Soil Temperature

By Dr. Jerry Parsons, Extension Horticulturist
Texas Agricultural Extension Service

ore important than moon signs and more predictable than weather is another variable which drastically affects how seeds and transplants grow - soil temperature. Soil temperature is a factor which few of us consider important enough to check before planting yet it is probably the most important factor affecting seed germination, stand establishment and seedling growth.

Soil temperatures at which vegetable seed will grow are classified into four categories - the minimum temperature required for seed growth, an optimum temperature, a “realistic” soil temperature and, maximized temperature above which little germination will occur. The realistic soil temperature is that temperature somewhere between optimum an minimum at which gardeners 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 degrees F, but the heat of summer will decrease yields and plant vigor. Yet if a gardener plants when the soil temperature reaches only 60 degrees F, the vegetable seed will germinate and grow, but not vigorously. Thus, there must be a “realistic” soil temperature. For the above-mentioned crops, the realistic soil temperatures (degrees F) for best plant production are: cucumber (64), cantaloupe (68), okra (73), pumpkin (75), squash (70), and watermelon (72).

Crops such as beans, beets, cabbage, chard, eggplant, pepper, radish, tomato, turnip and corn have an optimum soil temperature for seed germination of 85 degrees F. Yet the minimum soil temperature required for certain of these cold-tolerant crops such as beets, cabbage, hard, radish and turnip is as low as 40 degrees F. The realistic soil temperature (degrees F) recommended for these crops are: beans (72), beets (45), cabbage (54), chard (54), eggplant (75), pepper (64), radish (45), tomato (55), turnip (50), and corn (55).

As might be expected, those vegetables which are really the cold weather champs 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 degrees F, for parsley and lettuce is 75 degrees F, and for spinach is 70 degrees F. The minimum temperature required for these crops is 35 degrees F. The realistic soil temperature at which all of these crops should be planted is 45 degrees F.

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This article appeared in the April 2001 issue of Horticulture Update, edited by Dr. William C. Welch, and produced by Extension Horticulture, Texas Agricultural Extension Service, The Texas A&M University System, College Station, Texas 77843.