It’s hot. Really hot! Forecasters say that temperatures here in Rhode Island could reach 100oF (38oC) with humidity to match even though summer is just beginning. Fortunately, I live on the coast, so high temperatures inland create a sea breeze that cools our local area outdoors. But inside my greenhouse is a different story. If I forget to open the greenhouse roof windows, the temperature inside can easily exceed 110oF (43oC). Heat this high is terrible for plants.
to 100 degrees F if you forget to open the greenhouse.
What can you do to lower your greenhouse temperatures during heatwaves? First, make sure that all the greenhouse windows and vents are open on a hot day. A well-built greenhouse usually has at least 25% openable glazing, so open everything you can, including the door, especially if you have screens to protect against unwanted insects and other wildlife intruders. But what if you simply forget to open up everything and the temperature inside your greenhouse soars? After immediately opening up all the windows and vents, also spray down the plants, which are likely wilted from excessive transpiration. Spraying them will not only provide the water needed to help the plants recover. It will also lower their temperature and slow their transpiration.
Beside maximizing ventilation, what else can you do to lower summer temperatures inside your greenhouse? One option is painting the glazing with a shade paint, although I’m not a fan of doing that. The paint has to be completely removed in the fall, and with large panes of glass and my weary body, it’s a chore I prefer not to suffer. It’s also quite expensive for something that’s temporary. Online prices for the paint vary from around $70 to $100 per gallon.
Shade cloths for your greenhouse glazing are a different matter. They’re available in a variety of densities that shade out from 20% to 90% of sunlight. The fabric choice is knitted or woven. Knitted fabric is usually polyethylene and tends to be lighter than woven fabric, although woven weight depends on the tightness (the density) of the weave. Knitted cloths are usually purchased sized-to-fit and often have taped bindings with grommets for easy installation. Woven cloths are typically made of polypropylene and they, too, are usually cut and edge-taped by the manufacturer or the seller. (If you cut woven fabric yourself, it may unravel.) Both knitted and woven shade cloths come in a number of colors – black, green, tan, grey, blue, and white. Color choice can have significance besides just looks. Black cloth tends to absorb sunlight, so if installed inside the glazing it may add heat to the greenhouse. White cloth, on the other hand, reflects sunlight, so it may help to cool the greenhouse. For some specialized growers, other colors are chosen to absorb light of a particular wavelength and improve plant growth.
Should shade cloths be put inside or outside the glazing? Inside the glazing is the preferred location. Hartley greenhouses often have preinstalled rollers for shade cloths, but you can install shade cloths yourself using thin lathes either nailed or screwed (for easy removal) to the framework of a wooden greenhouse. For a metal-framed greenhouse, shade cloths may need to be tied into place over strategically positioned hooks. Installing shade cloths on the outside of the glazing is a relatively easy job, but it exposes the cloth to wind, which may cause it to flap and rip. In extreme cases, high winds can tear the cloth off the greenhouse and may even damage the structure. So if you care about longevity, inside installation is best.