Optimal power output range and luminous flux density can be confusing to calculate for different lamps, so here is a guide to calculating approximate power/ area and how this is affected by mounting height. Further on, we will compare LED, Incandescent, Fluorescent, and HID lamps, because all emit light with slightly different characteristics. Regardless of what you are growing, the correct power levels are absolutely vital to getting good results, and while modern LED and HID grow lights are now relatively easy and safe to use as grow lights for indoor plants, unnecessary mistakes can cost you time and money.
Let’s break this information down into two lots. First the simple, quick and dirty version for those of you who can’t sit still for more than 30 seconds, then the full colour, complex version that will help you make genuinely educated choices. Try and read them both and hopefully it will make sense…unfortunately there is a lot of misleading information on the net, so it’s worth educating yourself before making any purchase. No grow lights for indoor plants are perfect!
Grow lights for indoor plants: The simple version.
In order to effectively emulate natural light and stimulate vegetative growth, the following power outputs are to be taken as a rough guide. Power is given per square foot.
Incandescent light bulbs: Between 25 and 50 watts per sqft. Generally don’t exceed 14 hours of light per day, and be watchful for dried out leaf tips. This means a 25-50w bulb suspended about 1 ft above a 1 sq ft patch of growing medium – or about 1 regular sized…um… tomato plant. Because normal light globes (if you can still get them thanks to ridiculous new environmental laws) shine in all directions, use a reflector. It doesn’t need to be fancy – make your own from a thin sheet of Aluminum, or just use a white plastic lampshade. Or, simply use one of those silvered globes that have their own reflector, they are often used in outdoor security lights and have a 60 degree reflector.
Fluorescent Light tubes: Begin with 10w per sq ft at germination stage, and gradually increase this to 30w per sqft as plant reaches mature size. Can extend to 16 hours of light daily. Fluorescent tubes commonly come in 3ft and 4ft lengths, and generally put out 10w per ft of tube (different ones are available but a 4ft long 40w tube is the standard for interior lighting applications. Therefore, start with a single 4ft tube over a 1×4 ft growing box, at a height of 1 ft above the plants/ seedlings. This will give approximately 10w/sq ft. Later add another, then another tube along side – a really easy and bulletproof way to get your 10, 20, then 30w/sq ft levels in as the plants grow.
HID grow lights: Between 15 and 35w per sq ft. usually not recommended to exceed 16 hours daily, as they can get pretty hot. You are unlikely to actually find 15-35w HID grow lights for indoor plants, rather, 100w or more is the lowest common output (this has to do with the construction and physics of high intensity discharge technology. Usually for larger setups, where a 150-350w lamp would be suspended about 3-4ft above an 8-10sq ft growing box, or array of large pots.
LED grow lights: Between 5 and 10 w per sq ft. some people claim 20 hours a day as the light runs at low temperature, but conservative recommendation is no more than 16-18 hours maximum. When using LEDs as grow lights for indoor plants you cam lower the light quite close to the leaves – the low temperature will not scorch or dehydrate them (this is part of the reason why you can get away with a lower power output. A single 10w LED panel is around 5 inches x 5 inches (some are round, some square, but who cares) and you wil note it has a mixture of red, blue and maybe even white or yellow LEDs. These are dead easy to use. Just hang them over a 1ft sq pot or box, or buy larger panels for larger boxes. 10w is said to be enough to fully grow a good 2-3ft tall plant.
Now, that is a really quick and dirty guide, before the scientifically minded amongst you start to protest! As mentioned before, there are significant differences between the characteristics and quality of light that is emitted by LED, HID, Incandescent and Fluorescent grow lights for indoor plants, and to better understand how and why each type of light works best we need to get a bit technical.
Perceived light levels VS PAR (photosynthetically active radiation)
The problem we have is that light is not just light – it is composed of lots of different colors, or wavelengths, of light, or more accurately radiation. We as humans see a bright light which illuminates a room or object, and it looks white and bright – quite natural – but different species will look at the same source, and see something different. A reptile will observe a heat source, an insect will notice that flowers and other insects are brightly lit by ultraviolet light from the same source. In short, every species has adapted to use light in a range that is most useful, be it in the infrared or ultra violet – it just so happens that humans fall somewhere in the middle. Plants are similarly specialized – they like a certain wavelength (actually there are several peaks, so a RANGE of wavelengths). Basically, plants are best able to use light which we would consider to be in the blue and red color band. Green light, by the way, is next to useless…it is simply reflected straight off the leaves, which is of course why plants LOOK green to us. The wavelengths of light that the plant can actually use are referred to as PAR, photosynthetically active radiation. A light can look bright to me or you – but it might be quite dull and cold to a head of lettuce! This is why the measure of light in “lumens” is totally inappropriate in a horticultural environment. Lumens, the single unit being one lumen, is a measure of light visible to humans, and was only really intended as an aid to photographers to determine appropriate lighting levels. Be wary of grow lights for indoor plants that are rated in lumens…just because they LOOK bright does not mean they will impart energy to your plants efficiently.
How does this affect our considerations for grow lights for indoor plants? Because it helps us choose the most efficient lamps, thus saving power and money.
Recall that above we said that you need to apply about 25-50w of incandescent light per sq ft, or per plant? Well a lot of this light will be wasted, because only a relatively small proportion of the 25-50w of emitted power will fall in the red-blue area of the spectrum. Incandescent grow lights for indoor plants put out a great deal of heat (everyone has been burnt by a hot light bulb) so there is clear evidence that a huge amount of power is lost in the essentially useless IR (infrared) are a of the spectrum. So, the bulbs are cheap, but they are not very efficient. It doesn’t mean they don’t work, but they are not the choice of pro’s. HID lamps are somewhat better – they can be “tuned” by doping the ballast (the energized medium that emits photons, or light) to emit light that is far “brighter” in certain areas of the spectrum. Usually professional growers will use 2 different types of HID for the initial growing (leaf and stem development) phase, then switch to another for budding/flowering/fruiting. Fluorescent tubes or bulbs are naturally concentrated toward the blue area,emitting little if any heat, and are thus still a reasonably good choice. This leaves us with LED grow lights for indoor plants. Now, LED’s are somewhat unique in that they are very precise in the wavelengths of light they emit. You can actually buy LEDs rated at 440, 460, 0r 600 nm, the “nm” standing for nano meters, which is the length between the apex and the trough of a wave, so they are very precise – in theory, we could work out exactly what wavelengths a plant best responds to, and give it ONLY that frequency/ color of light…saving a ton of otherwise wasted power. In the practical world, it is not quite this simple (scientists it seems are constantly discovering the significance of specific colors/wavelengths of light) but we do come fairly close – you will note that LED grow lights for indoor plants usually are in a ratio of approximately 8x red, 1x blue, and 1x white or orange LEDs. Not 100% perfect, but LED grow light technology is definitely shaping up to be the new big thing for indoor plant growers.
So, now we know about visible light VS effective light, and can appreciate that those dull violet looking LED light panels are actually the equivalent of a hot summers day for our seedlings and clones.