Read the back of the seed packets chosen for the garden. Some seeds need soaking, while others require stratification, or a period of cold temperatures, before planting. Other seeds may need their hard outer shell nicked to help the germination process.
Determine the last expected frost date for your area. Seed packets generally have information about when to start seeds for outdoor gardening. If the packet states five weeks from the last expected frost date, use the Hardiness Zone Map to find the last known frost date for your area and count backwards five weeks. This is when the seeds need to be planted in the greenhouse trays. Different varieties will require different planting times.
Mix the potting soil with just enough warm water to make it moist but not soggy. Pre-moistened soil alleviates the need for watering during seed germination. The soil should form a ball when squeezed in your hand, but fall apart easily when crumbled.
Fill the greenhouse trays with the potting soil. Make holes in the middle of each cell to hold the seeds. Some seeds need to be covered with soil, while others require light to germinate. Refer to each variety's growing instructions to determine planting depth and lighting requirements.
Plant the seeds according to the packet directions and label the trays according to plant variety. Cover the trays with the cover or clear plastic and set them in a warm place with a good light source. If window lighting is not adequate for plant growth, place them under a grow light.
Use the heat mat where temperatures are below 70 degrees. Seeds need warmth to germinate, and the optimum temperature for most seeds is 65 to 75 degrees. Keep the greenhouse trays on the heat mats, if necessary, until the seeds have germinated.
Monitor the greenhouse trays daily to ensure the seeds are free from mold. Watering should not be necessary because of the greenhouse effect created by the covers or plastic. However, if the soil appears dry, spritz the surface with water or use the wick system, in which you place the growing tray into another container of water and let the water be absorbed up from the bottom.
Remove the plastic once the seeds have germinated. Continue moistening the soil by wick watering or spraying the surface lightly with a misting bottle. To avoid damaging the roots or plants, do not pour water directly onto the seedlings.
Turn the greenhouse trays daily once the seeds have germinated to keep growth even. Plants grow toward the light source and may grow uneven if the light is only located on one side. Seedlings growing spindly and seeming to stretch sky-ward require closer proximity to the light source. Move the tray up or the light down.
Harden off the plants before planting outdoors. Hardening off is a process of slowly acclimating the seedlings to the outside temperatures. Move the growing trays outside into bright, but not direct sunlight for an hour or two each day. Do this for at least a week for stronger plants. If the temperatures are cold, wait until the weather warms up. Protect the new plants until their stems are hardened off for planting.