Maintaining a greenhouse in winter means different things to gardeners. A greenhouse may simply be a cold frame where frost is kept off tender perennials; where the soil need only remain above freezing for most plants to survive. Many choose to actively grow cool crops such as cauliflower, carrots, cabbage and broccoli, requiring a minimum nighttime temperature of 45 to 50 degrees. Others may choose to grow vegetables or tropical plants, requiring minimum nighttime temperatures of 55 to 60 degrees. Heating requirements will vary significantly depending on how the greenhouse is used and the minimum outdoor temperature. When planning a winter greenhouse, heat is generally the first consideration, ventilation is a frequently overlooked component of a healthy winter greenhouse.

When spending time, energy and money to heat a greenhouse, ventilation (bringing fresh air from outside in) may not sound like a good idea. After all, outdoor air is cool, maybe even downright cold. So why bring in the cool air and let the warm air out? There are a few good reasons: high humidity is an enemy of a winter greenhouse, plants need a fresh supply of Carbon Dioxide (CO 2) or they become leggy, and oxygen is required for gas heaters and nighttime plant transpiration. It’s tempting to seal up a winter greenhouse to keep all of the heat in, but that’s likely to cause a whole host of problems.

In the summer months when the differential between the outside and inside temperature is small, humidity is not much of an issue. When the outside minimum temperature is below 50 degrees, however, many problems arise if the humidity is not controlled. Humidity levels above 70 percent may result in plant disease, mildew, and an uncomfortable environment. Problems may also occur with greenhouse structures, especially those made of wood. Condensation, often called sweating, is another result of high humidity. Droplets of water form when warm moist air comes in contact with surface areas cooled from the outside air. Greenhouses made with metal framing or single walled glazing will produce more condensation. Metal and single layer coverings conduct more heat than non-metallic and insulated materials, cooling the warm air much faster.

Depending on the size of your structure and the location of the greenhouse, you will likely choose between gas and electric heat. Gas greenhouse heaters are the most economical to operate and are a frequent choice for greenhouse gardeners, however, they require more ventilation than electric heaters. The warm moist air produced by gas heaters must be removed from the greenhouse before it comes in contact with the cold structure and cools below the dew point. Fresh oxygen laden air, required for the heater’s combustion, is then warmed to replace the stale air. Many gas heaters have a built in safety sensor. When low amounts of oxygen are detected the heater shuts off automatically. If a greenhouse is improperly ventilated, the heater may shut down when it’s needed the most. Too many people have lost their precious plants this way.

Taking the time to prepare now may save you hours of messy clean up and keep plants alive and healthy. Adequate ventilation and circulation will prevent the problems caused by high humidity and provide the necessary CO 2 and oxygen for optimal growth. Small greenhouses have a high percentage of exposed surface area to area under cover. That means that the environment can change rapidly. The more automated you can make your greenhouse, the easier it will be to control the temperature and the humidity.

Ventilation in is measured by the number of air changes per hour. The minimum recommended exchange in a winter greenhouse is two complete air changes every hour. Natural ventilation is created when warm air is exhausted from the greenhouse. The resulting change in pressure will draw in the cooler air at the base of the greenhouse to be heated. A thermostatically controlled exhaust fan will automatically vent the wet air out when needed. The same thermostat controller used with the heater will help maintain the exact temperature and conditions needed.

Using circulation fans will also help to keep the internal temperature consistent. Ideally, circulation fans should run constantly in a winter greenhouse. This will help to maintain a consistent temperature by keeping the warm air from rising and the cool air from settling to the floor. Circulation fans are best used in tandem to create on oval air flow pattern. It is interesting to note that circulation and ventilation are even more critical in a cool greenhouse because the cool air is not capable of holding as much water as warm air, making condensation a higher risk.

Operating a successful home greenhouse need not be difficult. A simple calculation will determine your structure’s CMF (Cubic Foot per Minute) requirement which then can be matched to the correct products. There are no calculations, however, that substitute for your observations. After a cold winter night, take a peak in the greenhouse to see how things are faring. If you see the signs of condensation, then you can increase the ventilation before the problems start. If the temperature is uneven in the greenhouse (all the warm air is at the top) you can increase circulation. Now that you know what to look for, you will be able to make minor adjustments before major problems arise. Spending time in the greenhouse is a joy when the environment is comfortable and the plants are happy.

Note: Much more ventilation is required in warmer months. It is advisable to purchase a ventilation fan that will handle one to one and one-half air changes per minute. Many companies that sell greenhouses have a calculator on their website. The calculation for CFM (Cubic Feet per Minute) is: Length x Height x Width=Greenhouse Volume

General Greenhouse Ventilation