Botany & Marijuana: Shedding Some White Light

In a 2001 book, “The Botany of Desire,” food journalist Michael Pollan referred to underground marijuana growers as “the best gardeners of my generation.”

Part of their gardening savvy, Pollan explained, involves the lighting they install in their basements or “grow rooms,” and their intense concern over the distinction between sodium and metal halide lights.

For those who haven’t been practicing underground botany: metal halide lights produce a wide-spectrum illumination by an electric arc within a mixture of gases often including argon or Xenon, and mercury. Sodium lights create an arc through vaporized sodium metal, though other materials and gases are employed in part to control the color.


Some Underlying Science

Most of us probably remember even from a K-12 science curriculum that white light, what we might call “God’s light,” breaks down into a spectrum of colors, and that the portion of this spectrum visible to humans runs from violet to red, shading off into ultraviolet on the one side, infrared on the other.

What botanists add is that plants “see” the world in much the same way humans do. That is, the light that matters most to plants, technically known as the photosynthetically active radiation, or PAR, portion of the spectrum, consists of the wavelengths from 400 to 700 nanometers which is more-or-less the same as the band of visible light.

Photosynthetic activity is not uniform along that band. Plants respond much more avidly to some portions of the spectrum than to others. The matter has been studied for a long time—it is, of course, not of interest uniquely to marijuana cultivation, but to agriculture and forestry generally.

Graph by Georgia State University

The above chart, from a website maintained by Georgia State University, looks at several variables. It tracks the absorption of light for three different pigments—chlorophyll a, chlorophyll b, and B-carotene—as well as looking specifically via the green curve at the output from photosynthesis.


Understanding a Cliché

Chlorophyll a is the most important pigment, and its absorption curve on this chart shows two distinct peaks. The first peak is at indigo/ blue light, just below 450 nms. Then the curve shows a sharp dropping off—the low value of green light for photosynthesis as reflected in the middle of this graph is, of course, why leaves are green or, in the language of biophysics, why photosynthetically active surfaces reflect back the green portion of the spectrum.

But, following the curve further, there is a rise in response and a second peak on the red side of the rainbow, just below 700 nms. The curve for chlorophyll b is quite similar, though its first peak comes a little later and its second peak a little sooner than do their cousins, the “green valley” is the same for both a and b.

This graph may answer the question: why has the “purple haze” cliché grown around the use of grow lights in basements? Purple is a mix of violet and red, so it results from an effort to capture both of the hills in this chart, ignoring the intermediate valley.

Another line that demonstrates the origin of the “purple haze” cliché is labeled “output from photosynthesis,” which starts at its high point, at the violet left hand edge, then slopes downward consistently through half the graph, until it moves more-or-less steadily upward to a second peak just about where chlorophyll b also peaks. And, indeed, a lot of basements seem to have benefitted from that intuition, but that doesn’t necessarily achieve outstanding results.


A New White-Light Product

Nick Brumm is the founder of NextLight, a company created to reflect his conviction that, as his website puts it, the quest for a “perfect spectrum” by the makers of purple light emitting diodes is a “gimmick rather than a scientific dispute.” In a recent exchange of emails, Brumm clarified his position. The competitors “are trying to develop lights that deliver the chlorophyll A/B curves, but they are missing the entire middle of the visible spectrum,” he said.

NextLight sells lamps designed to emit light as close to that from high pressure sodium lamps as possible, in the process delivering white light.“We think the missing link is the entire spectrum,” he wrote, “and we are trying to deliver the most area under the curve in the PAR region. We have by far the widest red and blue bands in the industry along with everything else the plants need.”

Brumm is convinced that it is a bad idea to omit that green mid-range. Yes, plants reflect back green light. But any particular surface is reflecting back only one specific shade of green, whereas the sun, or full spectrum light otherwise active, is providing it with a wide range of shades, even a wide range of shades of green specifically, all as part of the whole photosynthetic mix.

Green light, he explained, is processed with an efficiency quite close to that of blue light, and efficiency is what is important, especially in the lower bud sites on a cannabis plant, where the buds are shadowed by their upper-tier brethren.

In NextLight’s view, the bottom line for growers is simple: its white-light system gives growers a better bang for their buck than those of its competitors. NextLight offers, as its website claims, full spectrum white LEDs that are “the best LEDs money can buy,” while also reducing energy consumption in the process.

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