A greenhouse is an ideal place to start tomato seeds, whether you plan to grow the plants in the greenhouse or outdoors in the garden. Because of the humid environment in a greenhouse, use a soil-free seed starting medium, such as vermiculite, peat moss or a commercial seed starting mix.
Fill individual 2-inch pots with vermiculite, peat moss or other seed starting mix.
Sow 3 to 4 seeds per pot. Bury the seeds about 1/8 inch deep and lightly firm the growing medium over them.
Set the individual pots into a large shallow pan. Add enough water so it comes about halfway up the sides of the 2-inch pots. Allow the pots to sit in the pan of water until the surface of the growing medium looks moist, but not soaking wet.
Remove the 2-inch pots from the pan of water and set them on a tray to allow excess water to drain out.
Set plants on a shelf that is 2-4 feet above floor level in the greenhouse. This will keep them warm and out of drafts. Position them so that they are right next to the glass wall and receive the maximum amount of sun, unobstructed by other plants.
Water from the bottom (as outlined in Step 3) as needed to keep the seed starting medium moist until germination occurs in about 7 to 10 days.
When the tomato seedlings have two sets of leaves, thin to the strongest seedling in each pot. Snip the others off at soil level with a small scissors so as not to disturb the roots of the plant that remains.
Feed tomato seedlings every 2 weeks after you thin to the strongest seedling in each pot. Use a water soluble granulated fertilizer that is dissolved in water. Mix the fertilizer to half the strength recommended by the manufacturer. Apply fertilizer by watering the pots using a watering can with a rose sprinkler attachment. Allow excess fertilizer mixture to drain out of the bottom of the pots. Do not allow pots to sit in the drained fertilizer mixture.
When seedlings are about 4 inches high, water them using a rose sprinkler attachment on a hand watering can rather than by sitting the pots in a pan of water.
Transplant seedlings to gallon-size pots for growing in the greenhouse or transplant into the garden six weeks after germination.