For optimum growth, all transplants and seeds need to be planted within a certain soil temperature range. Along with your calendar, a soil thermometer will help determine the proper planting time.
Planting too early, before the soil has had time to warm up, can lead to seed rot, slowed germination, poor growth and disease. For example, cucumber seeds usually take less than a week to germinate in a soil of 70 degrees F. They could take two weeks at 60 degrees F. Tomato transplants need a soil temperature above 60 degrees F for growth. Setting pepper plants out before the soil temperature is 70 degrees F could stunt their growth for the entire growing season.
Correct temperature readings need to be observed on three consecutive mornings. Readings should be taken at a depth of 1 – 2 inches for seeds and 4 – 6 inches for transplants.
Use the following guide for minimum soil temperatures for seeds and transplants:
60 F – tomatoes, cucumbers, snap beans
65 F – sweet corn, lima beans, mustard greens
70 F – peppers, watermelons, squash, southern peas
75 F – okra, cantaloupe, sweet potatoes
Soil thermometers may be purchased at local nurseries and hardware stores or ordered from gardening catalogs.
Linda Scoggin, Smith County Master Gardener
Texas A&M AgriLife Extension Service