First, it's great to know I'm appreciated. I did a lot of reading to learn what I think I know and I'm glad some other people can put that to good use. But this is as much for me as for you because I learn as much as I share here.

The hormone that triggers flowering in cannabis has to build up
I think it is OK to think about it this way but the truth is actually more complicated. For a short-day plant, like cannabis and rice, long-day conditions cause a hormone to be produced which delays flowering by supressing the hormone which causes it. When the long-day conditions expire the flowering hormone is no longer supressed and is free to express itself due to the diminished existence of the other hormone.

Reference


I don't have many references for this next bit as I usually do because I'm tired and it is late. I can find some later if anyone makes the request.

Some nonsense about absorption spectra....
I think a lot of people have huge misconceptions about what the different pigments in plants are for. Lets say there are two classes of pigments: primary pigments and accessory pigments. For land plants, the two primary pigments are chlorophyll a and b. Everything else, which includes carotenoids, xanthophylls, phycocyanin etc., serves some other function in support of the primary pigments so that they can operate at peak efficiency. For example, carotenoids can act as antenna pigments to supply the photosystem cores extra energy in low-light conditions. At the same time, carotenoids can absorb excess energy from photosystem II when it outpaces photosystem I in unbalanced or high-light conditions. You may see other pigments in an absorption spectra for a number of reasons (including, as khyberkitsune said, because they are talking about algae) but what you need to know is that they are irrelevant. For a perfect light source there is no need to stimulate anything other than chlorophylls a and b unless (and this is the only caveat) we wish to evoke a specific response. This caveat has an example in cucumbers - no one has identified a pigment in the green range but cucumbers will simply not grow without some greenish light.

I'm going to assume that any spectra you have seen are wrong. Most of the ones floating around the intaerweb are absorption in vitro (in the glass). We want action in vivo (in the living). To illustrate the difference between absorption and action I want to talk about how we get a sunburn. The UV light from the sun is absorbed by our skin and kills the cells there then we have a burn. We know that it is UV light because if it were any other light then we would be getting sunburns inside from artificial light. Our skin absorbs more than just UV light because if we reflected all light then we would be perfectly white but most people are at least a little off-white and some of us are dark as night. So we have an absorption spectra for our skin which is much broader than the action spectra which causes sunburns.

For plants we are interested in only the action spectra - that is the spectrum of light for chlorophylls a and b which cause a plant to produce oxygen (or, as it is actually measured, to consume carbon dioxide). Action spectra always show the red peak (around 660 nm) higher than the blue peak (around 450 nm). Most spectra will show the blue peak almost as high as the red peak but others will show the red peak extremely higher than the blue peak. This difference comes from the different ways which light can be measured. From our basic physics class we learned that light is both a particle and a wave. Most scientific experiments measure light by counting the particles, or number of moles. Light also has energy. The longer wavelengths (660 nm, or red) have less energy than the shorter wavelengths (450 nm, or blue). Because of this, measuring in moles tends to favor more energetic light because for the same number of particles, blue light will have significantly more energy than red light - yet action spectra still show red wavlengths to have greater effect using this measurement. If we convert this action per photon to action per unit energy then most plants in blue light alone (450 nm) are performing photosynthesis at only ~60% the rate of those same plants in red light alone (660 nm).

Proper Action Spectra

This brings us back to why we are using LEDs. In order to stimulate a plant to grow most efficiently we must use deep red light (around 660-ish nm). This will allow us to use the least amount of energy in to get the maximum amount of growth out. Still, this is not quite enough. Certain other responses are only realized in other wavelengths of light. Fortunately, most of these for most plants happen to be in the blue with broad response ranges so that 10-20% of your red output added again as blue (around 450-ish nm) will be enough to take care of these. This has been found to be the minimum requirement for growing most plants and indeed applies to our favorite plant. LEDs fit the bill for specific wavelengths and given their other benefits seem the best choice for this application at the moment. But, that doesn't mean we can't minimize even further if another technology comes along (I'm still waiting for these induction lights to get more attention).

Anyway, I hope I have not bored anyone. For exactly how much light per unit area per day, look into Daily Light Integral (DLI).
thepaan Reviewed by thepaan on . Importance of 460nm red LEDs? Here is something very confusing to me. Many LED proponents loudly proclaim the need for 460-470nm red LEDs over the cheaper 630nm LEDs. I understand that this better coincides with the accepted chlorophyll B absorption peak. So far so good. LEDs veg very well given appropriate wattage & color. So far so good. LEDs do not compare (generally) to HPS flowering as far as yield. (Please - let's not go off in that direction here. Thank you!) Rating: 5