Guide To Weed Control For Warm Season Grasses

If you live in the South, then you are probably more than familiar with the climate. The summers are long and scorching hot as well as humid, while the winters are rather mild and much shorter than the rest of the country.

Warm-season grasses are grass types that are more tropical and actually thrive in the summer when the temperatures are high (between 80 to 90⁰F). Some of the traits of warm-season grasses are the thick and dense lawn cover which remain green all summer long and will begin to brown in the wintertime. The grass greens up again when it begins to get warmer. In general, they are green a little over half the year.

The most common of warm-season southern grasses are Centipede grass, St. Augustine, Bermuda grass and Zoysia grass. Each of these grasses has its own traits and maintenance needs so they can stay healthy and bright. But like most lawns, undesirable weeds can creep up and ruin the uniform look of your turf.

In this guide, we will look at how to manage weeds on warm-season grasses using proper cultural practices and the help of herbicides which will keep your warm-season grasses healthy and weed-free all year long.

Identification

The first step in tackling invasive weed species is proper identification. Since mowing the lawn removes flowers and seedheads, it can often be tough to identify the weeds that you have unless you let them grow and get a good idea of what weed is present. Below we have compiled a list of some of the more common weeds that show up on warm-season grass.

Chamberbitter

Also known as gripe weed, Chamberbitter is a small summer annual that was once a sought after ornamental. Since then it has been an invading weed that has gotten out of control on lawns. Chamberbitter is heavily branched at the base with ascending stems, often forming a mound 1 to 2 feet in height.

Doveweed

Doveweed is a summer annual that grows best in moist sunny areas and first germinates in late Spring. Doveweed looks similar to St. Augustine or centipede grass. They have linear leaves and can be up to four inches long with soft hairs on the sheaths and a rubbery texture. Doveweed grows clusters of pale purple flowers throughout the summer and its stems root at the nodes allowing it to spread by stem fragments.

Sandspur

Also known as burweed, Sandspur is a low-growing winter-annual broadleaf weed that has narrowly separated leaves and small noticeable flowers. Sandspur is perhaps most known for its small spiky stickers that can prick skin when you walk with bare feet over it.

Dollarweed

Dollarweed is a moisture loving perennial warm-season weed that is recognized by its round, lily pad-like leaves and five-petaled flowers that resemble stars.

Virginia Buttonweed

Virginia buttonweed is known for forming thick mats that spread and are deeply rooted in the soil. Virginia Buttonweed has branched stems that are covered in little hairs and grow white four-petaled flowers that are also hairy. Virginia Buttonweed can spread and generate in a variety of ways, making it particularly frustrating to deal with.

Wild Garlic/Onion

Wild Garlic and Wild Onion are perennial weeds that are similar to one another in that they have hollow, stem-like leaves and get their name from the bulbs they form resembling onions and giving off a strong garlicky smell.

Inspection

It is important to do a thorough inspection on your lawn when you notice weeds and invasive plants emerging on your lawn. An inspection will help to determine what weed you have on your property so you know where they are concentrated, what conditions may be helping the weed to thrive and what herbicides may best be able to control the weed.

Where to Inspect

Scan your lawn and look for the weeds we described in the identification section. The best time to inspect your lawn for weeds is in the early fall (October or November) for winter weed control and then following up again with an inspection in early spring (March or April). Inspecting in the early part of fall is best because theres a chance of early detection and subsequent treatment since weeds are most vulnerable to herbicide treatment when they are young.

What to Look for

Closely observe the traits of the invading weeds and determine what species it is. You should also look at the surrounding environment for anything that can tipoff why the weed is thriving. Is the soil compacted? Are their drainage issues? Is your grass undergoing a lot of stress? Finding these answers can determine how you approach control.

Treatment

Once you have pinpointed the weeds on your warm-season lawn and properly identified them, the next step would be to control them through the use of herbicides. It should be noted that there are two types of herbicides, post-emergent and pre-emergent.

Pre-emergent herbicides are a preventative measure and are used when you know about a particular weed that grows at a certain time every year and you lay out a pre-emergent to make it so the weed doesn't germinate.

Post-emergence herbicides are weed killers that kill weeds that have already sprung up and are visible. Young weeds which are in the two to four-leaf stage and those weeds which are actively growing are the most vulnerable to herbicide chemicals and require the least amount of herbicide to successfully control them.

The timing of application is essential for best results. The reason for this is because during this early stage, herbicide uptake and translocation takes place more easily and weeds have weaker root systems to provide much resistance. If you wait until plants have grown more mature, there will be a significant dip in the herbicide translocation, making it tougher to control mature weeds and raising the chances of injuring your grass if higher rates are needed to achieve weed control.

Thus, post-emergence herbicides should only be used when weeds are in active growth. This primarily occurs when temperatures are between 40 and 80 degrees and good soil moisture is available. Applying herbicides outside of this temperature range or when soil is on the dry side will result in the herbicide performing slowly and can lead to turf damage from using excessive herbicide.

Pre-Emergents For Warm Season Weed Control

Our top recommendation for pre-emergent control of most warm-season weeds is Prodiamine WDG. Prodiamine works by stunting the development of seeds at the beginning of the germination phase. It is labeled for most grasses but it may be harmful to Annual Bluegrass so if that is a desired grass on your lawn, use with caution.

Determine how much Prodiamine you will need by calculating the square footage of your yard (length x width = square footage). The maximum application rate can range anywhere from 0.185 oz. to 0.83 oz. per 1,000 sq. ft. per calendar year. Mix the appropriate measured amount with water based on your calculations and apply over your entire lawn uniformly.

Pre-emergent herbicides should be applied in the fall as most warm-season turfs go dormant when winter arrives and temperatures drop. An application of pre-emergent will protect the turf from weeds that may appear in the winter.

Post-Emergents For Warm Season Weed Control

Our top recommendation to deal with post-emergence broadleaf and grassy weed control in warm-season turfgrasses is 2,4-D Selective weed killer. This is a selective herbicide meaning it will only harm target weeds and spare desired warm-season grasses.

For small applications with a hand sprayer, mix between 0.72 to 1.1 fl. oz. in a gallon of water to treat 1,000 square feet. Fill your sprayer halfway with water, add the appropriate measured amount of 2,4-D and then fill the rest of the way with water. Apply the 2,4-D mixture to areas where unwanted vegetation is currently growing, preferably on a fan spray setting to ensure even coverage.

Once again, control depends upon how mature the weed you are targeting is. Younger weeds are easiest and the cheapest to control. With this being the case, to tackle winter annuals, applications should be started in November to take advantage of these younger, more succulent plants.

Waiting until March or April to attempt control may result in the need to do multiple follow up applications every 10 days or two weeks, resulting in a lot of herbicide being lost, which can be costly. Applying herbicides late also may affect your warm-season grass, hindering its ability to green-up and it may work slower to get the results you seek. For summer-annual weeds, mid to late-spring applications are recommended.

Prevention

To successfully prevent future weeds on your warm-season lawn, it will require a combination of good cultural practices and choosing the right herbicides necessary to create a thick healthy turf that will tackle the weeds and give your warm-season grass the advantage in keeping the weeds from thriving.

Dethatch your lawn in May when your grass is actively growing by raking it regularly. This promotes a healthy lawn by removing dead roots and stolons that may be stuck between the soil surface and your living turf, encouraging better grass growth.

Trim back tree branches to reduce shade and promote proper water retention in your soil. Give your grass a watering of 1 to 1.5 inches of water a week and mow your grass at a taller height of 3 to 4 inches to encourage root growth.

Finally, make sure you fertilize your lawn with the proper amount of Nitrogen it needs so it is equipped with the nutrition necessary to fight against invasive weed threats.

Key Takeaways

• Warm-season grasses like St. Augustine and Zoysiagrass thrive in warmer climates in the Southern United States. Common invasive weeds include like Dollarweed, Spurge, Clover and Sandspurs.
• We recommend Prodiamine 65 WDG for pre-emergent control of most warm-season weeds and 2,4-D Amine Selective Weed Killer for post-emergent control.
• Prevent future weed problems by implementing lawn maintenance practices like regular mowing, watering and dethatching that promote a thick, dense turf. Thick turf will minimize the opportunity for weeds to thrive in your yard.