Prep the Planting Tray Fill a planting tray with moist seed-starting mix, usually a soil-less product that combines perlite, vermiculite and peat moss. Sow the Seeds Sow the seeds 1/4 inch deep, firming the moist seed-starting mix over the seed. Place one seed per plug if your seed tray has inserts that hold the mix in separate 1- to 2-inch compartments. Otherwise sow seeds a few inches apart so seedlings will be easy to remove later.