Turf[TECH]Report - Pre-Emergent Herbicides: The Key to Controlling Weeds Before They StartMarch 6, 2018
Spring has finally arrived! Flowers have already started to emerge, and it won’t be long before the leaves really start to fill in the trees. Naturally, with warmer temperatures comes the sound of the lawn mower — the growing season has begun! Unfortunately, grass isn’t the only thing that grows in your yard. Scattered across the soil in and below the thatch layer are countless seeds that secretly landed in your yard last fall, and they’ve been waiting all winter for this opportunity. Fortunately, there is a solution to this problem; one that kills the weeds from the onset but leaves your lawn unharmed: pre-emergent herbicides.
What is a Pre-Emergent Herbicide and How Does it Work?
Pre-emergent herbicides are designed to target newly germinating weeds before they become problematic. Pre-emergent herbicides vary slightly by chemistry and specific mode-of-action, but in general, they are designed to stop newly germinating seeds from rooting in the soil. Most pre-emergents work to disrupt root systems from developing. When roots are unable to locate water and nutrition, newly germinated seeds desiccate and starve once the endosperm (nutrition source inside the seed coat) has been depleted.
Specifically, many pre-emergent herbicides are members of the Dinitroaniline family of herbicides. These herbicides function as microtubule assembly inhibitors. Dinitroanilines are water-insoluble and do not work with foliar applications; they must be integrated into the soil. One of the more popular members of this family, Prodiamine has low volatility and relatively quick half-life in the soil (the time it takes for this molecule to break down in the environment).
When Should Pre-Emergent Herbicides be Applied?
The vast majority of pre-emergent herbicides are applied in early-mid spring (depending on latitude), right before grasses begin aggressive growth. This strategy is employed to ensure that adequate chemistry is present in the ground right before most weeds begin germinating.The later a pre-emergent is applied in the growing season, the less effective it will be against controlling a large number of weeds. Most pre-emergent herbicides have no effect on weeds that have established a root network, even if the weeds are still relatively small. The map below shows roughly when to consider applying spring pre-emergent herbicides to your lawn.
Many perennial weeds also release seeds in mid-late summer in the hopes that the seeds might germinate and root before the arrival of winter. Additionally, while pre-emergent herbicides may be successful at preventing weeds from growing in your own yard (and producing more seeds…), many seeds are transported into your yard by the wind. For these reasons, a late summer/early fall pre-emergent application should also be considered. Many landscape professionals use a pre-emergent herbicide coated onto a high-nitrogen fertilizer to avoid having to make two separate fall applications (in essence, a fall weed & feed).
What Can Pre-Emergent Herbicides be used on?
Most pre-emergent herbicides are effective against many grasses (monocots) and broadleaf weeds (dicots). Considered to be a one of the most obnoxious lawn invaders, many pre-emergents also control crabgrass, however some products require multiple applications for effective control. If you have a crabgrass infestation, be sure to read the label carefully before purchasing to ensure the product will be effective.
List of the Most Common Weeds Controlled by Most Pre-Emergents.
Annual bluegrass (Poa annua), Barnyard grass, Carpetweed-04, Chickweed (common), Chickweed (mousear), Crabgrass (large and smooth), Crowfoot grass, Cup grass (woolly), Foxtails (annual), Goosegrass, Henbit, Itchgrass, Johnsongrass (seedling), Jungle rice, Knotweed, Kochia, Lambs quarters (common), Lovegrass, Panicum (browntop, fall and Texas), Pigweed, Purslane (common), Pusley (Florida), Rescue grass, Shepherd spurse, Signal grass (Broadleaf), Speedwell (Persian), Sprangle top, Spurge (prostate), Witchgrass, Wood sorrel (yellow, from seed)
What Grasses are Safe to Use with Most Pre-Emergent Herbicides?
Bermuda grass, Bahia grass, Centipede grass, Kikuyu grass, Seashore Paspalum, St. Augustine grass, Tall Fescue (including turf-type), Fine Fescue, Zoysia grass, Buffalo grass, Kentucky Bluegrass, Perennial Ryegrass, Creeping Bentgrass (0.5 inches or more in height).
NOTE: If in doubt on your grass species, refer to the Turf Grass Climate Regions Info Graph and consult with your local lawn care expert to determine the best pre-emergent product to use on your turf grass.
What Should a Pre-Emergent Herbicide NEVER be used on?
Pre-emergent herbicides should NEVER be used on or around newly seeded grass, newly applied sod, or any plants being grown from seed (such as herbs, vegetables, and wild flowers). If you want to grow anything that is being started from seed, don’t use pre-emergent herbicides on or near those plants.
What are Examples of Pre-Emergent Herbicides available at Turf Care?
TurfCare™ Supply has a wide variety of pre-emergent control products. These herbicides can be coated onto fertilizer, soil amendments, or carriers. Such examples include:
- Echelon 4SC
- Evade 4FL