Greetings, Richard and welcome to the world of indoor horticultural lighting!
Your question gets asked often by new growers who are trying to sort through the innumerable lighting products on the market today. However, you have successfully boiled it down to the two most prominent forms of high-intensity discharge (HID) lighting commonly used in indoor horticulture – high-pressure sodium bulbs (HPS) and metal halide (MH) bulbs.
To get right to the point, MH bulbs are typically used during the vegetative stage, while HPS bulbs are used during flowering. MH bulbs have a spectrum that is stronger in the blue wavelengths and these bulbs produce a “whiter” light. This helps keep internode lengths at a minimum during early development, which in turn keeps plant heights shorter — which is desirable for indoor growing, as space is usually limited. Additionally, the blue wavelengths of spectrum carry more light energy with them (in the form of photons) which is used by plants during photosynthesis. This is why you will sometimes see growers supplement their HPS bulbs with a MH bulb or two mixed in during flowering.
HPS bulbs, on the other hand, are much heavier in the orange and red wavelengths of spectrum. These bulbs give off an orange, yellowish hue in gardens. The red wavelengths encourage stretching, growth and flowering. This spectrum also mimics the late summer/ fall seasons when most varietals go into bloom for the harvest season. It is for these reasons that you may sometimes see growers use an HPS bulb for both vegetative and flowering phases, but rarely will you see a grower using a MH only for both cycles of growth.
All that being said, it is important to remember that the fuller and broader the spectrum can be during all growth phases, the better it is for the plants. Spectrum is like a like a multi-vitamin, providing “light nutrition” to your plants. The goal in indoor horticulture is to mimic the conditions found in nature, where our plants have evolved for millions of years. This, obviously, includes mimicking sunlight, which is full-spectrum, white light. We have not yet mastered this replication in our horticultural lighting applications, but we are close!
For more information on today’s newest indoor lighting technologies, please check out my latest feature in the September 2015 issue of High Times on newsstands now. This article features everything from LEDs, to ceramic bulbs, to plasma lights, to the newest digital HPS systems. Best of luck and enjoy!
Thanks for reading everyone and remember: Grow… And help the world grow, too!
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