Tomatoes prefer warm soil and air temperatures between 75 and 85 degrees Fahrenheit during the day and above 55 degrees at night. When temperatures drop below 55 degrees Fahrenheit, tomatoes suffer and may be set back a month or more. For the northern or short-season garden, temperatures may drop before tomatoes reach maturity, as the days to maturity from seeding to harvest ranges from 90 to 130 days. Starting them inside from seed makes it possible to grow tomatoes in nearly any home garden in the United States.
Fill seed trays or flats with moist seed starter six to eight weeks before the last expected frost in your area.
Plant tomato seeds to a depth of a quarter-inch and cover with seed starter. Firm the soil down with your hands to settle the seeds. Mist with water to moisten.
Cover with plastic wrap to hold in heat and humidity. Punch several holes in the plastic with the point of a pencil to allow for air circulation.
Place the seed tray in an area that receives bottom heat. The top of a water heater, TV or other appliance that provides gentle heat is ideal.
Check daily for the emergence of seeds and keep the soil evenly moist. Open the plastic to allow for air circulation if the soil becomes soggy or you notice signs of mold on the surface of the soil.
Prop the plastic up with a pencil or dowel to hold the plastic above the tops of young seedlings until all seeds have germinated and seedlings are 1 inch high. Seedlings appear in three to 10 days, depending on the soil temperature and variety of tomato.