Greenhouses are an effective way to start seedlings in the spring before the last threat of frost, or to grow heat-loving plants during cold weather. Greenhouses---whether made of inexpensive plastic sheeting or expensive glass panes---are designed to protect plants from the cold. But unless you understand some basic greenhouse gardening principles, you'll have little luck using a greenhouse effectively.
Place the greenhouse where it will receive as much sun as possible. This is a vital first step because the sun not only offers plants light necessary for growth but also regulates heats the greenhouse.
Place a thermometer in the shadiest spot in the greenhouse and check it at least once a day. If the weather fluctuates frequently, you may need to check it several times a day. Most plants will thrive if given temperatures of 60 degrees to 70 degrees Fahrenheit.
Adjust ventilation according to temperature. If the temperature in the greenhouse goes above 70 degrees Fahrenheit, open doors, windows and vents as necessary to keep the temperature lower. On hot days, everything may need to be open to prevent plants from overheating. On cool nights, usually only two vents should be opened an inch or two; this allows good air flow through the greenhouse.
Turn on heat if the temperature drops below 60 degrees Fahrenheit.
Act quickly if the greenhouse overheats. To quickly cool down the greenhouse, open all the doors, windows and vents and spray the floor with water.
Help ventilation by not clustering plants together. In stead, space them evenly throughout the structure.
Install an automatic misting system to control irrigation and humidity in the greenhouse. If this isn't possible, you'll need to check plant soil daily. Once the first inch of soil in each pot is dry, the plant should be watered.
Install grow lights for winter if you live in an area where winters are darker than summers. Fluorescent lights should be placed a few inches from the top of the plants, while 400-watt metal halide lamps should be kept several feet from plants.
Examine plants regularly to check for pests. In the open garden, pests can be a real problem, but pests in a greenhouse can quickly decimate everything in the structure. If you discover pests on any plant, promptly remove the plant from the greenhouse.