By growing hothouse tomatoes in a greenhouse, you can extend the tomato-growing season and enjoy fresh tomatoes "out of season." In southern states, plant hothouse tomatoes in the summer for harvest fall through spring. In northern states, plant hothouse tomatoes in winter for harvest spring through fall.
Poke drain holes in the paper cups or milk cartons with the nail. Fill containers with moistened potting mix. Sow three or four tomato seeds in each container. Cover the seeds with a thin layer of potting medium.
Check the seeds every few days and water them if the potting mix becomes dry. Germination should occur in seven to 10 days. Thin tomato plants to one per small container after the seedlings develop their first true leaves. Allow seedlings to grow for five to six weeks.
Put a layer of gravel on the bottom of each five-gallon container and fill about 2/3 full of potting mix. Take a 5- to 6-week-old seedling out of its small container and set it in the large container. Fill around it with potting mix. Repeat as needed until all of the seedlings have been transplanted into bigger pots. Water each re-potted tomato plant.
Water hothouse tomato plants with about two quarts of water every day or as needed.
Fertilize tomato plants with a 5-10-10 fertilizer mix according to package directions. Consider using half the recommended amount of fertilizer twice as often, as the North Carolina University Extension suggests. This approach makes over-fertilization less likely.
Keep the greenhouse warm, but not hot. Tomatoes set fruit when the growing environment is between 60 to 90 degrees. A heater may be needed in your greenhouse if you are growing tomatoes in the winter.
Provide adequate light for the tomato plants, which do best with several hours of full sun per day. Consider supplementing with artificial lights if you are growing tomatoes in the winter months.
Add a support cage to each container when tomato plants reach about 10 inches high. This eliminates the need for pruning and conserves space since it encourages vines to grow upward. Tuck any stray branches in if they begin growing out the cage holes.