KNOWLEDGE FOR LIFE

Soil Temperature and Vegetables

One of the most neglected tools for vegetable gardeners is a soil thermometer. Soil temperature is a much better measure of when to plant than air temperature or the calendar. Planting when soil is too cool can cause some seeds to rot and transplants to sit there.

A number of vegetables can germinate and grow at cool temperatures. For example, peas will germinate and grow well at a soil temperature of 40F. Though lettuce, parsnips, and spinach can sprout at a soil temperature of 35F, they prefer at least 45F for best germination and growth. Radishes also do well at a soil temperature of 45F. Even if the seeds of these cool-season crops are planted below their commended soil temperature, the seed will rarely rot.

Warm-season crops such as tomatoes, sweetcorn and beans are different. They prefer at least 55F for germination (or transplanting), but others such as peppers, cucumbers, melons and sweet potatoes need it even warmer, about 60F. If planted when soils are too cool, they likely will rot before germinating.

Taking soil temperature accurately is a bit of a science. First, use a metal soil thermometer, which is sold in many garden, auto parts and hardware stores. Take temperature 2.5 inches deep at about 10:00 to 11:00 a.m. Temperature variations throughout the day and night affect soil temperature, with lowest readings after dawn and warmest around mid-afternoon. The late-morning reading gives a good average temperature. If taking the soil temperature at this time is not practical, take a reading before you leave for work and a second when you return home and use the average. Also be sure to get a consistent reading for four to five days in a row before planting, and make sure a cold snap is not predicted.

For more information, please contact the local K-State Research and Extension Office.

K-State Research and Extension is an equal opportunity provider and employer.