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Lighting and HID Bulbs Explained

The end of the outdoor season is drawing to a close; gardeners are finishing up harvesting throughout the month of “crop-tober”.  When one door closes, another opens – referring to the end of the outdoor season and the beginning of the indoor season.  The first step towards beginning to grow indoors is to check your bulbs.  All the growth you get indoors is entirely dictated by the quality of the bulb that you put the plants under.  There are two spectrums of light bulbs that we use for indoor gardening – metal halide (MH) and high pressure sodium (HPS) bulbs.  Both of these are considered high intensity discharge bulbs (HID), because they require a lot of power to illuminate and the light that they put out is intense.  Plants do not see light like humans see light.  Think of it like this – humans can only see how bright a light is (measured in lumens), but plants can see a wide range of colors and only make use of certain colors (measured in PAR).  (On that note – never buy a bulb based off of the lumens that the packaging claims. Lumens matter not at all to plants.  The PAR spectrum is what you need to look at to determine which bulb to buy). That’s why there are two different bulb styles (MH and HPS) to garden under – each one has the different colors that plants use. What is the difference between the vast options of bulbs available for sale?

Metal halide light bulbs are best used for the vegetative stage (otherwise known as the foliar stage). These bulbs give off a blue/white color when illuminated. However, this is misleading. That is the color that the MH bulbs look to humans. Plants see these MH bulbs a little differently. To plants, they look bright blue, with some yellow and also what is called “incidental green”. A high quality metal halide will also give off some ultraviolet/purple photons.  The colors that come out of MH bulbs are formulated to mimic springtime sun colors. These specific colors keep the plant short and bushy.

High pressure sodium bulbs are the best bulbs for the flowering stage. They put out huge amounts of yellow/orange colors, with some additional red photons. These colors mimic the august/summer sun, which initiates and promotes flowering along with high yields. High quality HPS bulbs will also contain some blue/purple/ultraviolet photons, similar to a MH bulb. These darker colors are responsible for pulling out the essential oils during the flowering stage. Some people even replace their HPS bulbs during the last week of flowering with metal halide bulbs, specifically to pull out as much essential oils as possible.  Although not ideal, a high quality HPS bulb can be used for vegetative growth because of the small amount of blue colors it throws off, but there isn’t a bunch, so the plants will have a tendency to stretch tall.  You can get away with a cheap vegetative/MH bulb, but if yields matter to you - go ahead and buy the better HPS bulbs.   There's no hiding the difference between low and high quality HPS bulbs. Your yields are going to directly reflect the quality of the HPS lamp you decided to buy. An example of a very fine, high-quality HPS bulb is the Hortilux Super HPS. It's called "Super" for a good reason.

Another viable option for indoor plant lighting are LED fixtures. LED is an acronym for light-emitting diode. LEDs are tiny little bulbs (called diodes) that only consume a small amount of power to illuminate, and put out the perfect light spectrum for plant growth. LED fixtures contain many of these small diodes. Each diode only consumes but a few watts of electricity, making them very low energy consumers, as far as lighting fixtures go. Because of this low energy consumption, they are not as intense of lighting as an HID bulb. The lower intensity is not so much of a problem becuase the colors are so specific to plant growth.  During the vegetative stage, when light intensity matters less than the importance of perfect light spectrum, LED’s can be ideal. They run cool (because the heat gets vented out the top of the fixture, versus an HID reflector that emits the heat downward towards your plants), low power consumption and perfect light spectrum are all reasons that make LED fixtures another option for indoor plant growth. LED technology is still relatively new to the indoor gardening industry but is always improving.