I’m not much of a beach person. I enjoy splashing and diving in Lake Michigan, but the summer sun is too intense for me to lie around under it for long.
Many of my plants feel the same way. Some of the houseplants I set out in the garden this May quickly developed scorch marks. No big deal; I snipped off the browned leaves and they quickly grew new, unblemished ones. But it’s a reminder that as much as plants need light, there can be too much of a good thing.
A prairie grass that evolved in wide open spaces can handle all the photons the sun can throw at it, even when the days are longest and light is most direct and intense. But a begonia that evolved in the shade of a forest is constructed to make the most of limited light, not to process unlimited light all day long.
I depend on the leaves of the trees overhead to shade many plants. After all, that’s how most of my houseplants evolved, living in the understory of rain forests. My mistake was to move the plants outdoors too soon, before the trees had fully spread their leaves to filter the sunlight.
Plants in a greenhouse can have the same problem. During the short days of winter it’s wonderful for them to have as much sun as possible, but in summer, when the days are so much longer, some can suffer from sunlight overload.
The solution is usually shade cloth made of plastic fibers. Depending on how closely it’s knitted or woven, it can block out different proportions of light, such as 40 percent or 60 percent. Your choice should depend on the plants you have and the light they need.
You can buy a big rectangle of shade cloth like a tarpaulin, edged with a row of grommets for attaching it. Some gardeners will drape a cloth over the whole greenhouse and tie it to the structure or stake it to the soil, like a tent fly. That’s a simple solution, but it can interfere with vents and it works best on small hobby-sized greenhouses. The cloth also can blow loose in a windy storm.
In a large greenhouse, it can make more sense to hang the shade cloth inside, just under the ceiling, where it won’t block vents that open upward. Consider shading the sides too, at least on the south and west. Make sure to rig the cloth so you can easily take it down at the end of summer.
Shade cloth will help a little bit to reduce heat buildup in the greenhouse. But the big answer to keeping the greenhouse cool, or at least not much hotter than the world outside, is ventilation—lots of vents, low and high, and ideally a fan to move air out fast so that it doesn’t have time to heat up.
A high ceiling also helps, giving hot air space to rise so that down at the level of people and plants on benches, the air is at least a bit cooler. Summer sun is only fun up to a point.