Keeping a good airflow through the greenhouse is essential for healthy plant growth. It is especially important at the height of summer when temperatures outdoors can reach into the seventies and eighties (if we are lucky). To keep the air moving and to prevent extreme heat building up in the greenhouse you need good ventilation.
Windows, vents and doors
Quality greenhouses are made with adequate vents and windows to assist the movement of air. For good ventilation you need vents low down, such as louvre vents at ground level to draw in fresh air and top vents on the greenhouse apex to release the hot air as it rises. This creates a more natural movement of air through the greenhouse. If you are investing in a new greenhouse then make sure yours has plenty of ventilation. Extra vents and windows are a sound investment.
Ideally a greenhouse should have windows and vents that equate to at least one third of the roof area. They don’t have to be positioned at the roof, but ideally you need some vents at different levels.
Over crowded plants and poor ventilation can quickly result in botrytis and other fungal problems. An over-hot greenhouse can result in the death of some plants.
You can improve summer airflow by leaving the doors and windows open, but this may create a security problem if you keep precious plants or tools in your greenhouse.
Many greenhouse gardeners have pets that will take advantage of the warm dry greenhouse conditions, or may simply be over inquisitive. My cats simply love the greenhouse environment and are often hidden out of sight on a shelf under the potting bench. My chickens would create absolute havoc in the greenhouse given half a chance, so I have to make sure that they are secured in their run or that the greenhouse door is barricaded against large feathery creatures. Usually a panel of wire is enough, but it doesn’t keep the cats out!
You can install window screens and door screens to keep out unwanted visitors, but these may also preclude pollinating insects, which you may need if you are growing fruit, and fruiting veg in your glasshouse. Don’t let internal or external screens or blinds block the airflow and be sure to check the greenhouse regularly in case a bird or other creature has flown in through the roof vents and failed to escape again.
If you fear an invasion of small furry creatures, then install an ultrasonic device that will emit a sound wave that rodents find intolerable, but bear in mind that if your greenhouse has already become home to a family of mice or worse they may tolerate the sound as they raise their litter. These devices are better used as a deterrent rather than a solution. You can also use a cat deterrent to keep your own moggies or next door’s out of the greenhouse too.
Keep the air moving with a fan, many large, electrical greenhouse heaters can be used as air blowers in the summer months, simply by using the fan without the heating element.
Automatic vents are a godsend for the busy gardener and can work in his or her absence. Most are fitted to roof vents where they will open the windows as the temperature rises. They usually work by means of a cylinder of wax, which expands in the heat forcing the window open and contracts in the cool, allowing the windows to close at night. They can fail after several years of use, but replacement parts are readily available.
Damping down paths and greenhouse floors keeps the humidity high and reduces the temperature as the water evaporates from the surface. If you are unable to do this regularly then install an automatic sprinkler that will spray a fine mist of water onto the floor for a few minutes each day.
Don’t forget that not all plants need to be under glass for the whole summer. Move some out of the greenhouse onto the patio or into a shaded area for August and September if the weather is conducive. This will create more space in the greenhouse and improve the airflow.