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High Pressure Sodium Bulbs: The Choice of Cannabis Connoisseurs

August 3, 2016

Professional growers are well aware that cannabis requires a change in the duration of the light cycle to trigger flowering. But cannabis also requires a change in the bandwidth of light it receives to simulate the passing of the summer solstice and the shortening of the duration of the days as autumn approaches.

High intensity discharge (HID) bulbs are the most efficient means that an indoor grower has of providing the light that a crop needs to survive and thrive. To achieve the maximum yield and the highest possible quality, it is necessary to change the type of bulb used for each stage of the growth cycle. Metal halide (MH) bulbs are proficient at producing blue light, and high pressure sodium (HPS) bulbs are proficient at producing red light. So, HPS is the indoor lighting of choice among cannabis connoisseurs for use during the flowering stage.

Similarities to metal halide

Although the MH bulb is one of the most efficient sources of artificial white light, the HPS bulb is the single most efficient light source of all. HPS bulbs are available in 35, 50, 70, 100, 150, 200, 250, 310, 400, 600, 1,000-watt types. Like metal halide, HPS bulbs are available either with or without a phosphor coating, but those without a phosphor coating produce more light. In fact, a 600-watt HPS bulb produces 90,000 initial lumens.

In addition, like metal halide, HPS bulbs consist of an arc tube suspended in a vacuum (which reduces heat loss from the arc tube) contained within an outer bulb that serves to protect the arc tube from damage. Outer bulbs are available in different sizes and shapes to fit different reflectors.

Finally, like metal halide, HPS bulbs have a mix of vaporized gases—xenon, mercury and sodium—within the arc tube. This causes the bulb to emit its characteristic bandwidth of light when electricity is passed through the tube. As a result, HPS bulbs require a few minutes to “warm up” before they start emitting their full spectrum of light.

Characteristics of HPS bulbs

HPS bulbs operate on an entirely different electrical principle than MH bulbs. If the power surges or the light is turned off for any reason, the gases in the arc tube will need between three and 15 minutes to cool before the light can be restarted. But because starting a HPS bulb requires a tremendous amount of initial voltage—which stresses the system—turning the light on and off more than once a day is not advised because it can shorten life span of the bulb.

Unlike standard MH bulbs, HPS bulbs can be positioned at any angle without having to worry about the arc tube bending. So, there is no specific designation among HPS bulbs for horizontal, vertical or universal use, as there is with MH bulbs. In addition, not only are HPS bulbs the most efficient HID bulbs, they have an extremely long lifespan of approximately 24,000 hours, or up to five years of operation running 12 hours per day.

Over time, the sodium slowly escapes from the arc tube over long periods of daily use, and the sodium-to-mercury ratio within the arc tube changes—which eventually causes the voltage in the arc tube to rise beyond the point that the ballast is able to sustain. As a result, the light will eventually reach a point where it will start, warm up to full intensity and then extinguish repetitively. This is your indication that the bulb has reached the end of its lifespan. But in order to maintain peak efficiency in your garden, HPS bulbs should be replaced every 18 to 24 months, even though they have life spans more than twice that.

The main reason professional indoor growers use HPS bulbs for the flowering stage is that when a cannabis plant reaches the pre-flowering stage, hormones within the plant’s cells inform the plant that it has reached the age of sexual maturity. This causes the plant to cease concentration on the production of vegetative cells and instead start the process of preparing itself to produce seeds.

Because HPS bulbs produce a color spectrum that peaks in the yellow, orange and red wavelengths, they stimulate floral hormones within the plant and promote the formation and maturation of the buds. Thus, when the light cycle is reduced from 18 hours to 12 hours to induce flowering and the bulbs are changed from MH to HPS, the color spectrum emitted by HPS bulbs serves to enhance the process of inducing flowering. Then it continues to enhance the maturation of buds throughout the flowering stage.

Advances in HID technology

The invention of the 600 watt HPS bulb increased the efficiency of HID lights by 7 percent—making it the single most efficient HID lighting on the market. In addition, some HID bulb manufacturers, such as Philips and Iwasaki, have managed to produce a couple of enhanced-performance bulbs such as the Son-Agro (which is designed specifically to augment natural sunlight) and the Hortilux bulb. However, American growers haven’t developed a preference for a particular brand and type of HID grow bulb, though the Son Agro is widely favored throughout Europe.

Regardless of which HPS bulb you choose, it is important to remember:

  • Change your bulbs every 18 to 24 months in order to maintain peak efficiency.
  • Don’t turn your bulb on and off more than once per day, to avoid shortening its life span.
  • You should never attempt to remove a warm bulb from the fixture, because the base of the bulb expands as it heats, and causes the bulb to seize inside the socket.
  • Keep the surface of the bulb clean because it will collect both dust and dissolved salt residue, significantly reducing the amount of light the bulb emits. Bulbs should be periodically cleaned with water (or Windex) and a clean cloth.
  • Wipe off all fingerprints after handling a bulb, since the oils from your skin will weaken the outer envelope when the bulb heats.


By Bill Bernhardt
Image Credit: By Plantlady223 (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons
Cannabis Cultivation Today articles are for informational purposes only and should not be considered legal guidance or advice on grow practices. You should contact an attorney or a qualified cultivation consultant for specific compliance and cultivation advice.


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