Thanks, and a 12/12 light question, please!

by Kevin
(Yorkshire, UK)

I'd like to say a HUGE thank you to you for your sterling efforts on this website - I have to say it's by far the most accurate and informative hydro website I've come across in the last 5 or so years. Thank you, indeed.

I try to copy nature as close as I can. After all, plants have been around far longer than I have! I'm a little worried about switching from 18/6 to 12/12 light-on/light-off periods. My gut feeling tells me a plant will respond better to my gradually reducing the light by 30 mins each day until 18 hours of light becomes 12 hours of light, as opposed to a sharp, abrupt change of light.

What are your thoughts, Jason, please, on this?

Answer: Kevin- Flowering plants have developed a way to keep track of the growing season in nature....it is called a "chemical clock". When a plant finds itself in complete darkness, it begins to accumulate a particular light-sensitive hormone. Each morning, the sun comes out and begins to destroy the hormone that had accumulated the previous night.

As long as these hormone levels remain below a certain point, the plant continues to spend it's energy on vegetative growth. However, as the days grow shorter and the nights grow longer, the amount of hormone found in the plant each morning becomes larger and larger. At some point a critical level is reached. At this point, the plant wakes up one morning and has abruptly decided to put all of it's energy into flowering.

So, even though the photo period outdoors decreases very slowly over many days, the ONSET of flowering within the plant is triggered rather abruptly by very natural means. In my opinion, the onset of flowering triggered by going from an 18/6 light cycle one day to a 12/12 light cycle the next day is not as un-natural for the plant as you might initially suspect.

In all my years of gardening, I have never noticed my outdoor plants begin to change physiologically in preparation for flowering. Instead, outdoor plants remain growing vegetatively without any sign of the approaching flowering period (even in the face of shortening days) until a particular point is reached....a point that triggers the flowering cycle just as suddenly as if you cut the light cycle all in one day.

Furthermore, whether grown indoors or out my plants always display the same 10-14 day transition period once flowering has been triggered. This reinforces my idea that no physiological changes take place outdoors in advance of the flowering trigger, and also supports my assertion that flowering is triggered just as suddenly outdoors as it would be indoors (or very nearly so).

There is at least one reason I can think of why the sudden change from 18/6 to 12/12 would be a benefit to the grow cycle. Outdoors, a plant receiving only 13 hours of light may still be in a vegetative state. Indoors, your vegetative plants always have the benefit of 18 hours of light (or more) till the very day they are put into flowering. This means there will be more energy stored up in the plant tissue to be used in the first few days of flowering.

In short, there is no benefit for a "slow transition" to the 12/12 light cycle. As I have shown, plants are naturally switched into flowering rather suddenly due to hormones, in spite of the fact that the light cycle outdoors decreases very slowly. A slow transition will not prevent stress, it will not reduce shock, and it will not facilitate the onset of flowering in any way (in my opinion). I hope this sheds some light on your question....Happy Growing!