn. Most pre-emergent herbicide products also caution users not to apply the product to newly or recently sprigged lawns.


Which pre-emergent herbicide should I use?

As with post-emergent herbicides, the pre-emergent product you use depends on the species of lawn grass you’re growing. Certain active ingredients can damage some types of turfgrass. Refer to the Georgia Pest Management Handbook, Homeowner Edition (http://extension.uga.edu/programs-services/integrated-pest-management/publications/handbooks.html#home ) for weed control product recommendations for different types of lawns.

Also consider the type of equipment you have available to apply the herbicide, as well as the size of your lawn. Pre-emergent products come in both granular and liquid forms. The Georgia Pest Management Handbook shows the form of each product, the application rate per 1,000 square feet of lawn, and other information such as allowable frequency of application and recommended interval between applying the herbicide and reseeding the lawn. 

When using herbicides and other pesticides, always follow the directions that are printed on the product label. Application rates are legal limits that encompass the minimum amount at which the product is effective and the maximum amount at which the product is safe to use for you, your plants, and the environment. Also use the specified personal protection equipment indicated on the label of each product.


To fertilize or not to fertilize

“Weed and feed” products offer the convenience of preventing weeds and fertilizing lawns at the same time. But the combination isn’t always timely.

Lawn grasses and other plants should only be fertilized when they’re actively growing. If your lawn is tall fescue, a cool season grass, a “weed and feed” type of pre-emergent herbicide is fine to use in February-March, because tall fescue also needs fertilizer (the “feed” part) at that time. For the late summer application of pre-emergent herbicide, however, it’s still a little early to fertilize fescue.

For warm-season turfgrasses such as bermudagrass, zoysiagrass, and centipedegrass, weed-and-feed products are never a good idea. Active growth of these lawns begins in April or May, well after the fertilizer has left the root zone. In late summer, warm-season grasses need to prepare for winter dormancy; applying fertilizer at that time disrupts that process, and it can set up conditions for Spring Dead Spot in the following year.

You can find lawn care calendars for the different types of lawn grasses on the Forsyth County Extension website at http://extension.uga.edu/county-offices/forsyth/agriculture-and-natural-resources.html.


Heather N. Kolich is the Agriculture and Natural Resources Agent for the UGA Extension Forsyth County.