Soil Temperature: The Number That Times Your St. Augustine Lawn
Pre-emergent, fertilizer, and fungicide timing all follow soil temperature. Learn how to measure it, what the key numbers mean, and how to read the trend.
The short version
- Weed seeds, St. Augustine green-up, and fungus all respond to soil temperature, not air temperature or the date.
- Measure about 4 inches deep, in the lawn, mid-morning.
- Spring pre-emergent goes down before the soil holds around 55°F. The main spring feeding comes after it does and the grass is clearly growing.
- Go by the trend over several days, never one warm afternoon.
Why the soil matters more than the forecast
A sunny 80-degree day in February feels like spring, but the ground a few inches down warms much more slowly than the air. Weed seeds sitting in that soil don't care what the afternoon high was. They sprout when the soil they're in stays warm enough for long enough. The same goes for St. Augustine waking up from dormancy and for fungi like large patch becoming active in fall.
That's why every date on the Turf Method calendar is a starting point. The soil tells you whether this year is running early or late.
The numbers to know
| Soil temp (4 in.) | What's happening | What it means for you |
|---|---|---|
| Rising toward 55°F (late winter) | Summer weed seeds like crabgrass are about to wake up | Spring pre-emergent should already be down and watered in |
| Holding above about 55°F, grass greening | St. Augustine is coming out of dormancy | Time for the first real feeding once you see new growth and need to mow |
| Falling through the 70s (early fall) | Winter weed seeds start to sprout as it cools | Fall pre-emergent goes down |
| Falling into the 60s and 50s (fall) | Large patch fungus becomes active, especially when it's wet | Preventative fungicide should be down; water mornings only |
How to take a reading
- Get a soil thermometer. A simple probe thermometer with a long stem costs less than a bag of fertilizer. This is the one I use.
- Push it about 4 inches into the lawn, not a flower bed, and not the strip of grass baking next to the driveway.
- Read it mid-morning in a spot that gets normal sun. Use the same spot and the same depth every time.
- Write it down and check again over the next few days. Three or four readings in a row tell you far more than one.
Don't have a thermometer? The soil temperature tools on our Tools page let you look up estimates for your ZIP code.
Get reminded when it's time
A short email before each key St. Augustine task, plus the printable lawn calendar.
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Common mistakes
- Waiting for weeds to show up. By the time you see them, the soil already passed the point where pre-emergent could stop them.
- Feeding a sleeping lawn. Fertilizer on dormant St. Augustine mostly feeds the weeds and can set up fungus problems. Wait until it's greening, growing, and needs mowing.
- Trusting one warm day. A single reading after a warm spell can jump several degrees, then drop right back.
Everything in my St. Augustine program
The fertilizers, weed killers, fungicides, and tools I actually use on my own lawn, with links to buy.