According to the horticultural experts at the University of Missouri, plants germinated in a greenhouse flower sooner and produce an earlier harvest than seeds sown outdoors. If you plan to transplant your greenhouse-started plants outdoors when the season begins, plant them four to 10 weeks before they would normally be planted outdoors. The number of weeks a plant needs to reach transplant-able size largely depends on the species, so it pays to do a bit of research. Furthermore, not all plants tolerate greenhouse growth well and some are better sown directly outdoors.
Choose an appropriate container. Multi-cell seed trays are an easy way to start plants that will be transplanted outdoors as soon as they reach a few inches in height. Some plants do not like to have their roots disturbed. These plants fare better in 2-inch peat pots that can be planted directly into the soil once the plant is large enough. If your plant will remain in the greenhouse, seed it directly into a clay or plastic pot suitable for its adult size.
Fill the seeding trays and small peat pots with moistened commercial seed-starting potting soil. Keep the soil roughly 1 inch below the rim of larger pots. Press the soil down gently with your fingers to remove any air pockets.
Sow the seeds. The general rule for seed planting depth is three times the diameter of the seed. Tiny seeds can be cast on the surface, pressed in with your finger, and covered with a 1/8-inch layer of vermiculite.
Place the seeded containers somewhere where they will receive full, direct sunlight for most of the day. Keep them off the ground where the air is cooler.
Keep the soil moist (not soaking) at all times by moistening the surface of the soil with a fine mist from a spray bottle. The soil should not be allowed to dry out at any time.
Keep the temperature in your greenhouse between 72 and 76 degrees Fahrenheit (the optimal germination temperature for most plants) or at the optimal germination temperature for the seeds you have sown.