Every February, Tuscaloosa lawns fill up with henbit and annual bluegrass, and every February people go looking for something to spray. By then the fight is already lost. The application that would have prevented it was due the previous September.
Pre-emergent herbicide does not kill existing weeds. It establishes a chemical barrier in the top layer of soil that stops germinating seedlings. Timing is the entire product.
What Alabama Extension actually says
This matters, because a great deal of internet lawn advice attributes a soil-temperature rule to Extension services that Alabama Extension does not publish.
Alabama Extension gives calendar dates, not soil temperatures. Across their fall and winter lawn care guidance, their monthly gardener’s calendars, and IPM-0590 on chemical weed control, the guidance is by month:
- Fall, for winter and spring weeds: “apply them in mid-September or early October for the best results”
- September calendar: if winter or spring weeds were a concern this year, apply pre-emergent
- October calendar: same instruction repeated
- February: “Apply preemergent herbicides if forgotten in September and if spring weeds were a concern last year”
So the primary window in Alabama is mid-September to early October, with February as the backup.
The soil temperature rule, correctly attributed
The familiar “spray before soil hits 55 degrees” guidance is real and useful — it just comes from Georgia and South Carolina, not Auburn.
- UGA Extension B978: apply for summer annuals such as crabgrass and goosegrass prior to soil temperatures reaching 55 degrees. For winter annuals, apply when nighttime temperatures drop to 65 to 70 degrees.
- UGA’s climate blog notes crabgrass germinates around 50 degrees, and that weed scientists commonly use 52 degrees at half-inch depth as the trigger.
- Clemson HGIC uses air temperature instead: summer annuals when air temperatures hit 65 to 70 degrees for four consecutive days; winter annuals when nighttime lows reach 55 to 60 for four consecutive days.
Three respected land-grant universities, three different thresholds. Use whichever you like, but know which one you are quoting. For Tuscaloosa, the practical translation of all of them is: spring application late February to early March, fall application mid-September.
What each window is actually for
Fall (mid-September to early October) targets winter annuals — annual bluegrass, henbit, chickweed. These germinate in fall, sit small through winter, then explode in February when everyone notices them. Extension notes pendimethalin can be applied for annual bluegrass and chickweed in late August or early September.
Spring (late February to early March) targets summer annuals — crabgrass, goosegrass, foxtails.
Miss the fall window and February is genuinely too late for prevention. You are into post-emergent control, which is more expensive, less effective, and harder on the lawn.
The second application
One application does not last a full season. Residual activity runs roughly six to twelve weeks depending on product, rate and rainfall.
Extension’s IPM-0590 specifies that goosegrass control needs a second application eight weeks after the first. Clemson recommends a repeat at eight to nine weeks generally.
Doveweed is the one that catches people out. There is no Alabama Extension publication on it, but Clemson notes doveweed germinates late — when soil temperatures reach 65 to 70 degrees, noticeably after crabgrass. A single early spring pre-emergent has usually run out by the time doveweed shows up. Clemson’s timing for the SC Midlands is around April 1 with a repeat 45 days later.
Post-emergent, and the spring transition trap
For weeds already up, IPM-0590 says results are best when weeds are small and actively growing, applied when daytime temperatures are between 45 and 90 degrees.
The critical restriction, in Extension’s own capitals: DO NOT apply during “spring transition.”
Spring transition is green-up — the period when warm-season turf is coming out of dormancy. The grass is spending stored reserves to push new growth and has almost no capacity to metabolize herbicide injury. In Tuscaloosa that is roughly late March into April. Spraying then is how people damage their own lawns and conclude the product was defective.
Wait until the lawn is fully green and actively growing.
Weeds worth knowing by name
| Weed | Life cycle | When to act |
|---|---|---|
| Annual bluegrass (Poa annua) | Winter annual | Pre-emergent, late Aug to early Sept |
| Henbit | Winter annual | Pre-emergent, late summer to early fall |
| Chickweed | Winter annual | Pre-emergent, late Aug to early Sept |
| Crabgrass | Summer annual | Pre-emergent, late Feb to early Mar |
| Goosegrass | Summer annual | Pre-emergent, plus repeat at 8 weeks |
| Nutsedge | Perennial | Post-emergent; expect 3 to 5 weeks to full kill |
| Wild garlic and wild onion | Perennial | Post-emergent, Nov or Dec, again in Feb |
| Dallisgrass | Perennial | No Alabama-specific window published |
Extension describes henbit as emerging December through March and among the first weeds to bloom in winter and early spring, with seed that stays viable for years or even decades. Their recommended control is pre-emergent applied late summer to early fall — which is the same September window, again.
A note on what herbicide cannot fix
Thin turf grows weeds. Thick turf mostly does not.
If you are fighting the same weeds every year in the same spots, look at what is thinning the grass there: shade, compaction, scalping, poor drainage, wrong species for the site. Extension’s own framing is that excessive shade is a common reason for poor-growing turf. Pre-emergent buys time; it does not build density.
Quick reference
- Mid-September: the most important application of the year in Alabama
- Late February to early March: crabgrass and summer annuals
- Repeat at 8 weeks for goosegrass, and later for doveweed
- Alabama Extension uses calendar dates; the 52 to 55 degree soil rule is UGA
- Never spray post-emergent during spring green-up
- Wild garlic: November or December, then February
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