If you put fescue seed down this month, you have already made a decision about February’s weeds. Most people make it without realizing there was a decision to make.
Two jobs land on the same three weeks of the north Georgia calendar, and they cancel each other out. One is overseeding. The other is stopping the winter weeds before they ever come up. You get one or the other this year, not both, and which one you pick should depend on what your lawn actually looks like right now.
What germinates in a north Atlanta lawn in September
Annual bluegrass, Poa annua, is a winter annual. It comes up in late summer and early fall, sits low and unremarkable through the cold months, then makes its move in February and March while the fescue is still slow. By May the heat has killed it, and what it leaves behind are the pale, thin patches people usually blame on disease.
The trigger is soil temperature. UGA’s publication on annual bluegrass control puts germination at the point where soil temperature falls below 70 degrees, which around Dunwoody and Sandy Springs normally lands in the first half of September. Common chickweed, henbit and swinecress run on the same clock.
The part that catches people out is the seed. UGA notes that a single annual bluegrass plant can produce hundreds of viable seeds even when it is mowed short. So the plant you never noticed, because it spent all winter under the height of the blade, still restocked your yard for next year.
The pre-emergent window in north Georgia closed on September 15
UGA’s weed control guidance for home lawns gives August 15 through September 15 for north Georgia, keyed to nighttime temperatures dropping into the 65 to 70 degree range. That date is not soft. A pre-emergent herbicide works by sitting in the top half inch of soil and stopping seedlings as they try to establish, so putting it down after germination has started means treating for a stage the weed has already passed.
If you are reading this in late September, the window is behind you for this year. That is worth knowing rather than worth worrying about.
Why you cannot do both in the same fall
Fescue is a cool season grass, and September is when it wants to go down. The soil is still warm enough for quick germination and the air has come off its August peak, which is why our guide to overseeding a fescue lawn points at the same three weeks.
A pre-emergent cannot tell fescue seed from poa seed. UGA states that pre-emergence herbicides should not be applied to newly seeded turfgrass, and the annual bluegrass publication goes further: skip fall pre-emergents altogether if you expect to reseed or resod within the next several months. For pendimethalin the instruction is to stay off newly seeded ground until after the fourth mowing.
The mistake we run into most often is somebody doing both anyway. Pre-emergent goes down over Labor Day weekend, seed goes over the top two or three weeks later, and then the new grass comes in thin and blotchy. The product did exactly what it was sold to do.
What we would do in our own yard
Get down on one knee and look at the ground rather than at the grass. If you can see soil between the blades from standing height, the stand is thin, and thin turf is how poa gets its foothold in the first place. Seed it. Take the weeds on in late winter instead.
If the stand is already thick, you had a real choice this year, and the thing to do now is put the dates in next year’s calendar.
Either way, the strongest tool is not in a bag. UGA’s own summary is that weeds do not easily invade turfgrass that is fertilized and watered properly and mowed at the right height and frequency. Annual bluegrass is a weak competitor. It turns up in the strip along the driveway that bakes all afternoon, under the water oak where nothing gets four hours of sun, and along the line everybody walks from the drive to the back gate. Those are not weed problems. They are a light problem, a compaction problem and a traffic problem, and they will keep making weeds until somebody fixes them.
Mowing height does more than most people expect. Fescue kept on the tall side through fall and winter shades the soil surface, and the soil surface is exactly where poa needs light to get going. Scalping the lawn in November is an open invitation.
Then in March and April, bag the clippings. That is the opposite of what we tell people the rest of the year, because returning clippings feeds a lawn for free. But hundreds of seeds per plant is hundreds of seeds per plant, and every mowing you bag is seed leaving the property instead of dropping back into the seed bank. Two springs of that makes a difference you can see, and it costs nothing except the time to empty a bag.
If you seeded this month, wait before you spray
New fescue is not tough enough for herbicide straight away. UGA’s guidance for a newly established lawn is a post-emergence product at half the labeled rate after three to four mowings. Walter Reeves gives the same rule from the other direction: three or four mowings, or roughly 60 days from seeding, whichever arrives first.
For a lawn seeded in mid-September that puts you in November, which is convenient, because chickweed and henbit are still small then and small is when they are easiest to deal with. Spot treat the patches rather than blanket spraying a young lawn.
The short version for this week
Thin lawn: seed it now, skip the pre-emergent, plan on a spot treatment in November and a bagged mow in March.
Thick lawn: nothing to do this week. Mark August 15 in next year’s calendar and watch the overnight lows in early September.
Not sure which one you have: that is a five minute look at the ground, and we would rather you asked than guessed. It is one of the things we check on a maintenance visit, and you can get in touch if you would like somebody to walk the yard with you.
