Greenhouses provide an efficient and inexpensive way to extend your growing season in spring and fall and to grow hardy plants year-round. Almost any vegetable that grows in your garden outdoors can grow well in a greenhouse. In commercial greenhouses, tomatoes are a very popular cash crop, while amateur greenhouse growers often prefer cold-tolerant vegetables like Brussels sprouts and cabbage due to their cold tolerance.
Mix a sterile potting soil for your greenhouse comprising one part peat moss, one part compost, one part vermiculite and one half part composted manure. Never use garden dirt in a greenhouse. The dirt contains microbes that can reproduce too rapidly in a container and harm the roots of plants. Dirt may also carry soil-borne diseases.
Place a thermometer in your greenhouse and check the internal temperature of the greenhouse up to four times daily. Open your greenhouse doors or vents any time the internal temperature gets too high. Cold-tolerant plants such as lettuce grow best if it receives nighttime temperatures between 45 and 50 degrees F and daytime temperatures of 60 degrees. Tomatoes, melons, cucumbers and peppers prefer 60 to 65 degrees at night and daytime temperatures around 85 degrees. Tomatoes will not produce fruit abundantly when temperatures climb above 90 degrees.
Walk a path through your greenhouse each morning and check the soil of each plant in the greenhouse thoroughly. Water plants whenever the soil seems dry. Soil should remain as damp as a wrung-out sponge.
Examine each plant as you water it for signs of pests and diseases. Remove a plant from your greenhouse if it shows signs of mold, mildew or disease. Space each plant far enough apart that the plants have good circulation between them to prevent diseases such as mold and mildew. Wash a plant with insecticidal soap if it shows signs of common greenhouse pests such as aphids, spider mites or white flies.
Pollinate plants like tomatoes by shaking or tapping the stem gently. Hand pollinate melons and squash by picking male flowers and brushing the pollen over the inside of the female flowers.