Pick the right growing medium
You can buy bags of seed-starter mix or you can make your own seed-starting mix by blending equal parts of perlite, vermiculite, and peat. Add 1/4 teaspoon of lime to each gallon of mix to neutralize the acidity of the peat. You'll eventually want to repot most of your seedlings into larger containers before setting them into the garden. But lettuce, melons, and cucumbers are finicky about being transplanted and should go directly from the original containers into the garden. When starting these fussier plants, always add two parts well-aged, screened compost to your mix to give them a healthy beginning.
Moisten your medium in the containers before sowing the seeds. Next, drop seeds onto the surface of the mix, spacing them as evenly as possible. Cover the seeds to a depth about three times the thickness of the seeds. Some seeds, such as ageratum, alyssum, impatiens, petunias, and snapdragons, should not be covered at all because they need light in order to germinate.
Top it off
Lightly sprinkle milled sphagnum moss, a natural fungicide, over everything to protect against damping-off, a fungal disease that rots seeds and seedlings. In the case of seeds that need light to germinate, sprinkle the moss first and then drop the seeds onto the moss.
Keep seeds cozy
Cover the flats with plastic wrap or glass to keep the environment humid and place them near a heat vent or on a heat mat made especially for seed starting. Most seeds germinate well at about 70 degrees F.
Keep them damp
Mist with a spray bottle or set the trays into water so the mix wicks up the moisture from below.
At the first signs of sprouting, uncover and move the containers to a bright spot—a sunny window, a greenhouse, or beneath a couple of ordinary fluorescent shop lights (4-footers with two 40-watt bulbs). The lights are worthwhile, especially if you live in the North. They provide a steady source of high-intensity light. Short days restrict window light, and your seedlings need 12 to 16 hours of light a day. Suspend the lights just 2 inches above the plants and gradually raise them as the seedlings mature. If plants have to stretch or lean toward the light, they can become weak and spindly. To turn the lights on and off at the same time each day, hook them up to an electric timer.