Soil temperature by zip code
Air temperature is not what your lawn responds to. Roots, seed and crabgrass all answer to the ground, and the ground runs days behind the air in spring and holds heat well into autumn.
Why two inches, and why an average
Turf guidance is written against a probe pushed two inches into the ground, so that is the depth quoted here. The shallowest soil swings hardest: it can move fifteen degrees between dawn and mid-afternoon, which is why a single reading taken at the wrong hour opens a window that is not really open.
So this averages five days. Pre-emergent guidance says sustained above 55°F, not touched 55°F once on a warm afternoon in March, and the difference between those two readings is a wasted bag.
The numbers that actually trigger something
| Soil temp | Cool-season | Warm-season |
|---|---|---|
| 50–55°F | Crabgrass starts germinating. Pre-emergent goes down now or not at all. | Still dormant. |
| 55–65°F | Root growth at its best. Good window to feed. | Waking up. Pre-emergent window. |
| 65°F falling | Overseeding opens in autumn. | — |
| 60–70°F | Peak growth. | Growing, not yet at its best. |
| Above 75°F | Heat stress. Ease off nitrogen. | Approaching its peak. |
| 80–95°F | Real trouble. Water, do not feed. | Peak growth. |
The pre-emergent window, which is the one people miss
Crabgrass germinates when the top couple of inches hold above 55°F for several days running. A pre-emergent works by sitting in that layer before the seed breaks, so the entire job is to be early. There is no version of this you can fix afterwards: once it is up, a pre-emergent does nothing to it and you are buying a post-emergent and repairing thin turf.
Put it down as the five-day average crosses 50 to 55°F and is still climbing. Waiting for the number to reach 55°F means arriving with germination already underway, because the reading you are watching lags the warm afternoons that actually caused it.
Most labels allow a split application, and in a long spring it is worth it. Half rate at the 55°F crossing, the remainder six to eight weeks later, keeps the barrier intact through the second flush rather than letting it thin out in May. Check your own label for the season maximum before doing this: the total across both passes is what is regulated, and the label is the law.
One exception worth knowing: do not put a pre-emergent down where you intend to seed. It cannot tell your grass seed from crabgrass seed. Autumn overseeding and spring pre-emergent collide every year, and the usual resolution is to skip pre-emergent on the areas you seeded and accept a thinner spring there.
Seeding windows, which are not the same number
Cool-season grass
Bluegrass, tall fescue, perennial ryegrass. Seed germinates on soil between 50 and 65°F, and the best window is late summer into autumn, as the number falls back through 65°F. Autumn beats spring for a reason that is not obvious: the soil is still warm from summer while the air is cooling, so the seed gets fast germination followed by months of root growth without heat stress and without crabgrass competing for the same gap. A spring seeding gets the opposite of all three.
Warm-season grass
Bermuda, zoysia, St. Augustine, centipede. These want soil 65 to 80°F and rising, which puts the window in late spring through early summer, the opposite end of the year from cool-season. Seeding or sprigging these into cool ground does not slow them down, it fails outright.
This is the whole reason a single soil-temperature chart misleads. The same 68°F reading is a green light for one lawn and a closing door for the other. Which grass you actually have decides which half of every table on this page applies to you.
Feeding, and when to stop
Nitrogen goes on grass that is growing, not grass that is enduring. For cool-season turf that means soil in the 55 to 70°F band; above about 75°F you are pushing top growth on a plant whose roots are already shortening, which is how a July feed turns into disease. For warm-season turf the same logic sits roughly five degrees higher, and its peak feeding is the heat that shuts cool-season down.
How much is a separate question from when, and it is the one most advice skips. A rate is only meaningful against your actual square footage and what your soil already holds, which is a soil test rather than a guess.
Where this comes from
Modelled soil temperature at 6cm from Open-Meteo, averaged across the last five days, for the Census centroid of the zip you enter. It is a model, not a probe in your own yard: a shaded north side or a low wet corner will differ from it, sometimes by several degrees. That is an argument for knowing your own ground, which is the whole point of the app.
Common questions
What soil temperature should I apply pre-emergent at?
As the five-day average crosses 50 to 55°F and is still rising. Crabgrass germinates in the low to mid 50s and a pre-emergent has to be in place before that happens rather than after, so aim to be slightly early rather than exactly on 55°F.
Is soil temperature the same as air temperature?
No, and the gap is why timing from the forecast goes wrong. Soil has far more thermal mass than air, so it warms days behind it in spring and holds heat well into autumn. A warm week in March barely moves the ground, and a cold snap in October does not undo a summer of stored heat.
Why measure soil temperature at two inches deep?
Because that is the depth turf guidance is written against, and it is where seed germinates and most root activity happens. The surface can swing fifteen degrees between dawn and afternoon, so a shallower reading tells you about the weather rather than about the root zone.
What soil temperature does grass seed germinate at?
Cool-season grasses such as fescue, bluegrass and ryegrass germinate between 50 and 65°F, best in autumn as the number falls. Warm-season grasses such as bermuda and zoysia want 65 to 80°F and rising, in late spring. The same reading means opposite things depending on which you grow.
Do I need a soil thermometer?
A probe in your own lawn is the most accurate answer and costs very little. This lookup exists because most people will not take a reading at the same hour every day for a fortnight, and a five-day modelled average is closer to the truth than a single probe reading taken at 3pm.
Can I use soil temperature to time fertilizer?
For the timing, yes: cool-season turf feeds well with soil between 55 and 70°F and should be eased off above 75°F. For the amount, no. A rate depends on your square footage and what your soil already holds, and that needs a soil test rather than a temperature.
Knowing the number is the easy half
This page gives one reading for a whole zip code. A lawn is not one place: a shaded north side, a compacted strip by the drive and a low wet corner run at different temperatures, hold different amounts of water, and often want different treatment on the same weekend.
That is what YardScout does. It maps your property into zones and gives each one its own soil, its own water balance and its own timing, and it turns a lab soil test into pounds per zone rather than a rate you have to apply to a number you never measured. It has nothing to sell you, which is why it is capable of telling you to put nothing down this month.