Ten myths of growing under lights
I have been browsing some of the large number of internet pages devoted to growing plants under artificial light, some commercial, others purely informational. The amount of incorrect information is astounding. A few things crop up time after time, probably copied from page to page in true urban myth fashion.
Here is my top ten myths on this topic. Feel free to add more. Or, if you're that way inclined, defend one of my myths or just trash my explanation :)
1. HID lamps give off more heat.
All lamps of the same wattage give off more or less the same amount of heat. A 400W metal halide or HPS lamp gives off the same heat as ten 40W fluorescent tubes or four 100W incandescents. The HID lamps will be hotter because all the heat is being given off in such a small space, but the total heat is the same. A fluorescent tube will gently warm the whole shelf while a HID bulb will fry a few square inches of plant and hardly touch the rest.
2a. HID lamps have better "penetration"
2b. Light from fluorescent lamps drops off too quickly with distance.
Two ways of saying the same thing really, but both wrong. The same wattage of fluorescent light used at the same distance as a HID lamp provides equal "penetration". This myth arises because people tend to use fluorescent lights very close to plants (to compensate for having less power) and HID lights quite far away. The difference in light intensity between the top of a plant 2" from a light and the bottom maybe 12" from the light is very large, while the difference between 20" and 30" from a light is relatively small. No difference if you have the same amount of light, the fluorescents may actually perform better because the light is distributed more evenly.
3. GRO-LUX type lamps are less effective because they are less bright.
GRO-LUX fluorescents appear dim to our eyes because they emit light that our eyes are not sensitive to. However the total light power emitted is basically the same as an average fluorescent tube. The light emitted at wavelengths good for plants is as much as any fluorescent and more than many.
4. Plants only use red and blue light, green light is useless.
Plants use light at all wavelenghts from near ultraviolet to near infrared to convert water and carbon dioxide into sugars. They use red light near 650nm most efficiently, then blue light near 620nm, then light in between. Some plants are much less efficient at using green light, some use it almost as well as red and blue. All plants have some capacity to adapt to best utilise the available light.
5. Blue light is for growth, red light is for flowering.
Plants will grow under monochromatic red light. Plants grow faster with a little blue light (with the same total light power). Plants grow slower under monochromatic blue light because the same power produces fewer photons. Approximately 25% of the power in blue light appears to be best, but probably varies for different circumstances and plants.
6. LEDs are ("ultra-bright", "extremely low power", "used by NASA", "monochromatic", or pick your own catchphrase) therefore they are the best grow lamps.
LEDs look ultra-bright because they are small and emit all the light in one direction. This is great if you want to grow a single blade of grass and not much help for most anything else. LEDs are extremely low power because they are small and emit very little total light. This means you need a lot of them to grow anything useful and then they use a lot of power. NASA has experimented with LEDs because they are small, light, and nearly indestructable. None of these features are particularly valuable to the average plant grower. Many LEDs emit a very narrow range of light wavelengths, which means they can be calibrated to emit only the light most useful for plants. That's exactly what a GRO-LUX lamp does and the fluorescent GRO-LUX does it about five times more efficiently.
7. T5 fluorescents are the newest and best.
The new generation of T5 fluorescent tubes is extremely efficient and compact, both desirable features for a grow light system. They are as efficient as some metal halide bulbs. Unfortunately they are mostly being sold as premium items with a premium price tag and often with expensive custom fixtures. For the same money, a good HID setup would be better and for less money you can get the same light from standard fluorescents.
8. A windowsill isn't bright enough, you have to use artificial lights.
Direct sunlight through a window is far brighter than any grow lights most of us will ever see, use it when you can. The limitations are that there is very little direct sunlight in the middle of winter at temperate latitudes and there is only so much space directly in those south-facing windows. Indirect light in a south-facing window is poor, indirect light in any other direction is completely inadequate, and light more than a foot or so from a window is virtually useless to a high light plant.
9. Two fluorescent bulbs aren't enough, you need at least four.
The number of fluorescent tubes you need depends completely on how much light you want. Some plants need more light than others. Lighting a larger area requires more light. It is difficult to use more than two fluorescent tubes to light a strip the width of a seed tray, so two tubes are sufficient for a single row of trays. At the other extreme, four 40W fluorescent tubes are completely inadequate for lighting twenty mature tomato plants.
10. Cool white bulbs emit blue light and warm white bulbs emit red light, so mix them if you want both red and blue.
Warm white bulbs emit most of their light in the red part of the spectrum with only a small amount of green and blue light. Standard cool white (4100K) bulbs emit more blue light, but still more red than blue. "Daylight" 6500K bulbs emit roughly equal amounts blue and red light power, which still means about twice as many red photons as blue photons. So any of these tubes emit a lot of red light and some blue light, the difference is in the details.