Grow Hack: Does Switching from Metal Halides to HPS for Flowering Work?


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Indoor grow specialists all over the world use blue-heavy metal halide lamps (MH) for vegetative growth and switch to red-heavy high pressure sodium (HPS) lamps for flowering, but is this pricey, laborious process actually beneficial for cannabis?

Supposedly, the blue-light-intensive spectrum of MH lamps fairs best for vegetative growth, while the red-light-intensive spectrum of HPS lamps grow the best buds when it comes time to flower. Many growers light their gardens with this in mind, but others skip the process and light their gardens with the same spectrum from seed to harvest. As it turns out, switching between MH and HPS may be mostly unnecessary for cultivating cannabis.

When it comes to flowering, plants have divided themselves into three main categories: long-day plants, short-day plants and day-neutral plants. A long-day plant (LDP) flowers when it experiences the increasingly longer days of Spring and Summer. Most vegetable crops of interest to horticulturists happen to be LDPs: peppers, carnations, spinach, potatoes, etc. Cannabis, on the other hand, is a short-day plant (SDP), and therefore has different needs when it comes time to induce flowering in an indoor garden.

One of the most complicated subjects in plant molecular biology, photoperiodism, deals with plants’ response to seasonal light changes, and it just so happens that most of the research dedicated to photoperiodism has been conducted on Arabidopsis thaliana, a small mustard green native to Eurasia. Scientists use A. thaliana as a model plant for studying photoperiodism, and A. thaliana was the first plant to have its genome completely sequenced. Guess what, A. thaliana is an LDP.

Research on A. thaliana and other LDPs of interest shows us that, in addition to long days and short nights, LDPs respond favorably to red and infrared light when flowering. On the other hand, SDPs are notoriously insensitive to the spectrum of light they receive through a grow season, as long as it approximates that of the sun. At the end of the day, scientific research has shown us that SDPs are “dark dominant,” while LDPs are “light dominant.”

Plants use the ratio the red-absorbing pigment phytochrome in their “shade-avoidance” response. When phytochrome in its inactive state, pr, absorbs red light, it turns into its active state, pfr. Far-red light, a region on the spectrum between red and infrared that’s barely visible to humans, deactivates pfr and turns it into pr. At night, pfr slowly degrades to pr. When a long-day plant detects elevated levels of pfr in the morning, it knows that the nights are short and season has come for flowering. Supplementing LDPs with red light at the end of the day accelerates flowering by creating more pfr.

Despite the consensus that SDPs are “dark dominant” and do not respond much to light spectrum differences, some research done on chrysanthemums (an SDP) suggests that it too may respond to differences in light quality. This research on chrysanthemums has the most impact for, you guessed it, chrysanthemum growers. Flower greenhouses operate year round, and therefore require a way to keep crops from flowering until they’re needed, like during Christmas and Valentine’s Day.

What does the chrysanthemum research mean for cannabis? It suggests that short-day plants might not be as “dark dominant” as previously thought. Differences light quality during the day (provided by single-wavelength LED’s) affected the wavelength of light required to inhibit flowering responses during a “night break.” This means SDP must also be at least somewhat sensitive to the spectrum of they receive.

For years, the MH-for-vegetative-and-HPS-for-flowering dogma has been applied to cannabis, an SDP. Grow gurus have rationalized this arrangement by saying that cannabis would naturally receive a redder spectrum from the sun in the late summer months. The fact is, the light of the sun always has mostly the same spectrum. The winter months experience less intensity due to the angle of incidence, meaning the same amount of light get distributed over a larger area than during the summer. If it weren’t for cloud cover and other atmospheric conditions, sunlight in the winter and summer look pretty damn close. Any apparent differences in color are due to scattering effects, the same effects that make the sky blue, the sun yellow and the dazzling colors of a sunset.

When it comes time to configure the lighting scheme of your cannabis garden, your best bet is to stick with high-pressure sodium for the entire grow cycle. Some HPS lamps offer enhanced irradiance in the blue portion of the spectrum, giving you the best of both worlds. For rooting clones and seedlings, the best option continues to be fluorescent lighting. Fluorescent lights, poor in red, far-red and infrared, but rich in blue, recreate the cool atmosphere under a canopy small plants are used to and promotes short internodes and foliar growth resulting in bushy young plants. The lack of red light does not fare well for cannabis plants long term, but does not cause a significant shade avoidance response light metal halide or tungsten lamps do, which emit high amounts of far-red, which causes plants to stretch.

The idea that vegetative growth requires MH lighting while the bloom cycle requires HPS holds true for LDPs, but cannabis is not an LDP. This does not mean the idea is a complete fallacy for cannabis growers, but the idea should definitely be revisited. Until botanists perform controlled scientific studies on cannabis that experiment with different lighting schemes, and explain their findings using genetics and molecular biology, the world will never truly know.

Photo courtesy of the Oregon Breeders Group

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