Preventing Flowering in Marijuana Plants

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.
[image: Young plants in my garden [Photo by Jane Klein]]

Young plants in my garden [Photo by Jane Klein]

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.
[image: CFL bulb]

CFL bulb
[image: The compact fluorescent lights (CFL) were kept on all night to
prevent flowering so plants could get to a larger size.]

The compact fluorescent lights (CFL) were kept on all night to prevent
flowering so plants could get to a larger size.
[image: Hawaii has a long night cycle all year long. There lights were
turned on during the night to keep plants growing vegetatively. When legal
growing involves plant counts, large plants are prized.]

Hawaii has a long night cycle all year long. There lights were turned on
during the night to keep plants growing vegetatively. When legal growing
involves plant counts, large plants are prized.

*Here’s the science behind it:*

As I explained in *The 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.
[image: Phytochrome response chart shows phytochrome Pr-Pfr sensitivity
across the light spectrum.]

Phytochrome response chart shows phytochrome Pr-Pfr sensitivity across the
light spectrum.

*Warning:* Using marijuana may not be addictive, but growing it is.

Enjoy your garden
*Ed *
[image: The Girls are growing up [Photo by Jane Klein]]

The Girls are growing up [Photo by Jane Klein]