Planting Soil Temperatures
for Seeds
There's an old saying that Iowa farmers know that the soil has warmed up enough to plant corn when they drop their drawers and can sit on the ground comfortably. Me, I like to be a little more scientific (not to mention modest) about it. A soil thermometer can be a cold-climate gardener's best friend.
By Veronica Lorson Fowler
The Iowa Gardener
Most seeds—especially peas—are picky about the temperature of their soil, and especially during weird weather years, it can be hard to tell if the soil is warm enough. A soil thermometer ($7 or $8 at a garden center or on-line) tells you in just a minute or two.
Listed below are the soil temperatures at which various vegetables should be planted.* The temperatures are based on temperatures taken at 8 a.m. at 4 inches deep. (For beans, take the temperatures at 6 inches.)
Cool-Season Crops
Vegetable |
Germination Temperature °F |
| Beets | 40°/80°/90° |
| +Broccoli | 40°/80°/90° |
| +Cabbage | 40°/80°/90° |
| Carrots | 40°/80°/90° |
| Cauliflower | 40°/80°/90° |
| Leeks | 40°/80°/90° |
| Lettuce | 35°/70°/70° |
| Onions, green | 35°/80°/90° |
| Onions, dry sets | 35°/80°/90° |
| Parsnips | 35°/70°/90° |
| Peas | 40°/70°/80° |
| Potatoes | 45° and up |
| Radishes | 40°/80°/90° |
| Spinach | 40°/70°/70° |
| Swiss chard | 40°/85°/95° |
| Turnips | 40°/80°/100° |
Warm-Season Crops
Vegetable |
Germination Temperature °F minimum/optimum/maximum |
| Beans | 55°/80°/90° |
| Cantaloupe | 60°/90°/100° |
| Corn | 50°/80°/100° |
| Cucumbers | 60°/90°/100° |
| +Eggplant | 60°/80°/90° |
| +Peppers | 60°/80°/90° |
| +Tomato | 50°/80°/100° |
| Squash | 60°/90°/100° |
| Watermelons | 60°/90°/110° |
* Source: Colorado State University Horticulture Extension
+ Usually planted as established seedlings, not as seed.
