While sunlight is the best light available for plant growth, artificial lighting in the right wavelengths and intensity supports plant growth very well. Plants need light in the red and blue parts of the spectrum, from 400 to 700 nanometers. Ideal lighting for plants will contain mainly blue light supplemented with a small amount of red light. Light intensity can be supplemented by increasing the number of light fixtures and by using reflectors.
Metal Halide Bulbs
Metal halide bulbs are primarily used for leafy green plants and other plants during the vegetative phase of growth. They promote the growth of thick vegetation and large leaves, which use the light more efficiently, but plants grown exclusively under metal halide lights will not bloom or fruit well.
Metal halide bulbs produce more light in the blue spectrum that is close in spectrum to the summer sun. Many gardeners begin their plants on metal halide lights and change them over to high-pressure sodium lights to induce flowering and fruit formation. Leafy plants like lettuce, kale and collards can be grown on metal halide bulbs alone.
High-Pressure Sodium Lamps
High-pressure sodium lamps are high-intensity lights that produce more light in the red and yellow spectrums with less blue light. Red light stimulates flowering and is used most often during the flowering and fruiting phases.
Leafy green plants and plants in the vegetative phase need more blue light than is available from a high-pressure sodium bulb. This deficit causes plants grown under only high-pressure sodium lamps to grow tall and leggy, looking for more sun.
Cool white fluorescent lights produce light in the blue range valued by plants for vegetative growth. Warm-spectrum fluorescents produce more red wavelengths that induce flowering and fruiting. Fluorescent grow lights produce both red and blue--a combination of one warm-spectrum light mixed in with cool white fluorescents creates a healthy mix for the plants. The main drawback of fluorescents is the low intensity of light. The intensity is sufficient for plants with low light needs, but plants that need higher light intensity grow tall and lanky, with long stems and less vegetation. Leaves may be yellowed and the plant will not be healthy.
Light-emitting diodes, known as LEDs, are the newest grow light option available. LED lights are available in red, blue and dual wavelengths, and plant response will depend on the wavelengths and intensity chosen. Dual-wavelength bulbs produce the mix of blue and red that meets the needs of most plants, resulting in healthy compact plants that flower and produce a good crop.