While I'm not doubting this is true... I don't see how it could be really be effective to find the ganja in smaller outdoor grows. I mean what about someone who isn't growing MJ but has a huge garden with lots of tall green plants and fertilizes all the plants in it with high doses of nitrogen. Wouldn't those plants then also sit in the sun all day and have the nitrogen keep them hot at night and be seen by infrared?
My guess is that this type of infrared used to spot outside mj is only effective on monster grows composed of hundreds of plants all together in one location. I doubt FDD's 30 are going to put off enough heat to be noticeable. They are however certainly large enough to be spotted by a low flying helicopter.