Wattage vs. Color of CFL??

If you're gonna do one plant get a small hps or metal halide best bang for the buck and better lighting.. I think they make 150w and 200w systems

KaliKitsune said:

Let the actual Horticultural Science teacher weigh in.

Photosynthetic poisoning is quite uncommon in marijuana plants. At sea level, the sun outputs 150,000lux per square foot. A mile above sea level, it jumps another 100,000lux. Mature marijuana plants can tolerate around 350,000lux before they'll start getting poisoned by an overdose of photonic radiation.

With most any growing light, just bear a simple greenhouse rule of thumb - 60-85 watts of lighting per square foot of exposed area. This is enough for pretty much every single plant that you can grow in a greenhouse. Once you've successfully flooded each square foot with that much lighting, height of the lamps will not matter, you can keep them at the ceiling if you desired.

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I think you mean 150,000 lux, not "150,000lux per square foot". Wikipedia's article on Daylight (Daylight - Wikipedia, the free encyclopedia) says Brightest sunlight is up to 120,000 lux. Wikipedia's article on Lux (Lux - Wikipedia, the free encyclopedia) says Direct sunlight is up to 130,000 lux.

So maybe wikipedia's numbers aren't right, and yours are. One 1700 lumen CFL still produces 658,800 lux at 2 inches. You say light toxicity happens in a mature plant at 350,000 lux. My experience is closer to 200,000 but, I may be wrong - I'm also estimating this number since I don't have a lux meter but am relying on lighting simulations. The original poster's comments were about a three week old plant - not mature, and I still think light toxicity is a problem - of course, I haven't even seen pictures of the plant in question.


I also don't think using watts per square foot is a good metric. Why use watts when you have lumen ratings available? Lumens per watt varies on bulb type (CFL, HID, etc). Watts listed on light bulbs are a measure of energy consumed, not light output.

Regardless, your comments regarding watts per square foot in a green house probably don't apply to this posters problem. In this case, its a smaller area with lights much closer to the plants than in a green house.

Leeguana said:

My square footage is about 4.25. To reach 85 watts per square foot, I would need 8 or 9 42 watt bulbs. If my ceiling were 6 feet high and I mounted the bulbs on the ceiling for the entire grow, my plants would be stretched, skinny, and have minimal bud sites.

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Note that most greenhouses have their lights FAR above their plants, out of reach without a ladder. There's a good reason for this, I demonstrated why to my class back in '98. Especially given the small size of your grow area, if the entire ceiling were CFLs, you wouldn't have to worry so much about them stretching as getting side lighting. Trust me, once you put enough photons to the floor, the plant's not going to use much more, and it will grow like regular.

I also forgot to mention that you'd need to coat your walls with pure white mylar to help keep the photon density at a certain level as the light goes down to your plants, inverse rule and all that fun noise. Works just a tad bit better than flat white paint.

KaliKitsune said:

I wouldn't listen to Redeemer's words. People with experience know that LUMENS is a measurement of GREEN LIGHT, which pot plants do not use.

You're looking for photon flux density in certain nanometer wavelengths when you grow. CFLs do not have the photonic flux density to cause any amount of photosynthetic poisoning until you get to the 75+w CFL bulbs that are supposed to act as HID replacements.

Go buy yourself a light meter and if possible a photography book for beginners. You'll learn more about light reading a photography book than you'll even learn by listening to me, a teacher. Then go get a basic horticultural science book, even an older 80s college textbook will work. You'll be a growing pro in no time.

Good luck!

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Lumens is a measure of human visible light. Light bulbs are rated in Lumens since that is what most people care about.

You really want to know PAR (Photosynthetically active radiation) watts - this is the measure of the light applicable to photosynthesis. PAR ratings for lights cannot really be calculated and are generally not given - especially the cheap CFLs we buy.

Lumens is the best measure of light that we have access to, it's a good metric.

jelven said:

Thanks for the info.. just one question.. how come when the plant gets bigger it needs more lux than bright daylight? Don't most plants get the same amount of light regardless of size?

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As the plant gets taller, one CFL cannot provide enough >100,000 lux light for the whole plant.

Again, at 5" one 1700 cfl gives off 105,400 lux. But at 12", only 18,300 lux. As you move further away from the light source, lux drops very fast. Also, you need to make sure all sides of the plant gets sufficient light.

Anyway I just dropped down to one.. i really hope this doesnt slow growth as my plant is already super slow... do you think it will take longer to grow? Just better quality or what?

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Here's an easy safety guard if you need one:
- Measure the height of your plant now.

If you are providing too little light, it will begin stretching - growing tall but not producing new nodes.

If you are providing too much light, the plant will be stunted, turn light green, maybe show some curling, etc.

If it still doesn't seem to be growing, neither in height (stretching) nor new growth then lighting isn't your problem. Again - I haven't actually seen your plants, they may be doing fine or they could be suffering from another kind of problem.

so how far away should you keep you cfl's from the tops of your plants? i try to keep em 3 inches from the tops when their vegging. this thread has made me question everytime ive grown indoors. maybe im simply not using my lights correctly? should a 600w hps be 10 to 12 inches above the tops or is that too much light?