Soil temperature and vegetables
One of the most neglected tools for vegetable gardeners is a soil thermometer. Soil temperature is a better measure of when to plant than air temperature or the calendar. Planting when soil is too cool can cause 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 40 degrees F. Though lettuce, parsnips, and spinach can sprout at a soil temperature of 35 degrees F, they prefer at least 45 degrees F for best germination and growth. Radishes also do well at a soil temperature of 45 degrees F. Warm season crops such as tomatoes, sweet corn and beans prefer at least 55 degrees F for germination (or transplanting), but others such as peppers, cucumbers, melons and sweet potatoes need it even warmer, about 60 degrees F.
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 to 11 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 leaving for work and a second when returning 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.
An excellent guide sheet on this subject is published by the Alabama Cooperative Extension System and is titled “Soil Temperature Conditions for Vegetable Seed Germination.” It can be found at http://www.aces.edu/pubs/docs/A/ANR-1061/ANR-1061.pdf.