The best grow light for a plant is normally natural sunlight. When that isn't possible, there are other types of lights that work well with different kinds of plants. Factors to consider when choosing the best grow light for your plants are the color and heat the light emits. Always read the instructions that come with your plant to see any specific requirements that it may need.
Metal Halide bulbs are some of the best grow lights for plants, but are exceptionally good ones to use for helping keep plants compact. If you don't have natural light available, these lights are good ones to use as your primary light source. These bulbs can last more than 10,000 hours. However, after 10,000 hours, you should replace them because the light begins to diminish. To give you an idea of the kind of light these bulbs produce, standard fluorescent lights emit a mere 39 lumens per watt, while metal halide bulbs can pump out up to 125 lumens per watt.
High Pressure Sodium
These bulbs produce an orange-red kind of glow. Unlike the metal halide bulbs, high pressure sodium bulbs shouldn't be used as a primary source of light. It's best to use these bulbs as supplemental light, in addition to natural light. If used alone, these bulbs will cause plants to grow weak stems. These bulbs are perfect for growing plants in a greenhouse. If you're growing flowers, these are ideal lights to use. They encourage flowering and budding of your plants by triggering the hormones responsible for budding. They last about twice as long as the metal halide lights, up to 18,000 hours.
These are your everyday bulbs that are commonly used in homes. They're not a good light to use to grow plants for a couple reasons. One, they don't have enough blue (See the resource "How to Grow Seeds and Plants Under Artificial Light" to learn about coloring). Secondly, they produce too much heat and can burn your plants if placed too close to them.
Full-Spectrum Fluorescent Bulbs
Most regular house plants do well with these. This light has a good combination of both cool and warm colors, resembling the natural solar spectrum. Seedlings and culinary herbs also do very well with this type of light.