Metal Halide Vs. High-pressure Sodium: An Age-old Question For ...

While the technology behind indoor plant lighting has evolved rapidly, one thing has stayed fairly constant: Most growers still prefer to use high-intensity discharge lights in their grow rooms.

The HID lights that are most frequently used in growing plants indoors and in greenhouses are metal halides and high-pressure sodium (HPS). The reason both are commonly found in grow rooms – especially marijuana grow rooms – is because there has been a long-standing belief that metal halides are best for the early vegetative stages of growth, and HPS are more effective for flowering. So is there any truth to this?

Will metal halides lead to bigger, bushier, more robust plants, and are HPS bulbs the right way to go for flowering and bigger buds?

To get the answer, I did some research and decided the best place to find information on the efficacy of different types of supplemental lighting would be university studies.

Several major universities have done extensive research on plants grown indoors and in greenhouses. Utah State University and Michigan State University both have a wealth of information and studies on HID lights.

The university studies reveal there is no evidence to support the commonly-held belief that the extra blue light from a metal halide is best for vegetative growth. However, HPS is considerably more efficient for photosynthetic radiation (PAR) than metal halides.

And, careful university research has shown that the amount of light plants receive affects growth more than anything else.

At Utah State University, Professor Bruce Bugbee recently published a paper on the subject, and his research on a variety of lights and manufacturers demonstrates that (A) HPS puts out considerably more photons per watt of input power than metal halides, and (B) that there is no evidence to support the assertion that increased blue in a metal halide leads to more vegetative growth than HPS.

When I asked Professor Bugbee if the increased blue spectrum in metal halides led to bigger plants in the early grow stages, his answer was straightforward: “There is no theoretical or empirical evidence to support that claim. The apparent MH effect is likely caused by the amount of light, rather than the color of light.”

This latter point is important because so many in the marijuana industry buy metal halides for early stage “veg” growth, and switch to HPS for bud. However, if you trust the validity of carefully conducted, unbiased university studies using accepted scientific methods, then purchasing metal halides for “veg” may be a waste of money, and could lead to decreased yields.

Some points to ponder from my discussions with two university professors (Professor Bugbee at Utah State and Erik Runkle at Michigan State) who have studied horticulture and supplemental lighting for many years:

• HPS is about twice as electrically efficient as metal halide.

• Photosynthetically Active Radiation (PAR) is what really matters to plant growth, and delivery of usable PAR is better with HPS than metal halide.

• The enhanced blue in metal halide does not lead to increased plant growth compared to HPS in the “veg” stage.

• Metal halides deliver, on average, about 80,000-110,000 lumens per standard 1,000-watt bulb. For the same wattage, HPS bulbs delivers about 130,000-155,000 lumens. This increased light output far outweighs other considerations when trying to maximize plant growth indoors under electric light.

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