· by Michael Blair, Nashville's Weed and Feed
In Franklin, Smyrna and Mt. Juliet the story tends to go the same way: the weed killer went down, the weeds curled and browned, and two or three weeks later the yard looks the way it did before. It is tempting to blame the bottle. Most of the time the spray did what it was made to do, and what came back is one of four things: a fresh crop of seedlings, a perennial pushing up from roots the spray never reached, a grassy weed a broadleaf product cannot touch, or a spray that went down in weather that worked against it. Sorting out which one you are looking at is the first thing our weed control service does, because each has a different answer and only one of them is spraying again.
New weeds from seed, not the old ones surviving
A spray applied after weeds come up is a post-emergent. It works on the leaves that are there that day and leaves nothing behind in the soil to stop the next round. UT Extension’s W 1068 weed control guide draws the line between the two kinds of products clearly: post-emergents go on weeds you can see, while pre-emergents go down before the weeds appear and stop annual weeds as they sprout from seed. Once the plants are showing, a pre-emergent is too late.
Crabgrass is the best example of why this matters. UT’s W 146 crabgrass control publication says it starts germinating once the soil reaches 55 degrees F and can go on germinating for the next two to three months. Spray the first crop in May and a second crop is already on the way. The weeds you see in June are new plants, not survivors.
The same pattern runs in reverse in the cool months. W 1068 names annual bluegrass, chickweed, henbit and purple deadnettle as winter annuals, which sprout in fall and show up in late winter. A lawn sprayed clean in October can be full of them by February without anything having gone wrong with the October spray. The fix for seedling flushes is a properly timed pre-emergent treatment, and the 2025 UT Extension lawn maintenance calendar sets the spring one by the soil: 55 degrees at two inches for several days, about when the forsythia blooms, with a second application six to eight weeks after the first.
Perennials that regrow from what is underground
Some weeds die back on top and come right back because the part that matters is below the surface. W 1068 is direct about this group: pre-emergents will not control established perennials such as ground ivy, wild violet, sedges and bermudagrass. Ground ivy and violet often take a sequence of post-emergent applications, using products such as 2,4-D, MCPP and dicamba, made while the lawn is actively growing. One spray burns the leaves; the roots and runners send up new ones.
Nutsedge is the other common returner. It is not a broadleaf weed, so a broadleaf product does very little to it, and W 1068 notes that sedges usually mark ground that stays too wet or drains poorly. Spraying the sedge in a low, soggy corner of the yard every summer without dealing with the water is a cycle that does not end. Our post on getting rid of nutsedge and wild violet covers what each of those two needs, and identifying what is actually in your lawn helps you tell a perennial from an annual before deciding anything.
Grassy weeds a broadleaf spray was never going to kill
Most bottles sold as lawn weed killer are selective broadleaf products. They kill dandelion, plantain and clover and leave grass alone, and that also means they leave grassy weeds alone. W 1068 adds that a selective herbicide only stays selective when it is used the way the label says. Nonselective products, glyphosate and the vinegar and oil products sold as natural weed killers, kill the lawn along with the weed.
In a tall fescue lawn, which W 1068 calls the most common cool-season lawn grass in Tennessee, the grassy weeds that look like they survived the spray are usually bermudagrass and dallisgrass. Bermuda creeps in from a neighbor’s yard or a strip along the drive, stays green and wiry through July, and is untouched by anything made for broadleaf weeds. The UT calendar puts the start of bermudagrass and dallisgrass removal in fescue in July, finishing in August, as its own program rather than as part of the regular broadleaf spray.
A fescue front yard with a bermuda edge that keeps advancing no matter how often it is sprayed is not a case of the spray failing. The wrong tool went on the wrong plant.
Heat, rain and a stressed lawn on spray day
The last cause is the day the spray went down. W 1068 says post-emergents work best on young weeds that are actively growing, and that rain or irrigation soon after spraying can keep the product from being absorbed. How long a product needs to stay dry varies, and the answer is on its label, not in a rule of thumb.
Heat matters as much as rain. Turf researchers at the University of Nebraska-Lincoln, in UNL Turfgrass Science’s note on herbicides in high temperatures, put the best control at about 70 to 85 degrees F on plants that are growing hard. Above roughly 80 to 85 degrees, many broadleaf products start injuring the grass, and the heat thickens the wax on weed leaves and slows how far the product moves inside the plant. Their advice is to spray early in the morning. A Middle Tennessee afternoon in August can do all of that at once.
Fescue adds one more reason to wait. This is the transition zone, and fescue struggles through July and August heat and humidity here. W 1068 advises against spraying ground ivy in tall fescue during summer stress, and the UT calendar puts winter broadleaf control in fescue at air temperatures above 55 degrees F, which in practice means the cooler ends of the year. Holding off until fall usually does more than spraying a stressed lawn twice in summer.
Spraying only treats what is there. W 1068 calls a dense, well-kept stand of turf the best defense against weeds, and the UT calendar’s fall advice for fescue is to aerate and overseed to thicken it. Our aeration and overseeding service is usually where a lawn with returning weeds ends up once the weeds are under control. If you would like us to look at what keeps coming back in your yard and sort it into seedlings, perennials, grassy weeds or timing, ask for a free estimate, or read more about how we work in Franklin.
- New weeds in the same spot weeks later: a seedling flush, answered with pre-emergent timing
- Same weed, same spot, regrowing from the base: a perennial needing repeat applications
- Grass-like weeds untouched by the spray: a broadleaf product on a grassy weed
- Weeds barely affected at all: check what the weather did that day
Sources
- UT Extension W 1068, Lawn and Landscape Weed Control for Homeowners — Pre-emergent versus post-emergent; winter annuals; established perennials pre-emergents miss; sequential sprays for ground ivy and violet; sedges and wet ground; rain after spraying; selective use per label; tall fescue as the common Tennessee lawn grass; no summer-stress spraying on fescue.
- UT Extension PB 1903, 2025 Lawn Maintenance Calendar — Pre-emergent at 55 degrees F soil at two inches with a sequential application six to eight weeks later; winter broadleaf control in fescue above 55 degrees F air; bermudagrass and dallisgrass removal in fescue July to August; fall aeration and overseeding.
- UT Extension W 146, Crabgrass Species Control in Turfgrass — Crabgrass germinates at 55 degrees F soil and can keep germinating for two to three months.
- University of Nebraska-Lincoln Turfgrass Science, Be cautious with herbicide applications and high temperatures — Best control at about 70 to 85 degrees F; turf injury risk above roughly 80 to 85 degrees; heat thickens leaf wax and slows movement; spray early in the morning.