Winter Weeds in St. Augustine: Henbit, Chickweed, and Poa Annua
How to identify and control the common winter weeds in St. Augustine lawns, including henbit, chickweed, annual bluegrass (Poa annua), and burweed.
The short version
- Winter weeds sprout in fall, grow while St. Augustine is dormant, and show up green against a brown lawn.
- Fall pre-emergent is the best defense.
- Spot-treat what gets through on mild days in winter, while the weeds are small.
- Most die off on their own when it heats up, but not before dropping seed for next year.
The usual suspects
| Weed | How to spot it |
|---|---|
| Henbit | Square stems, scalloped round leaves stacked up the stem, small purple-pink flowers in late winter. |
| Chickweed | Low, sprawling mats of small, smooth, pointed oval leaves with tiny white star-shaped flowers. |
| Annual bluegrass (Poa annua) | Bright, light-green grass clumps that stand out in a dormant lawn, with white-ish seed heads even when mowed short. |
| Lawn burweed | Flat plant with lacy, finely divided leaves that grows spiny burs in spring. Full guide. |
Why they show up
These are winter annuals. Their seeds sprout when soil temperatures drop in the fall, and they grow through the cool months while your St. Augustine is slowing down or dormant. Thin areas, bare soil, and spots that stay wet are where they move in first.
Prevention: fall pre-emergent
A pre-emergent applied in late September to early October forms a barrier that stops most winter weed seeds from sprouting. It won't touch weeds that are already up, so timing matters more than product. Here's how pre-emergent works.
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.
Treatment: small weeds, mild days
- Treat in winter, not spring. Young weeds die much more easily. By the time henbit is flowering, it's near the end of its life and has already set seed.
- Atrazine is a common choice for winter broadleaf weeds and Poa annua in St. Augustine. Follow the label for rate and temperature limits. The atrazine I use is Southern Ag Atrazine.
- Spot spray. Treat the weeds, not the whole lawn.
- Pick a mild stretch. Many weed killers work poorly in cold weather, and some labels set temperature limits.
- Check every product label for "St. Augustine" before you buy. See weed killers safe for St. Augustine.
Don't mistake dormancy for damage
A brown St. Augustine lawn dotted with green weeds can look alarming, but the brown is normal winter dormancy. Don't fertilize to "green it up." Feeding a dormant lawn mostly feeds the weeds. Wait for spring. See when to fertilize in spring.
Everything in my St. Augustine program
The fertilizers, weed killers, fungicides, and tools I actually use on my own lawn, with links to buy.