How to Grow Tomatoes in an Unheated Greenhouse

Plant tomato seeds in your unheated greenhouse about six weeks before your final spring frost. Fill one or more nursery flats to within one inch of the rim with standard potting soil and sprinkle with water to moisten the soil surface. Broadcast tomato seeds on the soil and then cover with half an inch of potting soil. Keep the soil moist by watering daily and make sure that the seeded flat gets full sun at least six hours each day.

Transplant young tomato plants to larger pots when they are 3 to 4 inches tall. Use pots that are at least 12 inches in diameter and use only pots with drainage holes. Use the same type of potting soil that you used for your flats. Plant only one tomato plant in each large container.

Hang fluorescent shop lights above your potted tomatoes if your unheated greenhouse receives less than six hours of direct sun each day. Hang lights so that you can raise them as your plants grow taller: keep lights three to four inches above the tops of plants at all times.

Fertilize young tomato plants with a balanced plant food once before they begin to flower. Fish emulsion or another food with an N-P-K ratio of 8-8-8 (percentages of nitrogen, phosphorus and potassium) is suitable. Fertilize again after flowers begin to form, using a plant food that is lower in nitrogen, such as 8-32-16 or 6-24-24: this will cause plants to produce more flowers and fruit.

Allow the soil of potted tomatoes in your unheated greenhouse to dry out before you water them. If plants begin to wilt a bit, this is the time to give them a deep soaking by flooding the pot.