If you grow vegetables, one of the most valuable tools around is a soil thermometer. That’s because many vegetable seeds germinate and grow even when the soil is only 45 degrees. On April 6, I took my trusty soil thermometer and discovered that the top three inches of soil where I grow annual herbs was still only 41 degrees. I waited two weeks and it gradually warmed up so I could sow spinach, lettuce, onions and Swiss chard without having the seeds rot in the cold, wet soil.
Here are soil temperature ranges for seedling development. Sowing seeds of some warm-loving vegetables (like beans) before the soil is adequately warmed, is sure to make them rot before they get a chance to sprout. And warm-loving veggies (marked with an asterisk below) should not be sown outdoors before May 15—the average last day when a spring frost could settle in the Chicago area). Otherwise, the seeds may sprout, but the tender green growth is vulnerable to a cold snap and the plant won’t survive. You can start planting these edibles once the soil reaches these temperature ranges:
Snap beans* 65-80 Beets 50-80 Cabbage 50-80
Lima beans* 75-80 Squash 70-85 Turnip 70-85
Parsley* 60-85 Peas 50-80 Radish 50-70
Lettuce 45-70 Chard 50-80 Spinach 45-65
Pumpkin* 70-85 Onion 45-75
You don’t need an expensive digital thermometer. You can get one for about $15 and it will last a long time. Check your local garden center or visit Johnny’s Seeds catalog for a complete description of the optimal soil temperatures for sowing vegetable seeds outdoors. Happy Sowing…