H I D Spectrum
The best light for growing plants is the High Intensity Discharge (HID) light. These lights are highly efficient and offer intense light that is the correct color for plants (roughly 10 times the intensity of fluorescent lights). They consist of a highly efficient reflector, commercial grade bulb and a ballast. They come in two types, Metal Halide and High Pressure Sodium. Select the wattage based on the size of your garden, and the type of light based on the plants you wish to grow. Select the bulb to further refine the spectrum of light emitted.
This is a bluish light that most closely matches daylight. It is perfect for growing plants that are in a vegetative state of leafy growth such as lettuce and spinach. This is also the light of choice if you have a mixed garden with lettuce and spinach growing nest to chilies and strawberries. All plants will grow and bloom under sunlight, right? Note that seedlings are initially in a vegetative state of growth even if they will bloom later in life. Sodium conversion bulbs are available that deliver High Pressure Sodium light from a Metal Halide system.
High Pressure Sodium
This is a reddish light and is more efficient, emitting more lumens of light per watt of electricity used. It is commonly used to supplement daylight in a greenhouse, solarium or south facing window. It is switched on during cloudy days or when the sun sets to extend the daylight hours. It is also useful indoors if growing plants that are all in a bloom state such as chilies, strawberries or beans. High Pressure Sodium is poor for vegetative growth. Lettuce and spinach for example will grow very slowly, if at all under it. Halide conversion bulbs are available that deliver Halide light from a High Pressure Sodium system.