So now your seeds have sprouted. Move sprouted seeds to a 50*F spot with high light. I move sprouted trays when they are still “uncurling” or right after. Don’t let them dry to wilting, but gradually withhold moisture. Cool and dry from here on produces well-rooted, sturdy seedlings and minimizes damp off problems. You can recognize damp off if the seedlings’ stems are shriveling at the soil line, causing the plant to wilt and eventually die. When you must water, set the tray into a shallow pan of cool water and let the water wick up into the soil and place the tray somewhere to drain excess water. A laundry sink or bathtub works well and doubles for a drainage area. Keep the foliage as dry as possible.
When a seed sprouts, the first “leaves” are technically not leaves but cotelydons. When the first set of true leaves appear, transplant into a pot about sized about 1 cup. Transplant seedlings, burying the stem up to the cotelydons. Continue to grow on in high light and give moisture as needed. Occasional light feedings of fertilizer can be applied after growing approximately 2 weeks, but don't overdo. Seedlings will outgrow this first pot. You can tell this by looking and the root system. Tip the pot and gently remove the plant and soil ball. When the soil is completely laced with white roots and soil holds the shape of the pot, it is time to transplant to the garden or "uppot". This means planting into a larger size pot. For tomato plants, trim off the cotyledons and lower sets leaves, leaving only the newest 2-3 sets of leaves. Plant again all the way up to the remaining leaves. The tomato stem will develop roots! Some plants can be transplanted directly into the garden and this is a matter of timing, which will be the topic of Part 3.