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
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
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
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.¶