All gardeners want to enjoy their harvest as early as possible. But the ideal time to plant vegetable seeds depends on local growing conditions and the choice of vegetables. Like people, vegetables have preferences; some require a long, hot summer to ripen, while others thrive in the cool days of spring and fall. Choosing vegetables that will do well in local conditions and planting vegetable seeds at the best time for the particular area will get the best results.
The single most important element to consider in deciding when to plant vegetable seeds is the presence of frost. Most vegetables are planted after the last spring frost. The growing season is the time between the last frost in the spring and the first frost in the fall, and the choice of vegetables as well as seed planting schedule depends on the length of the growing season. In the United States, you local agricultural extension service can tell gardeners the average first and last frost dates for the area. The growing season is longer in warm regions, but gardeners must plan for the extreme heat of summer, which some vegetables cannot tolerate.
Gardeners are often told to plant vegetable seeds as soon as the soil can be worked or when the soil is warm because soil temperature and condition determine whether seeds will sprout or rot. In general, the soil should be pleasant to the touch, and a handful should be loose and crumbly. A simple soil thermometer will give accurate readings, and many seed companies suggest a specific soil temperature for each vegetable.
Some vegetables, like tomatoes and peppers, require a lot of sun, so they are unlikely to do well in regions with a high number of cloudy days. Others, like carrots, onions and peas, tolerate overcast skies or light shade. Day length is important for some vegetables because changes in the number of hours of sunlight cause the plants' growth habit to change. Onions, for example, are categorized as long-day or short-day varieties; long-day onions grow well in northern climates while short-day onions thrive in southern regions.
Cool Season Vegetables
Vegetables that thrive in the cool weather of spring and fall but stop producing or go to seed in summer's heat are called cool season vegetables. Lettuce is a good example. Typically these vegetable seeds can be planted very early in the season, while the ground and air are still cold. Often, a second planting can be started in the late summer so that these vegetables can be harvested in the fall; they usually tolerate light frost. In warm regions, these vegetables can often be grown through the winter.
Warm Season Vegetables
Many vegetables require the warm days of summer to ripen successfully. Often, they also require a long growing season. This makes them ideal choices for warm regions, but gardeners in cold regions may need to use growing methods that will extend the growing season.
Extending the Season
One of the best ways to extend the growing season is by starting plants indoors and transplanting them into the garden when the growing conditions are right. These vegetable seeds should be planted in pots long enough before the local last frost date to let them sprout and grow to a good seedling size; seed companies often publish guidelines for time from planting to germination and transplant size. In the garden, it is possible to plant vegetable seeds earlier in the spring and harvest later in the fall with the use of row covers, cloches, grow tunnels and mulch.