Planting Soil Temperatures

soil thermometer  copy
A soil thermometer takes fast, accurate readings of soil warmth or coolness.

The coolness or warmth of the soil (rather than the air) is key in knowing when to plant seeds or seedlings outside directly in the soil.

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. But it’s a bit more practical (not to mention modest) to be scientific about it. A soil thermometer can be a cold-climate gardener’s best friend.

Most seeds—especially early in the season—are picky about the temperature of their soil. Here in the Midwest, the weather can vary so wildly that ideal planting times can also vary by up to three or so weeks. So don’t leave it up to chance: Invest in a soil thermometer (about $10 and here’s a good one on Amazon) 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 temperature 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
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.

Other Links of Interest:

How To Direct Sow Seeds In Iowa

The Easiest Plants To Start From Seeds

Please do not use text, photos, or illustrations on this web page, web site, or newsletter without express permission from The Iowa Gardener. Many materials are copyrighted and reuse is violation of that copyright. Click here to request permission to use. Thank you!