Day Length in Veg

If you’ve ever cracked open a so called ‘grow bible’ you’ve undoubtedly come across sections discussing the merits of 18hr of light vs 24hrs. During these discussions, I’ve read things such as ’18 hrs of light is better because it’s more natural’, or ‘plants need the dark to do dark cycle reactions’, as well as things like ‘It just uses less electricity, so I prefer it.’ I can understand, and even see why ideas like these are simple and appealing, however, they are not quite right.

First, lets talk about why we must provide so much light during veg. You may have read about pFr, and its role in the signalling of flowering. At its most simple, flowering begins when there is a build up of the protein pFr, a form of phytochrome.  Phytochromes are soluble light-sensitive protein that react to red and far-red regions of light. The form of phytochrome that absorbs red wavelength light is called Pr, and the form that reacts to far-red is pFr. Phytochromes begin biosynthesis of Pr during dark periods and upon absorption of red light Pr converts to Pfr.1 What this means to us is that if we have a plant in the dark its beginning the produce Pr, which is converted to pFr during the day. The less time in the dark, the less Pr created, and thus less Pr to convert into pFr. This is the simple principle behind flowering starting. With that out of the way, lets go over the earlier discussed talking points.

In terms of being better or using less electricity, I think the following study was very elucidating. I think we can agree that the bigger the better when it comes to yield (as long as it never sacrifices quality). Keeping that in mind, GW Pharma actually did a study on the impacts of 18hrs vs 24hrs of light for veg.

“A GW Pharmaceuticals study compared the growth rates of eight varieties in day length of 18 and 24h. After three weeks the plant in the 18h day length were shorter and lighter than those in the 24h day length. …..
To produce a similar mass of foliage to the plants that underwent 24hr day length for 21 day, the plants in the 18h group required 28 days, at which point the light energy emitted was the same for both plants, thus saving no electricity. ‘ –Handbook of Cannabis – Oxford Press, pg. 75

Having now dispelled two out of three of the most common lines of reasoning on the subject, lets talk about the so-called dark cycle. Most importantly, the dark cycle is now called the Calvin Cycle or the Light-Independent Cycle, as scientist thought the name ‘Dark Cycle’ caused confusion, making people think darkness was required. Instead during photosynthesis Light Cycle reactions and Calvin Cycle reactions often happen side by side. While the Light Cycle does indeed require energy in the form of UV Photons, the Calvin cycle doesn’t care about the presence of light what so ever. I’ll talk about this in more depth in my next post ‘Photosynthesis in Cannabis’ where interesting things like what happens to chlorophyll during curing, why some plants express more coloration than others, and how/why that coloration can be brought out by certain lights.

I hope this helps those of you trying to decide between 18h and 24h for veg, and above all, satisfies a little bit of your curiosity.


1. Phytochrome-hormonal signalling networks – Halliday and Fankhauser 2003 – DOI: 10.1046/j.1469-8137.2003.00689.x