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 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 (╬╝mol) 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.

Many gardeners use only one type of HID light for their entire grow, and that includes MH, HPS, and CMH lamps. None of these gardeners has any trouble “flipping” their gardens from vegetative to fruiting/flowering. They simply changed the photoperiod-the length of time the lights are turned on. Plants that are sensitive to day length flower when their photoperiod changes, not when the color of the light they receive changes.

Myth 3: 90 LED Watts Can Replace 400-600 HID Watts

Oh, how you missed out on the fun of the early days of LED grow lights! When LED grow lights were first introduced, many manufacturers boldly proclaimed that a single 90-watt LED grow light would out-produce a 400- or 600-watt HID. These claims were laughable then, and they’re still laughable now. LED grow light manufacturers have typically been overzealous with their claims, which they”prove” by growing wheatgrass or lettuce instead of the light-hungry crops (e.g., tomatoes, cucumbers, herbs, or flowers) that indoor gardeners generally prefer.Testing revealed that these early “90-watt” units actually drew only 54-56 watts of power at the wall, on average. With a few watts going to power onboard cooling fans, these lights actually produced less usable light than 75-100 watts of HPS-not anywhere near the 400- or 600-watt HID performance claimed by their manufacturers.

At least the industry seems to have learned its lesson. These days, most LED grow light manufacturers provide realistic power ratings and coverage area recommendations for their lights. This combined with better, more powerful LEDs and more effective light designs are helping to end this myth. It would be ideal for LED grow light manufacturers to publish the power of their lights in micromoles at set height intervals so that we, their customers, could decide for ourselves how much HID these lights could replace in the actual conditions we face in our gardens.

Myth 4: This Could be the Last Grow Light You’ll Ever Buy

Because LED emitters have a 50,000-hour-plus life-span, which is about 10 years if used 12 hours a day, a common sales pitch is: “This could be the last grow light you’ll buy.” This pitch is intended to help the buyer overcome the high cost of an LED grow light. Unfortunately, it just doesn’t work that way.

Even though LED emitters have very long useful lives, continuing innovation in light design, such as secondary optics, better heat management, and still-better LED emitters on the horizon, will cause most growers to upgrade to a newer, better-performing light well before they’ve put 10 years on their first LED grow light. So while “the last light you’ll ever buy” makes a great sales pitch, don’t believe it. It’s not true.

Myth 5: LEDs Produce Little to No Heat

The next-most-common sales pitch for LED grow lights is that they produce little to no heat. When a manufacturer claims that an LED grow light produces almost no heat, it makes the experienced gardener wonder whether the manufacturer has ever used one for anything more than a photo shoot.

Sure, LED grow lights produce less heat than HID grow lights, but there is still heat, and that heat needs to be managed. See for yourself: garden temperature will drop immediately after an LED grow light switches off, just like in an HID garden. No heat-no way!

Myth 6: LEDs Won’t Burn Plants

One of the biggest myths about LED grow lights is that they won’t burn plants no matter how close they’re positioned to the plants. This myth is based on the light’s relatively low heat output and the idea “the more photons the better.” Early LED grow lights, with their lower output, could be positioned close to plants-as close as a fluorescent light, in some cases. With today’s high-powered units, it’s easy to exceed the light-gathering limit for plants.

When hung too close to plants, LED grow lights can cause photooxidation or “light bleaching.” This occurs when more light is absorbed than can be processed by the plant. Those portions of the plant that are closest to the light-often the biggest flowers, unfortunately-turn white because their chlorophyll is destroyed. Both LED and HID grow lights can bleach plants when improperly used, though the problem is less common with HIDs because their high heat output will generally cause the gardener to raise the light, eliminating the threat.

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