Vegetable seeds and seedlings require minimum soil temperatures to germinate and grow. Seeds and seedlings require optimal soil temperatures to thrive.
Soil temperature triggers not only seed germination but is an important factor in soil chemistry. Soil chemistry includes the release (dissolution) of mineral nutrients in soil moisture. Mineral nutrients are essential for vegetable plant growth and maturation to harvest.
The ideal or optimal soil temperature for planting and growing most vegetables is 65° to 75°F (18°-24°C).
Taking Soil Temperature
Soil temperature can be measured with a soil thermometer or gauge. Most home vegetable gardeners use a soil thermometer—a thermometer attached to a metal probe several inches long which is inserted into the soil.
The soil temperature for seed sowing should be taken between 1 and 3 inches deep. The soil temperature for transplants should be taken at 4 to 6 inches deep.
Commonly the temperature reading is taken after the thermometer has been in the soil for a couple of minutes.
Soil temperature is best taken in the early morning when the soil is coolest and not yet warmed under the day’s sun.
Take the soil temperature for at least three consecutive days and then average the results. Don’t depend on just one reading.
Planting and Soil Temperature
Vegetable seeds can be sown in the garden early in spring before the soil has warmed to optimal germination temperatures. If you sow early before temperatures are ideal, you cannot expect optimal germination.
Optimal germination and growing temperatures may not come until late spring or early summer. In regions where the growing season is short, waiting for optimal soil temperatures may not be practical or realistic. A 70 percent germination rate is often considered both practical and realistic.
You can use the minimum soil temperature for germination and the optimal soil temperature for germination to decide about what soil temperature you want to get started sowing seeds and setting out transplants.
Minimum Soil Temperatures for Seed Sowing and Germination:
- 35°F: lettuce, onion, parsnip, spinach.
- 40°F: beet, broccoli, Brussels sprouts, cabbage, carrot, cauliflower, collards, Asian greens, Chinese cabbage, fava beans, kale, kohlrabi, mustard, arugula, radish, Swiss chard, turnip, pea, radish, rutabaga.
- 50°F: asparagus, celery, celeriac, corn, tomato.
- 60°F: bean, cucumber, eggplant, muskmelon, pepper, pumpkin, squash, watermelon.
Soil Temperature Needed for 70% Germination:
- 45°F: beets, lettuce, parsley, spinach.
- 50°F: broccoli, Brussels sprouts, cabbage, carrot, cauliflower, collards, Asian greens, Chinese cabbage, fava beans, kale, kohlrabi, mustard, arugula, radish, Swiss chard, turnip, pea, radish, rutabaga.
- 55°F: cabbage, corn, Swiss chard, tomatoes.
- 65°F: cucumbers, peppers.
- 70°F: beans, cantaloupe, melons, squash.
- 75°F: eggplant, okra, pumpkins.
Optimal Soil Temperature for Germination (near 100% germination):
- 65°F: parsnip.
- 70°F: spinach.
- 75°F: asparagus, lettuce, onion, parsley.
- 80°F: bean, carrot.
- 85°F: beet, cabbage, eggplant, pepper, radish, Swiss chard, tomato, turnip.
- 90°F: muskmelon.
- 95°F: corn, cucumber, pumpkin, squash, watermelon.
Workable Soil Test for Direct Seed Sowing and Transplants:
Before soil thermometers were used in gardens and farms, the common method of determining when to plant was soil workability. (This is the tried-and-true old-fashioned way to know when to plant.)
The soil is workable and ready for seed sowing or planting if it passes the Workable Soil Test. Here’s the test: squeeze a handful of soil in the palm of your hand; when you open your hand if the soil remains a wet or very moist clump, it is not workable. Let the soil dry. If the soil crumbles from your hand with a touch, it is workable.
When the soil is workable in spring, you can:
- Direct sow: collards, kale, lettuce, parsnip, peas, radish, rutabaga, spinach, turnips.
- Transplant out: broccoli, Brussels sprouts, cabbage, onion.
Of course, once the soil is workable in spring, it will continue to warm. Often, old-time farmers would look to lilacs and other spring-flowering plants to decide when to plant. For more on this see the articles on Phenology: Nature Planting Signals for Vegetables.
More tips at: Pre-Warm Your Soil Before Planting Vegetables
See also: Soil: Making the Kitchen Garden