Seed trays, or flats, are shallow plastic trays that are single planting beds or may have numerous planting compartments. The home gardener can plant directly into soil-filled trays or can use the trays to hold individual peat pots or pellets. Smaller seeds, such as onion, are often started directly into seed trays, as they are easier to thin as the seedlings are established. Seed trays can be found anywhere garden supplies are sold.
Fill seed flats with potting soil to within 1/4 inch of the top of the tray.
Set the trays on a level surface and water until the soil is moist to the touch.
Sprinkle or place seeds on the surface of the moist soil. Follow the seed package instructions for spacing the seed.
Cover the seeds with an additional 1/4 inch of potting soil.
Cover the entire tray with plastic wrap. The wrap does not need to be secure around the tray, just loosely covering.
Remove the plastic wrap from the trays when the seedlings emerge from the soil. Place the tray in a warm, sunny area to allow the seedlings to grow.
Keep the soil moist around the seedlings and thin the young plants according to the label of the specific seed type.
Transplant the seedlings into larger containers after two to three weeks growth or when the primary two leaves are established on the plant.