10:55 - 12 May 2010
While the whole purpose of a greenhouse is to harness the energy of the sun and use it to warm an area to grow plants, sometimes the greenhouse can get too hot (or too cold) and actually damage the plants that are supposed to be protected by it.
The internal temperature of the greenhouse can become an obsession for the greenhouse gardener. Through the winter and early spring the glasshouse owner becomes fixated on keeping the temperature above freezing to protect their precious plants. A whole army of devices are employed for the purpose, from frost sensors and thermometers to alarms and heaters.
But then something strange happens. As the weather starts to warm and the sun rises in the sky for longer each day, the ritual changes to one of ventilation and airflow. While these are of course essential throughout the year, as spring develops a short warm spell can shoot the temperature sky high and this will quickly affect the plants enveloped within the greenhouse’s protective atmosphere. The battle then is to juggle ventilation with temperature and to keep things even. Too cold and the early started seeds will be checked, but too warm and they are quickly frazzled and dried out around the roots.
Ventilation is vital for good plant growth. Crowded plants and poor airflow quickly result in fungal diseases and sickly plants. In spring this can mean losing your whole crop of seedlings to damping off. Most basic greenhouses have the bare minimum of vents fitted and if the gardener wants more they become optional extras. This is false economy. You need to be in close control of the airflow and have some automatic vents fitted for those days when you can’t get up for sunrise or heaven forbid you’ve taken a holiday. Of course the biggest greenhouse vent is the door, but it isn’t alawyas practical to keep the door open. A hinged door can slam shut in windy weather and an open door is an open invitation to cats, dogs and other univited guests that may cuase damage to your plants.
To ensure good ventuialton in your greenhouse you need variety and a choice of options. It’s really something that should be considered and decided before you buy your glasshouse and a good manufacturer will help you to consider your options.
To facilitate good ventilation you need opening vents on the greenhouse ridge. Ideally these should be automatic, aptly because they are likely to be well out of reach. A minimum of two ridge vents ensures that hot air can escape and pull cool air into the glasshouse via lower vents. A ground level air vent will complete the cycle. Choose louvered vents that can be manually opened and closed. Don’t skimp on air vents they are an essential part of successful greenhouse gardening. To complete your set up choose some hinged vents that can be opened to bring fresh air into the greenhouse. You can also fit an extractor fan to remove hot air from the greenhouse, and don’t forget that most electric heaters have a fan option that moves air around the glasshouse during warm weather without the heater element activated.
You also need to consider that when the sun goes down the temperature can plummet; this is when automatic vents come into their own as they shut down as the air cools.
By early summer the greenhouse temperature can reach 80F very quickly. This is the time to fit shading that will keep the sun off the greenhouse glass. Ideally choose external blinds that can be rolled over the south facing slope of the roof and if necessary on the south side of the glasshouse. Internal blinds are another option or you can use shading paint that is painted onto the greenhouse glass and removed at the end of the season.
