7 Myths About Grow Lights

Most likely it’s the many myths, some started by the light manufacturers themselves, which have given LED grow lights such a bad reputation with indoor gardeners. It seems as if many LED lighting manufacturers don’t actually grow with their lights: their leadership team usually consists of a lighting engineer, plus an entrepreneur with an interest in gardening. Neither of them has much indoor gardening experience, if any. They’re chasing the next trend with the hope of turning a dollar, and with little practical gardening experience backing up their claims, they have inadvertently poisoned their market with misinformation.

To be fair, it’s not all of the LED guys, and it’s not just them. The indoor gardening industry itself has perpetuated these led lighting. myths out of ignorance. It’s easy to believe “facts” about LED grow lights when the same message comes from multiple trustworthy sources, including the distributors and magazines that serve the hydroponics industry.

What do you say we bust some of these myths?

Myth 1: Lumens = Photosynthesis

Silly grower… lumens are for humans! That lumens are an appropriate way to measure light produced by a grow light is the all-time number-one indoor gardening myth. Measuring light intended for photosynthesis in lumens is just plain stupid. Let’s be clear: a lumen (scientific symbol: lm) is a measurement of how much light the human eye perceives. It does not, in any way, measure the light that drives photosynthesis. Period. Simply put, lumens measure the total amount of human visible light that comes from a particular light source.

Plants and humans evolved under the same light, coming from the sun. But humans and plants use this light very differently. Humans use most of the “visible light range” between 400nm and 700nm, but our eyes are focused on 500-600nm, mostly the green and yellow portions of the spectrum. Plants have a completely different response to light, focusing their absorption around 400nm-500nm (blue) and 600nm-700nm (red). They also absorb some light in the rest of the visible spectrum as well as non-visible light in the ultraviolet and infrared bands.

Measuring grow light output in lumens is an artifact of the lighting industry itself. Since light bulb manufacturers focus mainly on illumination for humans, they publish their lamp specifications in lumens. Some countries require light bulbs to rated according to lumen output. Indoor gardeners have adopted this method for measuring the brightness of their grow lights since it’s generally available from the lamp manufacturers (at least up until LEDs came on to the scene).

When it comes to garden lighting, it’s time to stop thinking in lumens and start thinking about “photosynthetic photon flux density” (PPFD), which describes the density of photons reaching a particular surface area. PPFD is measured in “micromoles (μmol) per meter2 per second, ” which is a more useful measurement for the light your plants receive than lumens. You need a quantum flux meter to measure how much photosynthetically active light energy is actually reaching your plants. When testing LED grow lights, make sure to pick a quantum flux meter that is specifically designed for LEDs, or your measurements will be off. Unfortunately, these devices are very expensive.

Myth 2: Summer-to-Winter Kelvin Shift

A well-respected garden writer recently wrote this in one of the most popular indoor gardening magazines: “The [high-pressure] sodium light is very red and mimics the fall sun to induce flowering. ” HID lamp salesmen and hydro shop owners also claim that MH lamps are best for vegetative growth because they are “blue” like spring sunlight while HPS lamps are best for flowering because they resemble “red” fall light.

This is the second most widely held gardening myth: that the color of sunlight changes dramatically between seasons and that this color shift induces flowering. Ask yourself this: at midday, does a spring day look blue to you or a fall day look red? In a word, No.

Light “color” is measured according to the Kelvin (K) scale with blue having higher values and red lower ones. The world would look very strange indeed if the light temperature of sunlight changed from season to season by anything even close to the 2000-2500K difference between MH and HPS lamps. Don’t misunderstand: There is a seasonal shift in daylight color due to the depth of the atmosphere the sun’s light has to penetrate before reaching the earth. But this shift is small, 300-500K depending where you live, which is a difference that’s barely perceptible to the human eye.

On the other hand, daylight color definitely shifts across the duration of a single day. Sunlight starts out in the morning at approximately 2000K (orange), climbs above 5000K (white) at midday, then drops back to 2000K or lower at sunset. Daylight-sky color temp can climb as high as 8, 000-10, 000K (blue) on a sunny summer afternoon.

Why does this matter? Because indoor gardeners have been taught that changing from “spring blue” to”fall red” will induce flowering-in other words, will cause plants to shift from their vegetative growth phase to their flowering phase. This belief is likely the downstream effect of how HID lights found their way into indoor gardens. Initially, only MH lamps were available, and growers using them experienced results that were… OK. Then HPS lamps were introduced, and the gardeners who tried them found that these new lights significantly improved the weight of their harvests. Someone postulated that MH was better for vegetative growth and HPS better for flowering, and the myth was born. It’s become a mainstream “fact”: pick up any of the magazines distributed in hydroponics shops and you’ll find it. That doesn’t make it true.

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