Gardeners identify the proper time to plant vegetables through checking soil temperature. The standard for measuring soil temperature is to take the temperature 4 inches deep at 8 a.m., according to Colorado State University Extension. Many plants can be started indoors and then transferred outdoors after a period of hardening off. Vegetables are categorized by planting times determined by planting temperatures. For example, hardy vegetables, such as peas and cabbage, are cool-season crops planted when temperatures are between 60 to 80 degrees F. On the other end of the spectrum, very tender crops, including lima beans and watermelon, prefer temperatures between 70 and 95 degrees F.
Cool-season crops considered to be hardy include broccoli, cabbage, kohlrabi, onions, lettuce, peas, radish, spinach, leeks, parsnips, radishes and turnips. Plant when soil temperatures warm up to 35 to 40 degrees F. In general, cool season crops can be planted two weeks before the last frost date. Most cool season crops germinate at maximum soil temperatures of 90 degrees, except for lettuce and spinach, which require temperatures of less than 70 degrees.
Warm-season vegetables categorized as tender crops include corn, beans, celery, cucumber and summer squash. In general, these crops may be planted close to the last frost date for the area. However, when transplanting cucumbers and summer squash, wait until the temperatures warm to those recommended for very tender vegetables. Corn can be planted when the soil temperature reaches at least 50 degrees F. Other tender crops germinate at soil temperatures ranging from 60 degrees to 100 degrees, depending on the crop.
Very Tender Crops
Very tender crops are not tolerant of any frost or cool winds. Planting typically occurs two weeks after the last frost date and when air temperatures are consistently above 55 degrees during the daytime. Germination occurs for very tender crops when soil temperatures are between 60 and 110 degrees F. Very tender crops include watermelons, tomatoes and peppers.