A greenhouse can be a great asset when growing vegetables in your garden, extending your growing season and even providing you with fresh vegetables through the winter and early spring. Successful vegetable growing requires the right environment in your greenhouse. Environmental elements to be considered are heat (in cooler months), cooling (in warm months), ventilation, air circulation, shading, humidity and lighting.
Starting seeds: Seed starting for the summer or winter vegetable garden is a very common use for a greenhouse. With a greenhouse, you can get an early start to your garden or even have vegetables year round. Needed seed starting supplies are containers, sterile soil, fertilizer, water, and, especially for northern gardeners, heat and light. A relatively inexpensive way to start seeds is using a propagation mat under seed flats. This provides warmth directly to the soil to help with germination. An alternative to using flats is to plant seeds directly into a soil bench. Warmth can be provided with a heat cable buried about 6 inches in the soil.
Heating: Heat in the greenhouse can be provided with electric, natural gas or LP gas heaters. Electric heaters are flexible, economical, and easy to install. 240 volt heaters are generally more efficient than 120 volt. However, a 120 volt heater is generally adequate for heating a small greenhouse when controlled by a separate heavy-duty, moisture resistant thermostat. Natural gas and LP heaters should be properly vented, both providing fresh air for combustion and exhausting fumes, and equipped with a good thermostat. Other, less common heating methods include in-the-floor radiant heating or an extension of a forced-air home heating system to an attached greenhouse.
Lighting: Once the seeds come up they need light. If the natural light in your greenhouse is low (a common issue in winter), providing supplemental lighting is important to keep plants from getting spindly. A simple fluorescent shop light hung about 4 inches above your plants may be enough. However, many growers swear by the new High Output Fluorescent lamps or High Pressure Sodium or Metal Halide lights. These specialized lights provide strong, full spectrum light and can often cover a larger area than ordinary fluorescent. The high output fluorescent lamps are energy efficient as well.
Vegetables to grow in the winter greenhouse: Vegetables commonly grown in a winter greenhouse include lettuce, spinach, radishes, tomatoes, peppers, and cucumbers. However, what you are actually able to successfully grow depends on the night time temperatures you decide to keep. A cool greenhouse, with night time temperatures of 40-45°F, works for lettuce, spinach and radishes. Peppers, tomatoes, and cucumbers need warmer temperatures at night, around 65°F, especially when fruit is growing.
Pollination: Vegetables growing in a greenhouse often need help with pollination. Standard cucumbers will need hand pollination, taking the male blossom and gently rubbing the female blossom center. Tomatoes and peppers are self-pollinating but the blossoms should be gently shaken or vibrated on a regular basis. Circulation fans can help move plants. A preferred alternative is to look in seed catalogs for varieties of seeds that are appropriate for greenhouse production, often due to their method of pollination. Johnny's Selected Seeds is one seed catalog that clearly labels greenhouse appropriate varieties.
Watering: Water is needed but the amount and frequency varies with temperatures, day length, plant size and your growing medium. It is recommended that the plants be thoroughly soaked at every watering. In January, watering may be needed every 10 to 14 days. As the days get warmer, the frequency should be increased. A flat of seedlings being warmed by a heat mat will also dry out more quickly. When watering, avoid splashing foliage to prevent spreading diseases.
Ventilation: In the winter, it can be difficult to prevent the growth of mold and mildew. Do not over water. Help control humidity with proper ventilation and air circulation. For air circulation we recommend an oscillating fan running 24/7 all year long. For ventilation in the warmer months, the most gentle form of ventilation is through natural convection with base wall vents or jalousie (louvered) windows pulling cool air in down low, with roof vents allowing hot air out through the roof.
Cooling: Positive cooling is usually not needed in a greenhouse as long as adequate humidity and shading is provided on hot days. If positive cooling is needed we recommend evaporative air coolers which humidify as they cool. Air conditioners are not good for plants since they remove moisture from the air.