Check Soil Temperature
Soil temperature is a factor few gardeners check before planting, yet it is probably the most important factor affecting seed germination and plant growth.
Soil temperatures at which vegetable seed will grow include the minimum temperature required for seed growth, an optimum temperature and a "realistic" soil temperature. The realistic soil temperature is somewhere between optimum and minimum; it is the temperature at which you should plant to insure maximum success.
For instance, the optimum soil temperature for seed germination of vegetable crops, such as cucumber, cantaloupe, okra, pumpkin, squash and watermelon, is 95°F. However, if planted late enough during the year for soil to reach this temperature, summer heat will decrease plant vigor and yield. The minimum soil temperature for these crops is 60°F; yet, at this temperature the seed will not grow vigorously. Thus, there must be an intermediate soil temperature that is more realistic. Realistic soil temperatures for the best plant production are cucumber (64°F), cantaloupe (68°F), okra (73°F), pumpkin (75°F), squash (70°F) and watermelon (72°F).
Crops such as beans, beets, cabbage, chard, eggplant, pepper, radish, tomato, turnip and corn have an optimum soil temperature for seed germination of 85°F. The minimum soil temperature required for some of these cold-tolerant crops, such as beets, cabbage, chard, radish and turnip, is as low as 40°F. The realistic soil temperatures recommended for these crops are beans (72°F), beets (45°F), tomato (55°F), turnip (50°F) and corn (55°F).
As might be expected, vegetables which are really cold-tolerant, such as carrots, parsley, lettuce and spinach, have lower optimum soil temperatures for seed germination. For instance, the optimum soil temperature for seed germination of carrots is 80°F, for parsley and lettuce 75°F, and for spinach 70°F. The minimum temperature required for these crops is 35°F. The realistic soil temperature at which all of these crops should be planted is 45°F.¶