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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
minimum/optimum/maximum

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.


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