Shade cloth is the first and easiest recommendation. We often recommend a company that has a full complement of shade cloth options: https://shadyplains.com/
Plant a tree to provide some natural shade cover. If you can find a sturdy variety that gains leaves in May and drops leaves for winter, you would have the best of both worlds because the greenhouse will get more light in winter when it needs it.
Exhaust fan systems and evaporative coolers are mechanical options that move hot air out of the greenhouse.
Many of our customers spray down the greenhouse floor every morning to keep it cool too.
Move your plants to ground level where it is naturally cooler.
Prop the door open and make sure you have circulating fans moving the hot air
If you know that a certain wall is going to take most of the heat, think about a shade trellis. Vines that trail up the exterior of the greenhouse are a great way to create shade.
Whitewash paint is a very old technique that some customers use to prevent too much sun exposure hitting their plants.
A ceiling fan is an interesting option as you can place it directly below the roof vents, reverse the spin direction and pull hot air up and out of the greenhouse.
And lastly, some customers use a portable air conditioner which can be beneficial for maintaining a consistent and comfortable temperature for plants. Greenhouses can become very hot and humid, especially during the summer months, which can stress and damage plants. An air conditioner can help regulate the temperature and humidity levels in the greenhouse, which can promote healthy plant growth. However, there are a few considerations to keep in mind when using an air conditioner in a greenhouse:
Size and capacity: The air conditioner should be appropriately sized for the greenhouse. If it is too small, it may not effectively cool the space, and if it is too large, it may lead to excessive humidity and condensation.
Ventilation: It's essential to ensure proper ventilation in the greenhouse to prevent moisture buildup and maintain air circulation.
Energy consumption: Choose an energy-efficient model to avoid excessive energy consumption and costs.
Plant needs: Some plants may have specific temperature and humidity requirements and may not thrive in the cold air.