Growing tomatoes in the greenhouse allows you to enjoy "out of season" fresh tomatoes. In southern states, extend the tomato-growing season by planting greenhouse tomatoes in the summer, for fall through spring harvest. In northern states, plant tomatoes in the greenhouse in winter for harvesting spring through fall.
Use a nail to poke drain holes in the milk cartons or paper cups. Moisten the potting mix and then fill the small containers with it. Plant three or four tomato seeds in each container. Sprinkle a thin layer of potting mix on the top to cover the seeds.
Keep soil moist until germination takes place, which should be about seven to 10 days. After tomato seedlings develop their first true leaves, thin tomato plants to one per container. Water seedlings whenever soil becomes dry. Allow seedlings to grow for about five or six weeks.
Place a thin layer of gravel on the bottom of each five-gallon container you will use for transplanting. Fill containers about two-thirds full of moistened potting mix. Set a seedling into a large container and fill around it with potting mix. Repeat until all of the seedlings are in bigger pots. Water each tomato plant.
Water your tomato plants in the greenhouse daily, or as needed, with about two quarts of water each.
Fertilize tomato plants using a premixed 5-10-10 fertilizer, following package directions. The North Carolina University Extension suggests using half the recommended amount of fertilizer twice as often to reduce the likelihood of over-fertilization.
Keep the greenhouse temperature between 60 to 90 degrees, the temperature range at which tomatoes will set fruit. A shade cloth may be helpful for preventing the greenhouse from getting too hot; a greenhouse heater may be needed when growing tomatoes in the winter.
Make sure tomato plants receive adequate light. Tomato plants growing in the winter months may require supplemental artificial light since 10 hours or more of light will ensure proper plant growth.
Insert a tomato cage into the soil in each container as tomato plants reach about 10 inches tall. Cages eliminate the need to prune and help conserve space as they encourage vines to grow up, not out. Tuck in stray vines if they begin to grow out cage holes.