Every February we see lawns around Monticello that are brown on top and green underneath with a solid mat of poa annua, henbit and chickweed. By the time anyone notices, the weeds are flowering and the fix is a post-emergent spray on turf that is barely awake. The better fix happened five months earlier, in the fall, and most people never make it.
Why fall is the second pre-emergent window
Pre-emergent herbicides do not kill weeds; they stop seeds from establishing. That only works if the product is in the ground before the seed germinates. Winter annual weeds germinate in fall as soil temperatures fall through the low 70s, which in Jefferson County is typically late September into October. Miss that and the weeds are already up by the time the lawn goes dormant.
We wrote about the spring application and why a second spring application matters. The fall one is the third leg, and for a lawn that goes fully dormant every winter it is arguably the most visible.
What the fall application stops
- Annual bluegrass (poa annua): the pale, clumpy grass with white seed heads that dominates dormant lawns.
- Lawn burweed: the sticker weed. By the time you feel it barefoot in April it has already seeded. Fall pre-emergent is the only clean way to stop it.
- Henbit and chickweed: the purple-flowered and low white-flowered broadleaves that carpet bare ground.
- Some wild garlic and annual ryegrass volunteers. Partial control; perennial bulbs are not stopped.
Our winter weed guide covers what each of these looks like.
Timing for the Big Bend
Watch the soil temperature, not the calendar. When soil at four inches drops to about 70°F and stays there, apply. For Monticello that is usually the last week of September to the middle of October. Applying in early September on a hot year is too early, because most products only last two to three months and you want coverage through the December germination tail. Applying in November is too late for poa, though it still catches some later broadleaf germination.
A cheap soil thermometer pushed into the lawn in the morning is the whole tool. Check it a couple of times a week from mid-September.
Products that are safe on centipede
Centipede is sensitive to more herbicides than other lawn grasses, so read the label every time. The common pre-emergent active ingredients that are labeled for established centipede, St. Augustine, Bermuda and Zoysia include prodiamine, dithiopyr and pendimethalin. Each has a rate range on the label; use the low end on centipede unless weed pressure is severe. Do not use a pre-emergent on a lawn you plan to seed or sprig this fall, or on sod laid within the last few months. And do not use a "weed and feed" product in fall; the fertilizer half is the wrong nutrient at the wrong time, as our post on weed-and-feed on centipede explains.
Applying it right
- Mow first, so the granules reach the soil.
- Calibrate the spreader on a driveway. Over-application on centipede is a real risk and under-application does nothing.
- Apply in two passes at right angles for even coverage. Edges and strips along driveways matter; that is where poa shows first.
- Water in with a quarter to half inch within a day or two. The label will say. Without water the product sits on the leaf and breaks down in sunlight.
- Do not aerate or dethatch afterward. Breaking the soil surface breaks the barrier.
If you overseed with ryegrass
You cannot do both. A pre-emergent stops ryegrass seed as well as it stops poa. Choose. On a home lawn in the Big Bend we generally recommend skipping the ryegrass and using the fall pre-emergent; the dormant lawn looks tidy and the spring transition is easier. Our ryegrass post lays out the trade.
Where it fits in the year
Late February: spring pre-emergent. Late May: second spring application. Late September: fall application. Three applications a year is what it takes to keep a lawn genuinely clean without leaning on post-emergent sprays. Skip one and the gap shows up as weeds two months later.
FAQ
Can I apply the fall pre-emergent with the potassium feeding?
Yes, and the timing lines up. Use separate products rather than a combined bag so you can set each rate independently.
What if winter weeds are already up when I apply?
The pre-emergent will not touch them. Spot-spray with a post-emergent labeled for your grass, then keep the barrier in place for what has not germinated.
Is one fall application enough?
Usually. Poa germinates in a fairly tight window. A light second application in December helps on lawns with heavy burweed history.
Do you include pre-emergent in a plan?
Weed control timing is part of the custom plans we build around a property.
If your lawn turns into a weed patch every winter, get in touch before the end of September and we'll get the barrier down in time. We handle weed programs across Monticello and the Tallahassee side of the county through Chaires.