Plants respond to the relative lengths to light and dark periods as well as to the intensity and quality of light. Artificial light has been used extensively to control plant growth processes under various conditions. Plants differ in the need for light; some thrive on sunshine, others grow best in the shade. Most plants will grow in either natural or artificial light. Artificial light can be used in the following ways:

•To provide high intensity light when increased plant growth is desired.

•To extend the hours of natural daylight or to provide a night interruption to maintain the plants on long-day conditions.

Proper lighting not only extends the gardening day by enabling the gardener to work in the greenhouse during the dark evenings of winter and early spring, but it aids plant growth. Three basic types of lamps used in greenhouse lighting are:

Fluorescent lamps - These have the advantage of higher light efficiency with low heat. This type of lamp is the most widely used for supplemental light. It is available in a variety of colors but cool-white lamps are the most common. High intensity (1500 ma) fluorescent tubes that require higher wattage are also commonly used to reach 2000 foot candles. - These have the advantage of higher light efficiency with low heat. This type of lamp is the most widely used for supplemental light. It is available in a variety of colors but cool-white lamps are the most common. High intensity (1500 ma) fluorescent tubes that require higher wattage are also commonly used to reach 2000 foot candles.

Incandescent lamps - These vary in size from 60 watts to 500 watts. They are used to extend day- length in greenhouses. The grower can vary foot- candle levels by adjusting the spacing and mounting height above the plants. These vary in size from 60 watts to 500 watts. They are used to extend day- length in greenhouses. The grower can vary foot- candle levels by adjusting the spacing and mounting height above the plants.

High-intensity discharge (HID) lamps - These have a long life (5000 hours or more). With improvements made possible by the addition of sodium and metal-halides, the lamp has a high emission of light in the regions utilized by plants. These have a long life (5000 hours or more). With improvements made possible by the addition of sodium and metal-halides, the lamp has a high emission of light in the regions utilized by plants.