Starting your own seeds is very rewarding and allows you to grow many more varieties than are available at the average nursery. Most of us (myself included) have already started our seeds over the past few weeks. Now the focus should move from getting them to germinate to preparing them for life in the garden.
You should begin to fertilize your seedlings when they get their first ‘true’ leaves. The initial leaves that emerge from a seed are called the cotyledons, and they are usually rounded with smooth margins. The second set of leaves to emerge are the true leaves, and they begin to look like the foliage of the mature plant. Up until this point in their growth, the seed itself supplies nutrients to the developing plant. When they have fully emerged, it’s time to begin fertilizing and transplanting.
Many commercial seed-starting potting mixes contain some nutrients but usually in minimal quantities to prevent burning new seedlings. When those true leaves arrive, transplant the seedlings into larger containers or cell packs and use a potting mix that already contains a nutrient source. I like to use a locally mixed, certified-organic potting soil that uses compost and other organic sources to supply those nutrients rather than chemicals, but there are lots of different potting mixes available on the market or you can make your own.
Two or three weeks after transplanting, begin to fertilize the seedlings with a diluted kelp emulsion, fish emulsion or liquid worm castings. Dilute these products to half the recommended strength, and use them every two weeks or so. You can also make your own liquid fertilizer—aka compost tea—by filling some old pantyhose with compost and letting it ‘steep’ in a bucket of water for a day. Use the compost tea to fertilize the seedlings every few weeks.
One last thing to consider: Harden off your seedlings before placing them into the garden. May 15 is the last expected frost date at my house, so for two weeks prior, I work on acclimating my seedlings to outdoor conditions. I begin by placing them in a shady spot outdoors for just a few hours, then gradually leave the seedlings outside for longer periods of time and expose them to more sunlight until they are outside full-time. This hardening-off process is extremely important to young transplants and helps them gradually adjust to brighter light levels, wind and fluctuating outdoor temperatures. Once they are fully hardened off and the weather conditions are right, I move them out into the garden.