Greenhouse gardening is similar in many ways to gardening outside. The plants still need adequate nutrients and water, and protection from insect pests and diseases. You still must tie, prune, and tend to them.
But the greenhouse environment is also very different from that of a backyard garden. The very things that make greenhouse growing more controlled and convenient also make it more demanding. In a greenhouse, you control temperature, humidity, soil aeration, soil moisture and drainage, fertility levels, and light. This degree of environmental control gives you a tremendous amount of latitude as well as some new responsibilities.
Temperature: Heaters, vents, and fans are your allies in temperature control. Even in a well-designed solar-efficient greenhouse, outside conditions are sometimes so cold and cloudy that auxiliary heat is needed to keep plants growing at an optimum rate.
Vents and fans help to cool the greenhouse. On a sunny day, even at 20°F below zero, greenhouse air can heat up well beyond healthful levels. If the greenhouse is attached, you can move this hot air into your home. But in a freestanding unit, hot air must have a way to exit, and cool air a way to enter. Passive vents allow for this sort of movement, as do thermostatically controlled exhaust fans and intake vents. Manually operated vents are relatively inexpensive, but you’ll need to check them at least twice a day, and open or close them as necessary. Automatic ventilation systems are more costly, but they save time and reduce the chances of excessive cooling or heating.
Adjust air temperature in the greenhouse according to the level of light. In general, summer crops grow best at temperatures of about 75° to 85°F in the daytime and 60° to 75°F at night. On cloudy days, these temperature ranges should be somewhat lower, since the plant is not manufacturing as many sugars as usual.
Winter air temperatures can go as low as 45°F at night without damaging most leafy green crops and shouldn’t go much above 65° to 70°F during the day. Spring seedlings vary in their temperature preferences. Cool-weather crops, such as broccoli and lettuce, grow most vigorously at 50°F nights and 60° to 65°F days, while warm-weather plants such as tomatoes and squash require nights at a minimum of 55°F and days of at least 65°F but no higher than 80°F.
Ornamentals typically need night temperatures no lower than 55°F, and tender tropicals can require night temperatures of 60°F or even higher.
Investigate the temperature requirements of the plants you plan to grow before installing your greenhouse and heating system so you can match the heater to your plants’ needs. You can find a wide range of heater types and sizes, from freestanding propane heaters to powerful wall-mounted electric heaters. As with the greenhouses themselves, all greenhouse equipment is available through greenhouse and garden supply catalogs and Web sites.
Air circulation: Air circulation is extremely important to plant health. Good air circulation strengthens the woody tissue in stems and decreases the opportunities for fungi to attack your plants. Dense plant growth can interfere with air circulation and contribute to excessive relative humidity. Leave adequate space between plants and prune so that leaves from adjacent plants don’t touch each other.
Plants use carbon dioxide from the air to manufacture sugars. In a closed greenhouse, carbon dioxide can be so depleted that plant growth is slowed. Remember to ventilate to change the air supply at least once each morning, even if you have to add extra heat.
Besides vents and fans, one low-tech way to increase air movement is by installing screened windows and doors in your greenhouse. By opening a window on one end and the door panel on the other, you’ll have cross-ventilation. Positioning windows at the top and bottom of the greenhouse walls allows warm air to rise and escape from the upper windows, and cooler air to enter through the lower ones.