I originally posted it in the Absolute Beginners and it got moved here....
i meant you should post it in that particular thread, titled "so you want to grow marijuana? questions answered here." the mod is a grower of some 15 years experience, but he has been out for awhile with an illness so on second thought it might take some time. in the meantime, the reasons this is a bad idea are 1) growing cannabis (any plant really) requires a specific color temperature, corresponding to specific wavelengths of light, which appear at different points on the EM spectrum. not to say that christmas lights couldn't coincidentally have an appropriate spectrum, but it is unlikely. i haven't looked at a box of christmas lights recently, but i would venture a safe guess that the color temperature is not listed. 2) even if you found a perfect match for spectrum, the number of lights required for even one plant are enormous. i'm assuming you're wanting to use the LED christmas lights, so the CFL "rule of thumb" of 100 true watts per plant doesn't necessarily apply, but even commercial LED grow lights are barely climbing up just above the threshold of efficiency that makes them useful. the technology is advancing, but is nowhere near viable, and that's for LED's engineered to specific standards; christmas lights are for lighting doorways, not growing plants. actually, i bet if you used enough christmas lights to crowd out all the volume in your grow space, you could get some growth. cannabis is an impressively resilient plant, and it might grow. but would there be any bud to smoke? nah. even if you could you would be spending so much money on the required number of frigging christmas lights that you might as well just go get a real grow light and save some space. same goes for heat.. bulb for bulb, LED's produce less heat than HID's, sure.. but you would be using so many that you would have no advantage whatsoever in terms of heat control, and likely a similar story with electricity. those christmas lights are also extremely dangerous around moisture, and you can be damn sure there's going to be plenty of moisture in the air, on the plants, around the plants, on the floor, spills, etc. there's a reason those lights are cheap. they have barely any insulation around the wires, and so are prone to melting/overheating/shortages/potential sparks.
also, it's worth noting that christmas lights have been around at least as long as indoor pot cultivation. if there was any significant advantage to their use, there would already be tons of people using them.
lastly, please don't take this personally, but people who fuck off and try to get away with reckless shit like using cheap christmas lights to grow pot put other people's lives at risk. if someone got hurt in a fire caused by christmas lights in a grow tent, people would begin (continue, really) associating pot growing with people doing the proverbial concrete swan dive into the darwin awards. that doesn't lend any legitimacy to the practice.