When it comes to starting seed for the garden, do not start too early and do not start too late.
Seed started indoors should be ready for transplanting into the garden when the soil and weather are ready to take the seedlings into their proper growing season. That means the time to start seeds can differ widely. Some plants grow slower and need more time to develop than others. Depending upon where you live, the garden may need more time–the soil and weather may need to warm–to be ready for the crops you have in mind.
Most annual vegetable seeds can be started in the garden directly or started indoors 2 to 8 weeks before the last frost (most annual flowers can be started 6 to 8 weeks before the first annual frost). Most perennial seeds can be started 8 to 12 weeks before the last frost.
You’ll need a calendar and the approximate frost-free date for your region. The frost-free date is the estimated day in spring–based on local meteorological records–when nighttime temperatures no longer dip below 35°F (1.7°C). (If you are unsure of the frost-free date where you live, check with the county extension office or the public library.) The exact frost-free date can vary from year to year–by several days or even weeks.
Mark the frost-free date on your calendar and plan indoor and outdoor seed starting times counting backward and forward from that date depending upon the seed you plan to sow. So, for example, if you live in Birmingham, Alabama where the estimated last frost in spring comes on March 20, the first vegetable crops to be started indoors, say celery, which needs a 10 to 12-week indoor start on the season, can be planted about January 10.
Mid-winter is the right time to begin planning the garden season. When you know your frost-free date, you will be able to estimate the indoor seed starting date: you can divide your crops into those that need to be started 10 to 12 weeks before the frost-free date, 8 to 10 weeks, 6 to 8 weeks, and 2 to 4 weeks. Make a list of the dates to start each crop: sow basil, pepper, broccoli, and tomato 6 to 8 weeks before the last frost date; sow zucchini 4 to 6 weeks before the last frost date. Now mark those dates with the crop names on your calendar. You can check seed packets or planting guides to know when to start specific kinds of seeds.
If you plan to sow seeds directly in the garden, keep in mind that annual seeds grow best in the garden when the soil is around 60°F (15.6°C)–sow too early and the seeds may rot before they germinate.
Also of interest:
Average Last and First Frost Dates for Cities