Most gardeners plant by the calendar or by air temperature. In Missoula, soil temperature is one of the most useful numbers to watch too, especially for seeds sown straight into the garden. The soil warms up slower than the air, and a warm afternoon does not mean the ground is ready. Seeds dropped into cold soil can sit there, sometimes rotting before they germinate, while the same seeds in warm soil come up faster and more evenly.
Learning to think in soil temperature alongside dates takes a little practice, but it is a reliable way to stop losing seeds to cold ground. Here is what the numbers are and how to work with them in our climate.
Why the Date Alone Is Not Enough
Missoula’s median last spring frost is mid May (about May 16 at the airport, per NOAA’s 1991 to 2020 normals), and in about 1 year in 10 it comes after June 1. That frost timing still matters, especially for tender transplants, but it only tells you when the air is likely to freeze. It says nothing about how warm the soil is, and in a cold, wet spring the ground can stay chilly well past it. Two springs in a row can look completely different. A soil thermometer cuts through that guesswork by telling you what the seeds are actually sitting in.
Different crops have very different needs. MSU Extension groups vegetables into cool-season and warm-season types. Cool-season seeds germinate in cool soil, and most of the seedlings tolerate some frost. Warm-season seeds need genuinely warm soil, and the plants are damaged by frost. Plant each group when the soil is in its range and everything gets easier. Soil temperature is not the whole story, though. Frost risk, air temperature, and how many days a crop needs to mature all shape the planting date too.
Target Soil Temperatures by Crop
Here are the soil temperature ranges for germination that MSU Extension lists in its Can I Grow That Here? planting guide. Seeds sprout slowly near the low end of a range and faster toward the middle. These are germination numbers for seeds, not rules for transplants.
Cool-season crops like lettuce, spinach, peas, radishes, and other greens germinate in cool soil. MSU lists spinach at 40 to 75°F, peas at 40 to 70°F, radishes at 40 to 85°F, and lettuce at 45 to 75°F. They come up faster as the soil warms into the 50s and 60s, but you can start them early because cool soil does not stop them.
Root crops like carrots and beets have higher germination ranges than you might expect: 55 to 75°F for carrots and 50 to 75°F for beets. They will still sprout in cooler spring soil, just slowly, so a little patience here pays off with more even sprouting.
Warm-season crops need warmer soil before you direct seed them. MSU lists 60 to 85°F for snap beans, 55 to 85°F for sweet corn, and 65 to 85°F for cucumbers, squash, and muskmelons. This is the big one people get wrong. Beans and squash planted in cold soil often rot, while the same seeds planted two weeks later in warm soil can catch up and pass them.
Tomatoes and peppers are almost always set out as transplants rather than seeded in the ground. MSU lists 65 to 85°F for their seeds, but that is a germination range, and MSU gives no soil temperature for transplanting. Its timing rule is to set tomatoes out after the danger of frost has passed, which in Missoula usually means late May into early June, after a look at the 10-day forecast. MSU also notes warm-season crops grow best with air temperatures between 65 and 85°F. Set into cold ground during a cold spell, they tend to stall, and the leaves can turn purplish from the cold, so a soil thermometer reading and a warm forecast are both worth checking before you plant.
How to Measure It
You do not need anything fancy. An inexpensive soil thermometer, or even a meat thermometer kept for the garden, does the job. Push it about two inches into the soil where you plan to plant and take the reading in the morning, when the soil is at its coolest. Check it for a few days in a row rather than trusting a single reading, since you want to know the soil is reliably in range, not just warm for one sunny afternoon.
Once you have done this a couple of seasons you start to get a feel for it, but the thermometer keeps you honest, especially in a cold spring when it is tempting to plant warm-season crops too early.
Warming the Soil Faster
If you want to push the season a little, there are several ways to warm soil. Raised beds warm earlier than ground-level beds because they drain better and the sun reaches more of the soil, which is one of the reasons they work so well in our climate. A simple cloche or a row cover laid over a bed traps warmth and can warm the soil underneath, which may let you plant a little sooner. Pull covers off once the weather and soil have caught up so plants do not overheat.
Plastic mulch is another established tool. Clear plastic warms soil the most, up to about 14°F by University of Minnesota Extension’s figure, but weeds grow under it. Black plastic controls weeds and can warm soil modestly, though MSU’s trials in Bozeman found black film did not significantly raise soil temperature while red film did. Lay it on moist, smooth soil a week or two before planting. The tradeoff is that plastic has to be picked up and thrown away each season. Straw and other organic mulches work the other way, keeping soil cooler, so hold off on them around warm-season crops until the soil has warmed.
For planting dates by crop, see the Missoula Planting Calendar.
Sources
- Montana State University Extension, Can I Grow That Here? Vegetable Seed and Transplant Schedules
- Montana State University Extension, Successful Home Vegetable Gardening
- Montana State University Extension, Growing Tomatoes in Montana
- University of Minnesota Extension, Extending the Growing Season
- NOAA NCEI, U.S. Climate Normals 1991 to 2020 (Missoula International Airport)