How can I keep my cannabis plants from flowering? — Ed Rosenthal (2024)

This time of year, I’m often asked: “How can I keep my plants from flowering?” They’re referring to young plants germinated from seeds or clones that are hearty enough to bring outdoors. Now they want them to grow larger, but if they go by the natural light cycle there are over 10 hours of darkness which will trigger flowering.

Cannabis plants produce their own chemical trigger to begin flowering by measuring the hours of uninterrupted darkness. During winter and early spring, the plants respond to the short days/long nights by flowering. As the nights grow shorter in late Spring, plants respond by transitioning from flowering to vegetative growth. To keep the plants from flowering you can regulate the light cycle so the plants continue to experience short nights.

To prevent flowering, break up the dark cycle break with light. Think of it as if it were water and you wanted to get everything wet. You spray the light in the same way. The plants should be sprayed completely with light. Most lights, even a flashlight, can be used to break up the night cycle.

Preventing flowering takes less than a few seconds of light in order to break up the plant’s dark cycle. In my own greenhouse, I set up a system to give my plant’s light using two timers. The first is a repeating timer that has two dials. One sets the amount of time the light is on, I set it for a minute. The other dial sets how often the light goes on. I set it for every two hours. This timer is plugged into a standard 24-hour timer that turns on at 8 PM and stays on until 4 AM so the plants receive one-minute bursts of light every 2 hours between 8 PM and 4 AM. I use 20 watt CFL bulbs screwed into bowl reflectors since the light doesn’t have to be too bright.

It only takes a moment of exposure during the middle of the dark period for marijuana plants to remain vegetative, I have found that providing light at several intervals works best, so the dark cycle is broken up throughout the night.

With a bit of effort setting up you can manipulate the outdoor light cycle to grow and flower the plants at your convenience.

Here’s the science behind it:

As I explained inThe Marijuana Grower’s Handbook, the chemistry associated with the plant’s flowering involves a hormone called phytochrome (Pr). It has two versions Pr, which absorbs red light and Pfr which absorbs far-red light. During summer the plant receives far more red.light which is found in both daylight and lamplight than it does far-red light, so there is a buildup of Pfr. Towards dusk, there is more fr red light than red. This turns some of the Pfr to Pr and without red light, the Pfr slowly reverts back to its Pr state, When the than far-red light and time during long nights converts enough Pfr into its inactive form, Pr, the plant starts to flower. During the long nights of fall and winter, there is a long enough dark period for Pfr to be converted back to Pr. Pr is changed back to its active state, Pfr in the presence of even a few moments of light, which is why uninterrupted darkness is so important to flowering. Inversely, interrupting the dark period with a few moments of light can prevent plants from flowering.