Overview
Generic seed planting information is all based on the last average frost date; but the real date for the last frost can vary widely from year to year. And the last frost date has to do with air temperatures, not soil temperatures. In our rapidly changing climate, a calendar date is not the best predictor for an extra early onset of spring weather in March or an extremely late, cold and wet April. A better method to gauge when to put seeds in the ground is to test soil temperatures.
Difference between soil temperature and air temperature
Soil temperature is not the same as air temperature. Air temperature can change rapidly with a late winter storm or a warm, sunny day. Soil temperatures change much more slowly because of soil mass and moisture content. Whether soil is exposed (or not) to sun matters as well. A sunny day warms a dark surface like open ground, or ground covered with black plastic, faster than soil well mulched with light colored hay. Clear plastic also works to warm soil with a ‘greenhouse effect’, trapping heat and moisture. Even raised beds, if 4-8” tall or more will be warmer in the upper couple of inches of soil than soil at ground-level.
When do seeds break dormancy?
A seed itself knows when to break dormancy based on temperature. For example, yellow pear tomato ‘volunteer’ seeds dropped last fall in waste tomatoes (if they survive the winter) will wait until the soil gets above 70° F before putting out a root, so that rare warm March day doesn’t fool the seed. If you plant too early in cold soil, the seed may rot, but if it does geminate, it will be slow to do so and likely to produce stunted plants that are more prone to disease. Give seeds the optimum soil temperature and seedlings will emerge more quickly and grow rapidly in size and strength.
How to measure soil temperature
To take soil temperature, you can purchase a soil temperature thermometer and use just for testing soils. Take temperature readings on three consecutive mornings at the same time, usually mid-day (between 10 a.m.-12 p.m.). Insert the probe to a depth of 2 inches for seeds or 4-6 inches for transplants. Use the chart below to guide you on when to plant seed (measured at the two-inch depth) or set out transplants (measured at the 4-6 inch depth). Of course, there might still be a late freeze, so be ready to cover the bed if necessary.
Soil temperature minimums and optimal temperatures for seed germination
The following chart gives minimum temperature to plant, optimum temperature to plant, and a viable range.
| Crop Seed Type | Minimum Soil temperature | Optimum Soil temperature | Germination temperature viable range |
|---|---|---|---|
| Beets | 45° F | 85° F | 40-95° |
| Carrots | 45° F | 80° F | 40-95° |
| Lettuce | 45° F | 75° F | 35-85° |
| Parsley | 45° F | 75° F | 40-90° |
| Radish | 45° F | 85° F | 40-95° |
| Spinach | 45° F | 70° F | 35-85° |
| Asparagus | 50° F | 75° F | 60-85° |
| Peas | 50° F | 75° F | 40-85° |
| Turnip | 50° F | 85° F | 40-105° |
| Cabbage | 55° F | 85° F | 40-105° |
| Cauliflower | 55° F | 80° F | 40-100° |
| Corn | 55° F | 95° F | 50-105° |
| Swiss chard | 55° F | 85° F | 40-95° |
| Onion | 60° F | 75° F | 35-95° |
| Celery | 65° F | 70° F | 40-85° |
| Cucumber | 65° F | 95° F | 50-105° |
| Pepper | 65° F | 85° F | 60-95° |
| Cantaloupe | 68° F | 90° F | 60-100° |
| Squash | 70° F | 95° F | 60-100° |
| Tomato | 70° F | 85° F | 50-95° |
| Beans | 72° F | 80° F | 60-95° |
Author: Lisa Johnson, Horticulture Educator, UW-Madison Extension Dane County
Reviewers: Colleen Callahan, Carol Shirk and Doug Soldat
Revised: January 2025
References
- University of Nebraska-Lincoln Institute of Agriculture and Natural Resources – Extension Publications – Soil Temperature G2122 – https://extensionpubs.unl.edu/publication/g2122/soil-temperature
- The Iowa Gardener: https://theiowagardener.com/wordpress/planting-soil-temperatures/
