- Spring pre-emergent should be down before soil temperatures hold around 55°F, usually late February to mid-March in our area.
- A second application six to eight weeks later extends protection into summer.
- Fall pre-emergent in September to early October stops Poa annua, henbit, and chickweed.
- Pre-emergent does nothing to weeds that are already up. Timing is the whole game.
Every spring we get the same call: "I put down weed preventer, and the crabgrass came up anyway." Nine times out of ten the product wasn't the problem. The timing was. Pre-emergent herbicides are one of the most effective tools in lawn care, but they only work during a narrow window, and in Northeast Georgia that window arrives earlier than most people expect.
What pre-emergent actually does
A pre-emergent herbicide forms a treated zone in the top layer of soil. When a weed seed germinates and its tiny root and shoot pass through that zone, the seedling is stopped before it ever establishes. It doesn't kill seeds sitting dormant in the soil, and it doesn't kill weeds that have already emerged. That's why the application has to be in place, and watered in, before germination starts.
The 55-degree rule
Crabgrass, the most common summer weed in our lawns, begins germinating when soil temperatures at about a two- to four-inch depth hold near 55°F for several days in a row. Air temperature is a poor guide. A few warm afternoons in February don't warm the soil much, while a steady run of mild nights does.
In most years, that soil temperature arrives across our service area somewhere between late February and mid-March. It varies year to year, so we watch soil temperatures rather than circling a date on the calendar.
Jefferson warms up before Toccoa
Even within our compact service area, there's a difference. Lawns around Jefferson, Commerce, and Hoschton often reach germination temperatures a week or more ahead of shaded foothill lots around Toccoa, Clarkesville, and Demorest. South-facing slopes warm faster than north-facing ones, and a lawn next to a driveway or sidewalk warms faster than one under trees. When we schedule spring rounds, we generally work from the warmer southern end of our territory north.
A useful natural reminder: crabgrass germination tends to begin around the time forsythia finishes blooming. If the forsythia in your neighborhood is already dropping its yellow flowers and nothing has gone down yet, you're late.
Why one spring application isn't enough
Pre-emergents break down over time in warm, moist soil. A single February or March application can thin out by late spring, just as goosegrass and late-germinating crabgrass get going. A second application six to eight weeks after the first, often called a split application, carries protection into early summer. This is standard practice in our programs.
The fall application most people skip
Summer weeds aren't the only ones. Poa annua (annual bluegrass), henbit, and common chickweed germinate in fall as soil cools, generally when soil temperatures drop below about 70°F. In our area that's typically September to early October. These are the weeds that show up as bright green clumps and purple flowers in dormant Bermuda and Zoysia lawns in late winter. A fall pre-emergent prevents most of them.
Watering it in
Most granular and liquid pre-emergents need to be moved into the soil with about a half inch of rain or irrigation within a few days of application. Left on the surface, they can break down in sunlight before they do their job. We time applications around the forecast and tell you whether your irrigation should run.
Three timing mistakes we see often
- Applying in April. By then much of the crabgrass has germinated. Some products have limited activity on very young crabgrass, but control drops quickly as it matures.
- Aerating right after application. Core aeration pulls plugs through the treated layer and can open gaps in the barrier. Aerate before pre-emergent goes down, or plan the timing carefully.
- Seeding into a treated lawn. Most pre-emergents stop grass seed just as effectively as weed seed. If you plan to overseed Fescue in the fall, the fall pre-emergent plan has to change for those areas.
Why it matters who applies it
Product selection depends on your turf type. Some herbicides that are fine on Bermuda can injure Centipede or newly sodded lawns. In Georgia, applying herbicides on someone else's property for hire also requires a commercial pesticide license. Our applicators hold that license, which is one reason we can install a landscape and keep it clean. You can read more about that in why your grounds contractor needs a pesticide license.
If your lawn is a mix of grasses or you aren't sure what you have, start with our guide to Bermuda, Zoysia, Centipede, and Fescue.