Cover the seeds to a depth of three times their thickness by carefully sprinkling them with light, dry potting soil or seed-starting medium. Don’t cover seeds that need light to germinate (check the seed packet for special germination requirements). Instead, gently pat the surface of the mix so the seeds and mix have good contact.
Write a label for each kind of seed you plant and put it in the flat or pot as soon as the seeds are planted, before any mix-ups occur.
Set the flats or pots in shallow containers of water and let them soak until the surface of the planting medium looks moist. Or you can gently mist the mix. If you water from the top, use a watering can with a rose nozzle to get a gentle stream that won’t wash the seeds out of place.
Cover the container, using clear plastic or a floating row cover for seeds that need light, or black plastic, damp newspaper, or burlap for those that prefer the dark.
Finally, put the containers of planted seeds in a warm place where you can check them daily. Unless the seeds need light to germinate, you can save space the first few days by stacking flats. Just be sure the bottom of a flat doesn’t actually rest on the planting mix of the flat below. Check the flats daily; unstack as soon as the seeds start to sprout. Keep the soil moist but not waterlogged. As soon as you notice sprouts nudging above the soil surface, expose the flat to light.
To plan the best time to start seedlings indoors in spring, you need to know the approximate date of the average last spring frost in your area. Count back from that date the number of weeks indicated below to determine the appropriate starting date for various crops. An asterisk (*) indicates a cold-hardy plant that can be set out 4 to 6 weeks before the last frost.