There are several ways to cool a greenhouse during those summer months when you can’t stand to walk in the door. Note that the more of these you combine, the easier your temperature control will be. But, and it’s a big but, there’s only so much you can do when that sun is shining.
Also please understand that outdoor temperatures will also influence the effectiveness of these techniques. When it’s 95F outdoors, there are few ways to drop the temperature inside the greenhouse other than expensive commercial cooling systems.
But for our backyard greenhouses, here are several systems that work nicely.
Use fans. See this article on how to size the fan system for your greenhouse.
This acts like a giant screen put over the greenhouse. The fabric is rated by the percentage of light it stops and you’ll commonly see 30% and 50% fabric for sale.
This works nicely in the heat of the summer and we used to use it in the nursery sales areas to drop the summer heat, provide shade for both the plants and customers. Remember the soil temperatures in the pots also determine plant growth so having shade fabric is a good idea to bring that temperature down while the plants in smaller pots (anything in the eight-inch pot or less) are actively growing.
A word to the wise though, while it’s not as subject to the wind as straight plastic (it is like a big screen door) it does get blown around so do ensure it is well fastened down and secure.
A DIY Recipe for Painting the Covering Yourself
Covering the greenhouse plastic with a liquid shade solution works really well. You can buy this at greenhouse supply companies or you can use a homemade system like I used to do.
Mix a cup of white latex paint into a gallon of water. Apply this to your covering.
You can experiment with slightly more or slightly less to get the density of shade you want.
You apply it either by flinging it on with a cup or through a sprayer that shoots a jet of water (open up the nozzle on your garden sprayer all the way) The cup flinging method was my choice as it was faster and I could reach most of the roof area in the larger houses I used in my nursery. BUT it was messy for me – never toss into a wind. 🙂 And it left the greenhouse looking like a flock of rather large birds had flown over. This greatly amused my customers later in the season I note.
But it cooled down the greenhouse and it wore off after several rain showers. This meant I could get through a growing and retail season with temperatures that were good for the plants and customers and then let the greenhouse heat up during mid-summer (when all the plants were outdoors) to kill off weeds and pathogens.
Those are the three ways to cool a greenhouse – the cheapest is the paint but the best looking is the shade fabric.