Weed control is never ending in Florida landscapes. If there is a bare area of soil, weeds take advantage of that void and make themselves at home. Ideally, we would use good cultural practices that minimize weed invasion, but if prevention is ineffective and you need to use an herbicide, make sure you are using that tool properly.
When I talk to homeowners and commercial applicators about weed control, many times they have selected the appropriate product but are still struggling with management. While troubleshooting the problem, many times I discover, the reason is related to poor uptake because the client is not making herbicide applications to what I like to call “happy weeds.”
When using an herbicide, you need to think about how it works. Not the deep level chemical reactions, but rather consider how the active ingredient is going to be delivered to whatever physiological function of the plant it is targeting.
Does the herbicide need to be absorbed by the leaves/stems/roots?
Once absorbed, does it need to travel through the vascular system of the plant (translocation)?
What effect will temperature, moisture, mowing/trimming have on product uptake?
All these factors are very important because if a plant is stressed, the primary response is survival. During excessively hot weather plants close off stomata, form waxier surfaces, and do everything they can to retain moisture – which also includes reduced absorption of herbicides. In cool weather, the plant may be dormant or have slowed growth that will reduce translocation.
Mowing and trimming reduces leaf surface area minimizing uptake. Overcoming injury also triggers the plant to go into water conservation mode which also limits product uptake.
So, what do I mean by “happy weeds”? For herbicides to be effective, they should be applied when growing conditions are ideal for your target plant. Optimum soil moisture, soil temperature, ambient temperature, and minimal stress lead to “happy weeds” that are primed to accept herbicides and translocate if needed. Plants in survival mode will have their defenses activated and this decreases herbicide efficacy.
Always read and follow entire label instructions of all pesticides.
It’s the most asked question of Extension agents everywhere. “What is this weed and what herbicide should I buy to kill it?” The first part of that question is straightforward. Between personal field experience, formal plant identification training, and a team of weed science specialists to call on, Florida Extension Agents can get your weed accurately identified. The second part of the question is slightly more nuanced. Many weed problems can be avoided by following lawn and landscape Integrated Pest Management (IPM) practices, like not overwatering, mowing at the correct height, using mulch where you can, etc., but sometimes herbicides are required. For these situations, if you follow the “homeowners only need four post-emergent herbicides rule”, you’ll be ready for any weed challenge you face!*
Doveweed seedlings just emerged on July 9, 2021. Photo courtesy of Daniel Leonard.
The first weed scenario you should be prepared for is spot spraying weeds in landscape beds and around hardscapes like concrete pads, sidewalks, pools, driveways, etc. For this situation, you’ll turn to the backbone of the four herbicides rule and a staple in any weed management program, glyphosate. Glyphosate is the active ingredient (AI) in many non-selective products ranging from the infamous Roundup brand to cheaper generics like Killzall and Cornerstone. Glyphostate is extremely cost-efficient and very effective on a huge range of emerged weeds including grassy weeds, broadleaf weeds, and even sedges (also called nutsedge or “nutgrass”). Just be sure not to get any overspray on desirable plants or they’ll be seriously damaged!
The second situation we face occurs when “bad” grassy weeds invade landscape beds, shrubs, and vegetable gardens. These cases call for a grass-selective herbicide that you can spray right over the top of your broadleaf annuals, perennials, and shrubs to take out the unwanted grasses. In this case, there are two options at your disposal: sethoxydim (AI in many products like Fertilome Over the Top, Hi-Yield Grass Killer, Poast, etc.) and fluazifop (AI in the product Fusilade). Both products work well in removing weeds like crabgrass, bermudagrass, goosegrass, and others and can be safely applied over the top of many ornamentals.
Seeds from annuals like Chamberbitter easily get into mulch and turfgrass from surrounding areas. Photo by Beth Bolles, UF Extension Escambia County
From time to time, we need to treat a broadleaf weed like dollarweed, doveweed, chamberbitter, or any number of others, that have invaded our lawn grass. Some of these weeds are tougher than others, but almost all of them can be managed with the proper rates of 2,4-D, Dicamba, other similar products, or a combination of several of them. These active ingredients have been on the market for decades so there are many generic options at your disposal. However, if you have a truly tough broadleaf weed problem, the newer product Celsius WG from Bayer (a combo of Dicamba and two newer AIs), while a little pricey and only available online or at specialty chemical dealers, is well worth the expense and knocks out the most gnarly of lawn weeds.
Finally, there is a category of weeds that aren’t broadleaves and aren’t quite grasses either. These are the sedges. Commonly known around the Panhandle as “nutgrass”, sedges are a serious pest of lawns, particularly those that stay a little on the damp side, and vegetable gardens. Sedges, with their glossy leaves and distinctive flowers, stand out in lawns and gardens, are very unsightly in an otherwise well-maintained area, and can outcompete the desirable plants they invade. Fortunately, there are several AIs that work very well on sedges and are safe to use around turfgrass and many other plants. The most effective sedge herbicide AI for homeowners is halosulfuron-methyl (AI in Sedgehammer in lawns and Sandea and Profine in vegetables). Sedgehammer works very slowly (results can take up to a month) but is very safe in turfgrass and ornamentals and highly effective! If Sedgehammer and other halosulfuron products are difficult to obtain, Imazaquin is a slightly less effective but more common substitute. This AI can be found in the product Image Kills Nutsedge and is safe for use in turfgrass and most ornamental plantings.
The challenge of controlling the many types of weeds in your lawn, landscape, and vegetable garden seems daunting, but having just four basic classes of herbicides on hand can greatly simplify things! Whether you need to control a broadleaf, grass, or sedge weed problem, putting together a weed control toolbox containing a non-selective herbicide like glyphosate, a broadleaf selective like 2-4,D, a grass selective like sethoxydim, and a sedge selective like halosulfuron-methyl can allow you to handle most any weed you come across at home. For more information on controlling weeds in home landscapes and gardens or any other horticultural topic, contact your local UF/IFAS Extension office. Happy gardening!
*Most homeowners discover weed problems after the weeds are already up and growing, making post-emergent products necessary. Pre-emergent products have a place in weed management programs but are not the focus of this article.
Over the last decade or so, the Panhandle has been overrun, and I don’t just mean by the summer beach traffic. Rather, by an aggressive, exotic perennial grass that quickly displaces all native species, is not useful as a forage to wildlife or livestock, can spread by roots or seeds, and has no natural enemies. If you own property in the Panhandle or spend any amount of time on its roads, chances are you have become acquainted with this worst of invasive species, Cogongrass (Imperata cylindrica).
A native of Southeast Asia, cogongrass was introduced into the US in 1912 around Mobile, AL as a hitchhiker in orange crate packing. Then the 1920s, 30s, and 40s, it was intentionally introduced from the Philippines into other Gulf Coast states, including Florida, as a potential pasture forage for livestock. Since then, cogongrass has become one of the most economically and ecologically important invasive species in the US and worldwide, infesting nearly 500 million acres and is now found on every continent.
Cogongrass in Calhoun County, FL. Photo courtesy of Daniel Leonard.
Cogongrass is easily identified in late spring, when the grass throws easily spotted fluffy, white-colored seedheads above the mats of grass beneath. Additionally, patches of cogongrass are almost always noticeably circular in nature, radiating out indefinitely from the initial infestation. A closer inspection of the grass will reveal light green leaves up to 4’ in length, with an off-center, silvery colored midrib (the primary leaf vein that runs from the base of the leaf to the tip) and serrated leaf edges. Underground, cogongrass exhibits a dense underground root system that can reach as deep as 4’. This feature is the primary reason cogongrass outcompetes other plants, withstands any drought, fire, or soil condition thrown at it, aids in its resistance to herbicide activity, and generally makes it very difficult to manage.
The first step in managing cogongrass is prevention. If your property or the property you manage doesn’t have cogongrass, do everything you can to keep it that way. While the species can spread distances through seed dispersal, it is much more frequently moved around by fragmented rhizomes hitching a ride on equipment. If you or a contractor you’ve hired are working in or around an area with cogongrass present, avoid disturbing it with equipment and be diligent in monitoring the site for outbreaks following the job’s completion.
If you find cogongrass on your property, effectively eradicating it requires patience, persistence, and several years’ worth of herbicide applications. Currently, of the hundreds of herbicides available for purchase, only two chemistries have been proven to be very effective in destroying cogongrass, impazapyr (Arsenal, Stalker, etc.) and glyphosate (Roundup, Cornerstone, etc.).
Imazapyr is an extremely effective non-selective, residual herbicide that controls a wide variety of weed species, including cogongrass. Just one or two applications of imazapyr can provide 18-24 months of effective cogongrass control, with follow up treatments required as needed after that. However, Imazapyr has a major downside that limits its use in many settings. Because it is a non-selective herbicide with significant soil residual activity, it cannot be used around the root zones of desirable plants. Oaks, other hardwood trees, and most landscape plants are especially sensitive to imazapyr. This herbicide is best limited to use in fields, waste/fallow areas, natural areas, and monoculture pine plantations – it is not appropriate in most residential and commercial landscapes.
The other option, glyphosate is a non-selective herbicide with no soil residual activity. It is often a better option where severe injury or death of desirable hardwood trees and ornamental plants cannot be tolerated. However, due to its lack of residual soil activity, glyphosate applications on cogongrass patches will need to be repeated on an annual or biannual basis for up to five years for eradication of the infestation.
*Regardless of which herbicide you choose, controlling cogongrass is a multi-year affair requiring diligence and patience.
For more information on cogongrass and for specific herbicide recommendations and application rates/timing for your site, please contact your local UF/IFAS County Extension Office.
Hot, summer months are not the time to be using most lawn herbicides.
One issue is the heat of summer. Most lawn herbicide labels include statements such as the following.
“Do not apply when temperature exceeds 90°F.” “Do not broadcast apply this product when air temperatures are above 90°F (85°F for St. Augustinegrass) unless temporary turf injury can be tolerated.”
Every year we’ll see lawns that are injured or killed because of lawn herbicides being applied when temperatures are too hot.
Summer is usually a rainy and windy time of the year. Many lawn herbicide labels include statements such as the following.
“Allow 12 hours after application before watering lawn for maximum effectives on listed weeds.” “Apply only when wind is no more than 10 mph.” “Applying this product in calm weather when rain is not predicted for the next 24 hours will help to ensure that wind or rain does not blow or wash pesticide off the treatment area.”
It is critical to read and follow the label directions and precautions for any pesticide you use. Pesticide labels, including herbicides, include the following statements.
“To the extent consistent with applicable law, the buyer assumes all risks of use, storage, or handling of this product not in accordance with label directions.” “It is a violation of Federal law to use this product in a manner inconsistent with its labeling.”
Crabgrass growing in centipedegrass lawn. Photo credit: UF/IFAS Extension
By the time summer arrives, many of the lawn weeds are mature, which means they are well established with extensive root systems. These mature, well established weeds are much more difficult to control. They are more susceptible to herbicides when they are small, young, and not well established. Also, these mature weeds have been allowed to produce countless numbers of seeds as they move into summer. Most weeds are prolific seed producers. For example, a single crabgrass plant (a common summer lawn weed) can produce 150,000 seeds.
Applying a preemergence lawn herbicide in February to help prevent summer annual weeds such as crabgrass or applying a postemergence lawn herbicide during spring while the weather is mild and before the weeds are out of control simply makes more sense than waiting until summer.
The best options now with lawn weed control involve continuing to follow good mowing practices, maybe hand removal of some weeds, and just simply waiting it out until next February and spring to worry with the use of lawn herbicides.
In the meantime, you may want to read the following UF/IFAS Extension publication on lawn weed control.
Nearly everyone likes turfgrass lawns. They’re pretty and green. They filter water, chemicals, and nutrients before they enter our groundwater systems. They provide a recreation spot for people and pets. But lawns also come with maintenance tasks, one of which is weed control. Fortunately, keeping our common Centipedegrass lawns relatively weed free is as simple as smart management and utilizing herbicides effectively. Though the number of herbicides available for purchase can be overwhelming, you only need three to keep weeds at bay – a selective grass herbicide, a strong broadleaf herbicide, and a sedge herbicide!
Dollarweed, one of the toughest broadleaf weeds for homeowners to control. Picture courtesy of Julie McConnell, UF/IFAS.
First up on the essential herbicide list is the selective grass herbicide Sethoxydim. While most folks’ weed focus is centered on broadleaf weeds, grassy weeds like Crabgrass, Bahiagrass, Goosegrass, and others can be just as problematic and make for a very unsightly lawn. Enter Sethoxydim. Cheap and easy to find, Sethoxydim is offered as the active ingredient in many branded products like Fertilome Over the Top Grass Killer, Hi-Yield Grass Killer, and many more. These products control weedy grass species without seriously harming Centipedegrass or broadleaf ornamental trees and shrubs (Centipedegrass may temporarily be yellowed after sethoxydim application but will recover). Not only will it kill out the unwanted grass growing in your Centipede, but it will also remove these weeds from your flower beds!
Second, having a strong broadleaf herbicide on hand is necessary. I say “strong” because many of the homeowner grade products available at garden centers simply don’t have the “juice” to control tough broadleaf weeds like Dollarweed, Doveweed, Virginia Buttonweed, and others. For this job, I prefer to use a commercial grade 3-way product like Celsius WG by Bayer. Celsius WG is a 3-way combination herbicide with a healthy dose of Dicamba as its primary ingredient. Though Dicamba is a notoriously volatile chemical known to cause damage to unintended plants through drift in hot weather, combining it with the two other products in Celsius WG makes it safe to use in lawns, even in the heat of summer. While strong broadleaf herbicides like Celsius WG are expensive on the front end, don’t let that deter you. These products wind up being very cost effective in the long run due to minute mixing rates (one bottle goes a very long way in most residential lawns) and effectiveness – you simply will not need to waste time and money spraying lawn weeds over and over to obtain control like is necessary with lesser products – one or two applications will solve the toughest broadleaf weed problems.
Finally, any good lawn weed control program will include a quality sedge control herbicide. Sedges (often called “nutgrass”) look like grasses but are a completely different category of plants and as such, require specialized herbicide chemistries to achieve control. Sedge weeds prefer wetter areas of lawns, though they can occur in pretty much any lawn site and are very unsightly. For this weed category, there are several options available to homeowners. The one that consistently provides the best control in lawns is Halosulfuron-methyl, the active ingredient in the aptly named product Sedgehammer. Conveniently coming in individually pre-mixed packets for small lawns or a larger bottle when more acreage is to be treated, Sedgehammer couldn’t be easier to mix and use. While Sedgehammer and similar products are extremely effective in controlling various sedge weeds, they tend to work very slowly, and patience is required. Weeds immediately stop growing following a Sedgehammer application, but it can take up to three weeks to notice the sedges dying.
While having and using the above three herbicides can control almost any weed homeowners may encounter in their lawns, it is important to remember that herbicides are not substitutes for proper lawn management. When good cultural practices in lawns are followed, such as mowing at the correct height, only watering when necessary, following UF/IFAS fertilizer recommendations, etc., chemical weed control may not even be necessary in many cases! Also, once the decision to purchase and use chemical herbicides has been made, it is critical that one always read the label before using any herbicide product. This ensures safe and effective use of the product; the label is literally the law!
For assistance in choosing the correct herbicide for your lawn and other lawn care concerns, contact your local UF/IFAS Extension Office! Happy Gardening!