Ask Alex #1: Which light do I use to grow my cannabis?

WSJ Team September 12, 2011 2

One of the biggest questions asked, and one of the most difficult ones to answer. We will focus on our favorite medical crop for the purposes of answering this question.

Which light you choose depends on several factors: The amount of space you have for growing purposes, the amount of ventilation you have for your growing space, and your planned crop height are the most important factors to consider when choosing a growing light. A secondary consideration is whether you are using soil, soilless, or hydroponic methods.

If you have a lot of space, good ventilation, and are planning on growing large plants, the HID is the choice lamp. The debate of Metal Halide versus High Pressure Sodium will rage on and on, but in my professional opinion, the Metal Halide is the choice lamp, and as soon as Ceramic Metal Halide can be used in powers ranges past 400w, that will likely be the king HID for gardening. These lamps put out more photosynthetic radiation versus the High Pressure Sodium, which emits mostly green and infrared. The additional UV bands increase production of flavors and cannabinoid content, as shown by John Lydon back in the mid-80s. Don’t let the lower lumen count fool you into thinking these lamps are not as good. Lumens is rated at green wavelengths (550-555nm,) not red and blue wavelengths which plants favor. Metal halide emits much more red and blue light, as well as IR and UV.

If you do not have much growing height to work with, or have poor ventilation capability, Fluorescent lighting or LED lighting are your main options. With fluorescent lighting, go with the most efficient – T5HO lighting. There are many bulbs available for T5HO lamps, even single-color lamps for targeting more of a single wavelength. Your best bet is a mixture of cool and warm white, unless you are going for a red/blue specialty setup, in which case you want about 25-30% blue and 70-75% red.

LED is an entirely different beast, and there are tons of myths that need to be dispelled first before we can discuss this. First myth, also the biggest, is that LEDs have no penetration. The problem with this is that this observation was made with human eyes. Sure, the first few inches seem brightly lit, but then the lower section looks almost black as if no light were getting to it. This isn’t lack of penetration, this is efficient absorption of the light. Plants utilize very very little green light, and the vast majority of this light either passes through the leaf or is reflected back to our eyes. This is why even a T5 seems to ‘penetrate’ and light up a plant more than an LED light. Our eyes are playing tricks on us.

The next myth about LED – more bands = better light. This is the equivalent to the more blades = better shave going on in the razor industry. A well-tuned LED light using three or four bands can perform just as well as some hodge-podge LED lamp with 11 wavelengths plus white (which is a HUGE waste and used only to really boost an LED panel’s lumen count, which brings us to our next myth.)

Next Myth = More Lumens = better. Not for LED. Real LED panels will not include green light, as they only target the most efficient wavelengths. Since the lumen is weighed by green light, and a good LED panel will have no green emissions, rating an LED panel by luminous flux is pointless. In photobiology, the proper measurement is photon flux density – or how many photons are hitting a given area per second. The measurement for this is the umol. To make this something we can all understand, on a clear day, at noon, at sea level in the Mid-West United States, the average photon flux density is 2,000 umol per square meter per second. This may seem like a small number but that is an insanely large amount of photons in reality. I have an LED panel that draws about 150w hourly, it can hit that 2,000 umol intensity about 10 inches from the panel, dead-center. My 600w MH hits that about 15 inches away, dead center in a good reflective hood. LED is very rapidly surpassing HID, the issue is that the majority of LED sellers don’t have a good product and are just reselling, they’ve done no actual ‘research’ beyond a couple of test grows with no controls or decent documentation process.

There are plenty of cheap LED panels, they’ll work, but you won’t get optimal results. Same thing with expensive LED panels – plenty available, most of them garbage.

One other thing to bear in mind about LED – panels that do perform are going to be costly, but you will have the advantage of being able to use that light for a long, LONG time before needing to replace it, and the payoff in both yield and energy costs is typically seen within the first run, especially is used in a hydroponics system, where LED will supercharge growth rates in conjunction with an oxygen-rich root zone and CO2-rich atmosphere.

Bear in mind that high-power LED panels will generate heat. We are not escaping thermodynamics any time soon. You do need adequate ventilation for these panels regardless.

If you are growing outdoors, well, you’ve got the sun, you don’t really need any other light!

Any questions? Just Ask Alex!


  1. Mike September 21, 2011 at 3:52 PM - Reply

    So, are there any LED panels I can buy today that match the umol of a 800/1000W HID more or less? If so, where? If not, can I build a panel myself (I know to soder and build electronics). Thank you.

  2. askalex September 28, 2011 at 6:07 PM - Reply

    I would look for a 600w actual draw LED panel. That will do what you need. They are large and pricey, however. Make sure you get a good ratio of red/blue and make sure you aren’t using so many wavelengths. A good quad band will work just fine.

    You could build it yourself, but it’s quite a hassle. You’ll need to find the right drivers for it, then you’ll need to find the LEDs you want, pre-mounted to star thermal boards. If you have SMD experience, you can probably build it from a raw thermal PCB instead and lay your own circuits.

    I don’t know where to point you out. I would have to say do some research. It will be pricey, my estimate for a GOOD panel pulling 600w actual will run you around $3600 or so.

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