To maintain comfortable greenhouse temperatures, you may need to keep some light out of the greenhouse. Overheating problems are actually more common than under heating problems in greenhouses. Late spring temperature of 110° F have been recorded inside a greenhouse on a sunny day.
You can use various methods to block some of the sun's rays.
hanging commercial shade cloth (made of spun or mesh vinyls) inside the greenhouse
Remember that if you create shade to cut down on heat, your plants will also receive less light, and may grow more slowly.
Venting is critical both to draw out hot air and to provide air circulation to reduce problems with pests and diseases. Vents can be manual, electric, or solar (these are triggered to open and close by the heat of the sun.) Ideally, vents should be placed both high and low to allow for proper airflow. Exhaust fans placed high in the greenhouse help push hotter (upper) air out, while allowing cooler (lower) air to enter.
Carbon Dioxide: A Breath of Life
Carbon dioxide (CO2) is essential to the process of photosynthesis. If it's scarce, plant growth slows. If your greenhouse air is stagnant, plants can deplete the carbon dioxide in the layer of air surrounding the leaves, even though there may be plenty in the rest of the greenhouse. Keeping the air moving, via vents or fans, is important for providing necessary CO2 to plants. Some home greenhouse growers have tried increasing CO2 by composting right in the greenhouse, since the composting process produces carbon dioxide as well as heat.
Greenhouse Bed Gardening
Soil-filled greenhouse beds, near ground level or raised to 24 inches, are ideal for creating indoor planting habitats. Since beds (particularly raised) typically contain more soil than containers do, they promote good root growth.
The soil also provides thermal mass that retains some of the heat captured during the day. You can construct greenhouse beds from wood, bricks, stone, or recycled materials. Don't use pressure-treated wood or most wood treatments, since they often contain substances that can harm plants. Use either untreated hardwood or wood treated with a plant-safe preservative containing copper napthenate. If the beds will be accessible only from one side, build them no wider than 2 1/2 feet. They can be somewhat larger if you will be accessing them from two sides.
One disadvantage of using beds is that if you do have a problem with soil-borne fungi or other pests, you may have to dump an entire batch of soil to remedy the problem.
If you're growing in containers, conventional plant pots or recycled materials like cardboard milk cartons or yogurt containers will do the job, as long as you put holes in the bottom for drainage.
Unglazed clay pots are good because they're porous; however, they are heavy and breakable. Plastic pots, on the other hand, are lighter and easy to clean, but can be more easily over-watered.
Clean containers well to avoid pest and disease problems. Soak them for 1 hour in a solution of 1 part chlorine bleach to 9 parts water, or use warm soapy water.
Be sure that the container size is appropriate for the plant you're growing. A vegetable plant in a pot that's too small for it will be stressed and susceptible to attack by disease organisms and pests. On the other hand, a plant in a pot that's too large will also grow poorly, because it may get over-watered.