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A greenhouse can never really have enough ventilation. It’s easy to forget in the depths of winter here in the UK in the summer how hot it can get in the summer. Twin that with the fact that a greenhouse is on average 4 degrees warmer than the outside temperature. It’s on these days in particular that any extra greenhouse ventilation will come in handy. This is usually when your plants are at their peak, large leaves transpire in the heat so watering is critical. On days like this you can also damp down the greenhouse, this is the action of watering the greenhouse in it’s entirety – especially the base. Look at it like taking a dip in the pool – adding cold water helps keep things cool in the heat of the day. Care should be taken not to get water on the leaves of your plants when doing this to avoid scorching the leaves. This in turn will bring up the humidity in the greenhouse and good ventilation will help keep this in check. The door is often overlooked as a means of ventilating the greenhouse but if you have a means of holding this open (all cultivar greenhouses have a nifty catch for this) then this is one the best means of getting the air temperature down within the structure.
Most greenhouses come with a couple of vents in the roof and usually one in each side to. A cultivar greenhouse comes with full length roof vents on both sides, on the greenhouse sides there is low level vents running the entire length. This is more ventilation than your average greenhouse for sure but you can have more. On medium to large greenhouses I would recommend putting a vent at the plain end of the greenhouse. Then on those hot days you can open the door and plain end vent and get a good draft running through the greenhouse. On a cultivar glasshouse you can also put vents at eaves level anywhere down the length of the structure, perfect for customers who grow alpines that depend on good air movement.
The wax filled cylinders that open greenhouse windows automatically are great if you expect to be away from your greenhouse on warm days. In this respect you know the vents will open giving your plants fresh air in the heat of the day. There are times however, particularly in early spring when the ambient temperature is not quite high enough for automatic vents to open and it’s on days like this that a manual vent or two will also prove useful. This ensures air movement throughout the greenhouse but you should remember to close these at night to avoid the risk of frost damage to your plants.
I trust you’ve found this guide useful, the bottom line is that you should put in as much ventilation as you can afford.