A small greenhouse can allow you to extend the summer growing season, protect your frost sensitive plants in winter and get a leap on your spring planting. Different plants have different needs for light, but in general you'll want to provide the most light possible for most of your plants. In winter the skies are often cloudy or overcast much of the time, so you might need to hang grow lights above the plants that need the most light.
Providing Adequate Light in Your Greenhouse
Build your greenhouse in an area where it will receive a maximum amount of sunlight. Keep it away from tall trees and buildings that will create shade for the plants inside. When you build your greenhouse, use glass or clear fiberglass designed to allow a maximum amount of sunlight to get through.
Determine the light needs of the plants you will be growing in your greenhouse. You might want to measure the amount of foot-candles of light your greenhouse receives with a light meter. (See Tips and Resources.)
Cover your greenhouse roof with shade cloth if you are growing orchids or other plants that require low light and your greenhouse receives full sun.
Hang grow lights or fluorescent shop lights above plants that need additional light, such as vegetables, citrus trees and germinating seeds. Special lights are available that can give your plants the maximum amount of light (see Resources).
Experiment with different methods of providing light to your greenhouse plants. If plants begin to get "leggy" (too tall and spindly), it means they are not receiving sufficient light. If they develop burned areas on their leaves, they are getting too much light. The plants will tell you what they need through their appearance and general health, so when you begin to see changes in their appearance, change the lighting situation for them.