The purpose of a greenhouse is to keep plants warm and protected. However, too much warmth can do more harm than good. If a greenhouse overheats, the plants will show signs of stress and can eventually die. Even if you frequently water them, they will not thrive. There are several ways to cool a greenhouse, however, ranging from easy and inexpensive to more involved investments.
Research the plants you plan to grow in your greenhouse. Find out their optimal growing temperature. Aim to keep the greenhouse at this temperature during the day.
Use a min/max thermometer (See Resources) to determine how hot the greenhouse is during the day when heated by sunlight. The thermometer monitors the low and high temperatures and provides a read-out. Buy one at a nursery or plant store. If it's too hot for the plants to thrive, you know it's time to cool the greenhouse.
Buy shade cloth as an easy solution to an overheating greenhouse. It will block out some of the sun, which causes most of the heat. Lay the material over your plants, during June and August. If you live in a hotter climate, you can extend that time period.
Add circulation fans to the greenhouse to promote even warmth, and add exhaust fans to push out the stale air and allow for fresh air. Ventilation is crucial during hot temperatures. It also prevents pests from infesting stressed plants.
Use an evaporative cooler if no other cooling method works. It's a small unit that lets the heat escape the greenhouse, cooling down the plants. It's an investment that can run close to $600.