Soil texture and temperature are the most accurate indication of when to plant vegetables. As a general guideline, vegetables that grow in spring should be planted in late winter, summer vegetables in early spring, and autumn vegetables in summer when temperatures are beginning to fall. Again, though, what type of weather determines “spring”?
For most vegetables, spring means when soil temperatures are consistently above 50 F (10 C). Even if the soil is warm, though, avoid planting if it’s still muddy, as wet soil tends to harden and make it harder for plants to put down roots. Soil is ready for planting when it can be formed into a ball that crumbles easily. Once soil temperatures are between 65 and 75 F (15-25 C), summer vegetables can be planted. Keep in mind, too, that the terms "cool-season" and "warm-season" refer to a plant’s preferred growing conditions, not planting times. If you live in a desert or tropical climate, some cool-season vegetables like beans and lettuce won’t survive the summer and should be planted as early in the year as possible. The good news, though, is that a second crop can be planted when the weather cools to the 70s F (20s C).
While it’s risky to sow too soon after the frost, the long growing seasons required by some vegetables make it critical to get these plants started early. Another benefit of an early start is that the earlier vegetables are planted, the less likely they are to experience insect problems. In areas with short growing seasons, plants can be started indoors so that they’re ready to be transplanted to the garden as soon as the appropriate weather arrives. Sow indoors just early enough to give the plants time to become strong seedlings. Plants sown indoors too early may become spindly from lack of light and space.
The Old Farmer’s Almanac and phenology are particularly useful for vegetable gardening. While the Almanac’s information is conveniently gathered in one book, phenology resources usually have to be sought locally. Some guidelines are relatively well-known, though. For example, tradition holds that Native Americans in New England planted corn when the oak leaves were the size of squirrels’ ears. Potatoes should be planted when the soil temperature is in the 50s F (10-15 C), which is approximately the temperature in which forsythia comes into bloom.
If you’re taking guidance from the moon, the new moon is the best time to plant fruits or vegetables that bear their seeds on the outside, such as strawberries, lettuce, spinach, and grains. Melons, peas, tomatoes, and other plants that carry their seeds inside should be sown or transplanted when the moon is waxing, between the first quarter and full.