Myths About Grow Lights

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.

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.

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.

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!