To sow the seeds, fill a flat with moist seed-starting medium. (Johnson shares the recipe for her organic seed-starting mix here.) Level the medium and use a ruler or piece of lath to press shallow rows about 1/4 inch deep and 2 inches apart.
Sprinkle seeds into the depressions. Space seeds about 1/2 inch apart. If planted too closely, the seedlings will be more difficult to separate at transplant time. Label each row to identify the variety planted.
Once all rows are planted, gently scatter more of the seed-starting medium to cover the seeds (unless the packet specifies leaving seeds uncovered). Water the surface with a gentle mist and cover the flat with a plastic dome or sheet of clear plastic to maintain moisture.
Some seeds germinate best in cool soil, and others in warm; check the seed packet for temperature requirements. If the seeds demand warmth, place the flat near a heat vent or on an electric heat mat to warm the soil. Watch daily for emerging seedlings, and as soon as they appear, remove the plastic cover and move the flat to a sunny window or grow lights. At this point, many types of seedlings prefer cool growing temperatures around 65ºF.
When seedlings have grown their second set of leaves, they are ready to be transplanted to individual containers. Use a chopstick or letter opener to lift and loosen the soil beneath a row of seedlings. Gently tug the seedlings apart. Hold the seedlings by their leaves instead of their stems, which are more easily damaged by rough handling. Don’t worry if some roots are torn in the process, or if most of the potting mix falls away from the roots.
Add some potting mix to the bottom of a container (for the tomato seedlings shown, we’re using 3-inch-square pots). Center the seedling in the pot, then add more soil to fill.
Press lightly to firm the soil around the roots.
Using a rose on the watering can, soak thoroughly.
Seedlings grown in a mix that includes compost probably won’t require fertilizer. In compost-free mixes, feed the seedlings lightly every 2 or 3 weeks after moving them to individual pots. Johnson recommends liquid kelp and fish emulsion, mixed at half the recommended rate.
Starting about 2 weeks before you plan to transplant the seedlings into the garden, move them to a sheltered spot outdoors for a few hours at a time. This process, called hardening off, allows the seedlings to adjust gradually to outdoor conditions such as wind and sun. “Slow down on water to let them toughen up,” Johnson says. “Be careful not to put them in direct sunlight right away.” Gradually increase their time outdoors and light exposure.
Indoor seed starting requires the gardener to develop new skills, but in the end, it’s worth it. As Johnson says, “Gardening is more rewarding when you start from seeds.”