According to Oklahoma Cooperative Extension Service Educator Roger Williams, soil temperature is key to growing a successful vegetable garden.
Each spring, the warm air temperature puts most gardeners in the mood to plant. The best way to determine the time to plant vegetables in your garden is soil temperature.
This year, the soil temperature is running about the same as this time last year, about 50 degrees. When the soil temperature is within an acceptable range, transplanted plants and seedlings have the best chance to develop into healthy plants. When the soil is too cold plants are much more susceptible to stunting, fungus, and insect pests.
Soil temperatures at which vegetable seeds will grow are classified into four categories: the minimum temperature; the optimum temperature; a realistic soil temperature; and the maximum temperature. For instance, the optimum soil temperature for seed germination of cucumber, okra, and squash is 95 degrees. If you waited until the soil temperature was 95 degrees, the hot summer would reduce germination, plant vigor, and yields.
Here are some crops and realistic soil temperatures at which they should be planted: carrots, 45; cabbage, 54; squash, 70; parsley, 45; chard, 54; beans, 72; lettuce, 45; corn, 55; watermelon, 73; spinach, 45; tomato, 58; okra, 73; beets, 45; pepper, 64; pumpkin, 75; radish, 45; cucumber, 64; eggplant, 75; turnip, 50; and cantaloupe, 68 degrees.
In the spring, it is best to plant at the beginning of a warming trend when the soil temperature has been increasing for at least three days. The hardiest vegetables could be planted at this time but many would not flourish in this temperature. During this time of year, the average soil temperature can increase several degrees each week and vegetables planted at the proper temperature reflect that preferred temperature range.