Pre-Emergent for St. Augustine: The Two Dates That Matter
How pre-emergent works, when to apply it on St. Augustine (by February 1 and late September to October on the Gulf Coast), what it stops, and how to apply it right.
The short version
- Pre-emergent stops weed seeds from sprouting. It does nothing to weeds already growing.
- Two key applications: spring (by February 1 on the Gulf Coast) and fall (late September to early October).
- Spread it evenly at the label rate and water it in.
- Don't use it where you plan to plug, sod, or seed soon.
How pre-emergent works
A pre-emergent herbicide forms a thin barrier in the top layer of soil. When a weed seed sprouts, the young root hits that barrier and the seedling dies before it ever breaks the surface. That's why timing is everything. Put it down after the seeds sprout and it's useless.
The two windows
| Application | When (Gulf Coast) | What it stops |
|---|---|---|
| Spring | By February 1, before soil temps hold around 55°F | Summer weeds like crabgrass, spurge, and sandbur |
| Fall | Late September to early October | Winter weeds like henbit, chickweed, Poa annua, and burweed (stickers) |
The Turf Method program also adds a third application in early May to stretch summer protection, since most pre-emergents wear off after a few months. Inland and farther north, push spring later and fall earlier by a couple of weeks, or use the plan builder. The real signal is soil temperature. I check it with a long-stem soil thermometer.
Get reminded when it's time
A short email before each key St. Augustine task, plus the printable lawn calendar.
A short email when it's time for the next lawn task. Unsubscribe anytime.
How to apply it
- Measure your lawn so you buy and spread the right amount. Use Google Earth and the bag calculator.
- Read the label. Rates vary between products and bag sizes. Use the rate for St. Augustine. I weigh out the amount on a digital scale.
- Spread evenly with a broadcast spreader. Going over the lawn twice at half rate in crossing directions helps avoid gaps and stripes.
- Water it in within the time the label gives, usually about half an inch. It doesn't start working until it's watered in.
- Sweep granules off the driveway and sidewalk back onto the lawn.
Things to know
- It also stops grass from rooting. Skip areas where you'll put down plugs or sod in the next few months, or check the label's waiting period.
- Avoid disturbing the soil right after applying. Core aerating breaks the barrier, so aerate first, then apply.
- Already have weeds? Pre-emergent won't help with those. That's a job for a post-emergent spot spray. See weed killers safe for St. Augustine.
- Buy it early. Shelves get thin right before February.
Everything in my St. Augustine program
The fertilizers, weed killers, fungicides, and tools I actually use on my own lawn, with links to buy.