Keeping A Plastic Greenhouse Cool: Siting, Shading & Ventilation
Plastic greenhouses can suffer from poor airflow and, as a result, a lack of temperature control especially on hot sunny days and freezing cold nights. They are also prone to wind damage as they are very light and are not fixed to the ground.
Placing your plastic greenhouse where there is morning sun in both the winter and spring but evening shade in the late spring, summer and early autumn, and keeping the door open during the day but closed at night is the best way of preventing extreme temperatures and damage to your plants.
While they are more work than glass greenhouses, you can still have great success raising seedlings and growing food in plastic greenhouses if you ensure that your plants have the right conditions by following these steps…
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Erect Your Plastic Greenhouse Near A Wall, Hedge or Tree
Due to the lack of ventilation, plastic greenhouses build temperature very quickly and during the summer require shading from a nearby structure. This is the opposite to glass greenhouses with multiple windows which should be sited in full sun for as much of the day as possible.
From April onwards, I find that I need to keep the plastic greenhouse door open. Otherwise if the morning sun has been shining for an hour and the door is still shut, then the temperature inside will be about 35°C, despite the outdoor temperature only being around 5°C.
On nights without any risk of frost, I prefer to leave the plastic greenhouse door open to limit damage to my plants because I have become a very sleepy boy in my old age (I’m 30). In the summer it’s even worse and the greenhouse door stays permanently open from May onwards, right up until the first frosts in December. Even with the door open, it’s hard to get the temperature below 30°C during the day.
But by placing my plastic greenhouses under a tree, at the edge of its reach, I can limit the worst of the heat as the greenhouse starts to receive dappled shade by midday and full shade by the late afternoon when the air temperature is highest.
In the Winter, you may wish to move the plastic greenhouse to a location with full sun to increase the daytime temperatures, even on overcast days. Alternatively, you could move it to a sheltered position with evening sun and fill the base with heat absorbing materials like bricks, stone or a large body of water, to enhance the overnight temperatures and minimise any frost risk.
Add Additional Shade During Heatwaves
Greenhouses really do suffer from heat-build up and during extreme heat waves, like the one in 2022 where it reached 40°C in my garden, you really don’t want to do anything to increase the temperature in there!
Many plastic greenhouses only have the front door and the slide flaps for ventilation. And on the hot summer days, when the air is usually quite still, you want to limit the amount of light that gets into the greenhouse.
Many viewers have used shade-cloth and wrapped that over the top of the greenhouse but, in an emergency, I find draping a towel over the top or tucking it in between the metal frame and the cover can reduce the temperature in the greenhouse.
Raise The Flaps On Still Days
In the summer, you want to increase airflow into the plastic greenhouse to limit how hot it gets inside. And the easiest way of doing this is to limit how much air the plastic cover keeps out.
If you lift the bottom of the cover up slightly along the length of the base and use string or tape to hold it in place, then you can let more air in around the sides and prevent the air temperature inside the greenhouse from getting too hot.
It will still be much warmer towards the top of the greenhouse, but the plants on the shelves will have a much better time with the temperature closer to ambient.
Add Additional Windows or Doors
If you really wanted to increase airflow, you could modify your plastic greenhouse and add an extra window or door to the rear or sides. There are kits out there that are designed for adding zips to this material and viewers have written in to suggest that they have done just that to allow a through-draft and limit temperature build-up.
But I think that is a substantial modification to a very low-cost item and buying a different model or a poly-tunnel with additional ventilation routes may be an easier option. Otherwise that could be a good fall-back plan if your DIY skills are as rudimentary as mine.
Close The Door On Windy Days
No matter how hot it gets, you’re not going to want the ventilation when it’s very very windy. Plastic greenhouses are not solid and stable structures – they simply aren’t heavy enough and this is where glass and poly-carbonate greenhouses start to outshine the cheap alternative.
If you leave the door open on plastic greenhouse on a very gusty day, the plastic cover is going to act as a giant sail and create enormous forces on the metal frame. This is how collapses happen and, if the wind blows violently enough, it can lead to the plastic cover detaching from the frame and blowing away in the wind.
While you can follow these steps to make your plastic greenhouse storm-proof, you will still want to close the door when winds get too high to protect your plants and stop them being thrown across the floor.
But don’t panic! When the winds really are this high, I find that ventilation around the base is still usually sufficient to prevent the most extreme temperature build-ups in the summer.
Monitor Temperature and React Accordingly
The best thing you can do to ensure that your plastic greenhouse is sited and ventilated correctly is to monitor the temperature inside and adjust the position and shading as needed.
I use two of these small greenhouse thermometers from Amazon* because they let me record the maximum and minimum between each reset.
So if you reset the thermometer in the morning, you can go back the next day and see if your changes fixed the problem and, if not, keep iterating until you find the right location for your plastic greenhouse in your garden in the current season.
And once you’ve found the perfect spot, you’ll be able to get a head-start on the growing season and enjoy earlier harvests from your outdoor plants and even start producing more exotic fruits from the warmth of your plastic greenhouse.
Happy Gardening!
