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PRICE: Prevent summer weeds on warm season turfgrass

Feb 6, 2021

If your lawn is not in the greatest shape and there are thin spots, weeds will be a problem. Even well-kept lawns will have some weeds. Preemergence herbicides should be one of your main defenses to prevent weeds.

Most lawn weeds fall into two categories, annuals or perennials. Annual weeds complete their life cycle in less than one year and only reproduce by seeds. Perennial weeds are persistent in the winter and summer and live more than two years. They reproduce vegetatively and by producing seeds. These are the toughest weeds.

A thick healthy lawn is the best defense against unwanted weeds. There are four main types of warm season turfgrasses used in our area, and they all have different requirements for proper growth. 

The most common is centipedegrass followed by St. Augustinegrass. Bermudagrass and Zoysiagrasses are also used. Knowing what type of turfgrass you have and how to manage them is the key to preventing weeds.

Annual weeds are further divided into summer annuals and winter annuals. Many summer annuals germinate in early March and complete their life cycle by fall. Most winter annuals germinate in late September and complete their life cycle before May.

Now is the time to apply a pre-emergence herbicide to prevent summer annuals from germinating. It is better to put them out too early than too late. If you don't apply these herbicides before weeds germinate, pre-emergence herbicides will probably not be effective. Also some pre-emergent herbicides should not be applied to lawns while they are greening up.

Atrazine is a commonly used pre-emergence herbicide in Centipedegrass, St. Augustinegrass, and Zoysiagrass. If properly used, Atrazine will help prevent many broadleaf and some annual grass weeds. Atrazine can also be used on dormant Bermudagrass.

Atrazine is different from most pre-emergence herbicides because it has activity on weeds that have already germinated. Atrazine should not be applied during green up or around the root zones of ornamental plants.

If you have problems with grass-like weeds such as Crabgrass, pre-emergence herbicides containing Benefin, Oryzalin or Pendimethalin are better choices. Products containing these chemicals are safe on Centipedegrass, St. Augustinegrass, Zoysiagrass and Bermudagrass.

Oryzalin is commonly sold as "Surflan" and is safe to use near ornamental trees and shrubs. Benefin is commonly sold as "Balan" which is a granular. Pendimethalin is sold as "Halts." All three are excellent for pre-emergence of Crabgrass.

Some pre-emergent herbicides are attached to a fertilizer granule. Make sure you use these products correctly. For instance, if you have Centipedegrass, it should not be fertilized before April and by then most summer weeds will have already emerged. The herbicide may not be effective when applied this late.

If you fertilize Centipedegrass too early when soil temperatures are below 65 degrees, you can stress the grass and cause iron deficiencies. Fertilizing Centipedegrass too early can lead to a condition called "Centipede Decline" which can cause the turf to die.

Since there are no post-emergence controls for Crabgrass on St. Augustinegrass, using one of the above pre-emergence herbicides is essential for control of Crabgrass. There are fewer post emergence herbicides that can be used on St. Augustinegrass so a pre-emergence herbicide is a good weed management practice.

Some weeds will emerge even with use of pre-emergence herbicides. Spot spraying these weeds with the appropriate herbicide when they are young will give best control.

After May, when temperatures climb into the 90s, weed control is not as effective.

Jake Price is the University of Georgia extension agent/coordinator, Lowndes County. More information: Call (229) 333-5185, or email [email protected].