By AJ Binkley, UT Certified Lawn Care Professional
Every summer I get the same phone call, most often out of Spring Hill. Somebody walks out after a mow, sees a clump with blades twice as wide as everything around it already standing back up, and wants to know what it is and how fast it can be gone.
Almost everybody calls it crabgrass, because crabgrass is the one weed grass name most people know. It usually is not. What people mean by wide blade grass is almost always dallisgrass (Paspalum dilatatum), and the difference is not academic. Crabgrass is a summer annual that dies on its own at first frost. Dallisgrass is a warm season perennial that has been living in your yard longer than you have known about it.
You do not need a hand lens or a plant key. Here is the tell I use standing in a yard.
Dallisgrass blades are straight. Crabgrass blades are curly.
Dallisgrass runs its blades out flat and stiff from a central crown and they hold a line, close to a starburst looking straight down. Crabgrass blades kink and twist and lay over on themselves, and the plant sprawls sideways instead of sitting up in a clump. Once you have seen the two together you cannot unsee it.
One sorting question before we go on, and it is just the season. If the thick coarse grass shows up in cool weather while the rest of the lawn is growing hard, that is a different plant, and I wrote it up in the guide to thick green grass blades and quack grass. Dallisgrass is the July and August version of the same complaint.
This is the part nobody enjoys, so I will just say it. In most of the lawns I walk, the dallisgrass was planted there with the lawn, by whoever seeded it.
Two things put it there. The first is construction straw. Straw spread as seeding cover is chopped up dried farm grass, full of exactly the plants you do not want in a lawn. Dallisgrass and Johnson grass ride in on it, and both spread by rhizome once established, so a handful of seeds becomes a colony.
The second is cheap seed. Contractor mix is low grade tall fescue cut with annual ryegrass, and annual ryegrass is about the most contaminated grass seed you can buy, because its legitimate job is erosion control and nobody in that supply chain counts weed seed carefully. Kentucky 31, still stacked at every big box store in Middle Tennessee, is the same story with an older cultivar. Dallisgrass gets a break the others do not: seed labs count it as other crop seed rather than weed seed, because Oregon once grew it on purpose. That is a higher allowable threshold, so a bag can pass its own tag carrying more of it than you would ever accept.
So the map is predictable. Dallisgrass is thickest in Spring Hill and the newer construction along the I-65 corridor, in yards seeded with whatever was cheapest the week before final walkthrough. The contractor gets paid when the house closes. What the yard looks like nine months later was never on their list.
The lawn gets seeded in fall. Dallisgrass is a warm season plant, so those seeds sit through the winter doing nothing, and spring looks fine. The following June the soil is warm enough, the dallisgrass comes up, and it hits mowable height as a clump you can see from the driveway. From where the homeowner stands, that grass appeared out of nowhere a full year after the house was finished, so it must have come in on a mower deck or blown over from the neighbor. Neither one is usually true. It came with the lawn. It just took a summer to show its face.
Selective herbicides work by hitting broadleaf plants and leaving grasses alone. That is how I spray a whole yard without killing it. Dallisgrass is a grass and so is your tall fescue, close enough relatives that most chemistry cannot tell them apart, which is why the shelf that handles dandelion and clover has almost nothing on it for this.
That pushes people toward glyphosate, and I want to head that off. Roundup will brown it. It will not finish it. Dallisgrass and Johnson grass both bounce back, and plenty of perennial weeds have drifted toward outright resistance. You know what does not bounce back? The tall fescue right beside it that you also killed. Even where glyphosate eventually wins you are repeating it for two or three years and collecting dead holes the whole time.
Dallisgrass gets killed late summer into early fall. That is the window. Not May, and not June when you first notice it and want it gone that week. Late summer into early fall, when the plant is pulling resources back down into the crown and will carry chemistry down with them.
The tool is topramezone, sold as Pylex by BASF. It is a Group 27 herbicide, meaning it shuts down carotenoid production, and the symptom is unmistakable: treated tissue turns white rather than brown, within days. It is labeled for tall fescue and the other cool season grasses, which is the whole reason it exists as an option here. The label also wants an oil adjuvant, no mowing for two days afterward, and repeat applications rather than one and done.
I will be straight about something the manufacturer is straight about. BASF claims suppression of dallisgrass, not control, from multiple applications at the end of the growing season. Suppression is the honest word. You knock the plant back hard, repeatedly, and you keep coming back.
The other half is discipline, and it is the half that gets skipped. This is spot treatment work, plant by plant, the same way I handle Johnson grass. Somebody has to find the clumps and hit them, then come back later and find the survivors.
On a lawn that came in heavily contaminated, clearing dallisgrass takes me three to five years. UC IPM lands in the same place in different words: difficult, tedious, usually several years. I would rather tell you that in August than sell you a season and disappoint you in June.
So if you have clumps in the yard right now, this year you get your passes in while the window is open. You will still have dallisgrass next June, less of it, and next fall you do it again. Anybody promising a clean yard by spring is either not looking at the same plant or is planning to spray glyphosate and hand you the bare spots.
The reason I am particular about seed is sitting in these yards. If you are seeding this fall, do not use straw as cover, and do not buy Kentucky 31 or contractor mix. Read the tag, look for 0.00 percent other crop seed, and buy blue tag certified at minimum. If the cultivar names on the bag do not turn up in NTEP trial results, that bag does not belong on a lawn you intend to keep. I publish the exact cultivars I put down so anybody can check them against the trial data.
That is the mechanism, not a preference. Dense fescue keeps the next round of weed seed from finding open soil, and a thin stand is how all of this starts. The aeration and seeding work and the weed control side of the program are the same job from two directions.
Go look at a clump. If the blades radiate straight out of a tight crown and run wider than everything around them, it is dallisgrass, the window is open for a few more weeks, and it is not your mowing service's fault.
I treat this across Columbia, Spring Hill, Franklin, Thompsons Station, and the rest of the I-65 corridor. If you want it confirmed in person and an honest number of years for your yard, call (615) 490-4082 or email [email protected].
If you've noticed thick, fast-growing grass blades that look different from the rest of your lawn this year, you're dealing with contaminated seed issues affecting homeowners nationwide. Learn how to identify and eliminate quack grass and Johnson grass.
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