7. Give them space.
Those well-watered, well-fed, and well-fanned seedlings will soon need more root space. Shortly after the second set of true leaves appears, take a deep breath and thin your seedlings to one per pot. Use small scissors to clip off the weaker plants at the soil line, leaving only the stockiest plant.
Next, carefully “pot up” the survivors into larger, 3-or 4-inch pots. Squeeze the sides of the smaller containers all around, turn them upside down, and the plants should come out easily—soil and all. Immediately set them into the larger containers and fill with a mixture of 3 parts potting soil and 1 part your own screened compost.
(If you started your seeds in peat pots or homemade newspaper pots, you can plant both the seedling and its pot in the larger container; the pot eventually will decompose.)
Plant tomatoes deep in the new container to encourage them to develop a larger root system to support these often top-heavy plants. With most other plants, the soil level in the new pot should be about the same as in the smaller container. After you’ve finished repotting, water the plants well and set them back under the lights.
About a week or two before you plan to transplant your seedlings to the garden, begin taking them outdoors to a protected place, such as inside a coldframe or near a wall, for increasing lengths of time on mild days. This will help them adjust to the conditions outside—a process known as hardening off. Start with just a couple of hours each day, work up to a full day, and then leave them out overnight.
When you finally transplant the seedlings to the garden, be careful not to disturb their roots. Gently pop them out of their containers, keeping as much soil attached to their roots as possible. Again, plant tomatoes deeply, but set other plants at about the same depth as they were in their pots (or just slightly deeper).
9. Seal it with a K.I.S.S.
Most important, relax! Don’t worry if you forget to do something or don’t follow all the “rules.” Except for hardening off, all of these rules are flexible. Before long, you will learn what works best for you—and will have a few secrets of your own to share with fellow seed starters.