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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 ventitalation and air flow. Whil;e 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 atmsphere. The battle then is to juggle ventiation 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 crp 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 contol of the air flow and have some autimatic vents fitted for those days when you can't get up for sunrise or heaven forbid you've taken a holiday.
To facilitate good ventilation you need opening vents on the greenhouse ridge. Ideally these should be automatic, aptly because they are like;ly 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 louvred vetns that can be manually opened and closed. Don't skipm on air vents thy are an essential part of successful greenhouse gardening. To complete your set up choose some hinged vents that can be opend 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 electruc 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 greenhyouse glass. Ideally choose external blinds that can be rolled over the south facing slope of the roof and if necessary on the southside of the glasshouse. Ineternal 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.
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