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 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.
Other Links of Interest:
How To Direct Sow Seeds In Iowa
The Easiest Plants To Start From Seeds
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