A closed greenhouse in the sun heats up quickly, even on a cool spring day. Without enough vents, a summer afternoon gets far too hot for seedlings and tomato flowers. Good ventilation is the cheapest way to better crops.
Why ventilation matters
- Heat: too much heat makes tomatoes drop flowers and scorches seedlings.
- Damp: still, damp air invites grey mould, mildew and other diseases. Moving air keeps leaves dry.
- Fresh air: plants need carbon dioxide from outside air to grow.
Roof vents and louvres
Warm air rises, so roof vents do most of the work. A louvre low in the side wall or the end lets cool air in at the bottom, and the warm air flows out at the top. One roof vent plus one louvre works better than two roof vents next to each other.
Many budget greenhouses come with a single roof vent. That is enough for a 6x4, but tight for a 6x8 or larger. Our score gives 1 point for one vent, 1.5 for two and 2 for three or more. Some examples from our list:
- Halls Magnum 8x10: 4 roof vents
- Vitavia Jupiter 8300 8x10: 4 roof vents
- Rhino Premium 8x10: 2 roof vents and 3 louvres
- Palram Canopia Harmony 6x10: 1 roof vent for 10 ft of length
You can often buy extra vents and louvres from the maker later.
Automatic vent openers
An automatic vent opener is a cylinder of wax that expands as it warms up and pushes the vent open, then closes it again as it cools. It needs no electricity. If you are out at work all day, it is the most useful accessory you can buy. Check that its lifting force suits the weight of your vent.
On hot days
- Open the door as well as the vents.
- Fit shade netting or use shade paint on the sunny side.
- Damp down the floor with water in the morning. It cools the air as it dries.
In winter
Ventilate a little on sunny days to let damp air out, then close up before it gets cold in the afternoon.
Our picks for automatic vent openers are below.
