Greenhouses are a wonderful way to extend your vegetable growing season, to grow plants for your garden that would otherwise only thrive a few months or to grow lush tropical plants. The internal conditions of the greenhouse must be maintained to prevent plants getting too cold or too hot, to keep the humidity at the right level to prevent mildew or fungus growth and to keep oxygen and carbon dioxide levels balanced with proper ventilation.
Greenhouses are either free-standing structures or can be constructed as an addition to an existing structure such as a garden shed or even the house. The basic plan is a frame of either metal, plastic, or wood with glass or plastic sheeting for the walls and roof. The greenhouse can be as simple as an inverted-U "tunnel" made of PVC piping and plastic sheeting, or as grand as a wrought iron frame "cottage" with glass walls.
The function of the greenhouse is to create a warm humid environment to grow plants of tropical origin either year-round or to start the plants earlier than would be allowed if planted directly outside. The clear walls of the greenhouse allow sun in. This warms the greenhouse much like a car's interior warms up when sitting in the sun. Watering and plant metabolism makes the greenhouse humid and this helps keep the inside warm even overnight.
If you are using the greenhouse year-round or are starting plants in late winter, you may need to provide a heat source for the greenhouse. Heat can be provided either with an electrical heater or stove. You also need to make sure the greenhouse doesn't get too hot on days with intense sun. Temperatures above 85 can cause some plants to halt their growth and stop fruiting. Having an opaque cover to partially cover the greenhouse during the hottest times of day and providing ventilation helps control the temperature.
If the humidity is too high inside the greenhouse, it can prevent plants from being able to take up carbon dioxide and this will hinder growth. If plants are kept in too humid conditions they also cannot release water through transpiration or release oxygen. this is the equivalent of plant constipation and can cause a build up of metabolic waste products in the plants. In addition, overly humid areas encourage the growth of diseases, mildew, and fungus. Keeping the humidity at 25 to 80 per cent is a healthy range depending on what you are growing.
To maintain the heat and humidity levels it is important to have good air circulation in the greenhouse. Circulating air also moves the heat around in the winter to prevent spots that are cold while other areas are too hot. Installing a fan is the best thing you can do for your greenhouse. The moving air also brings in outside air to keep the air inside the greenhouse from growing stale.
Disease and pest prevention are also important in the greenhouse. Overcrowding plants is tempting to create a jungle-like feeling, but this hinders air flow and encourages hiding places for pests. If any plant shows signs of disease it should be removed instantly and you should be vigilant about keeping an eye on other plants that were near it. Destructive insects can be kept in check by culling infested plants and keeping a few beneficial bugs such as praying mantis in the greenhouse.