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Forcing Cannabis to Flower

Forcing Cannabis to Flower

Posted by Celia Daly on May 26, 2016 12:00 am
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What causes cannabis plants to flower? Any successful cannabis cultivator must know the answer to this question in order to generate revenue from a crop. Alternatively, the reverse knowledge, how to prevent cannabis plants from flowering, must be used to keep a Mother plant in a perpetual veg state.

The answer is simple and lies in the biological makeup of the cannabis plant. Cannabis plants will flower when they receive more than 12 hours of continuous darkness. This is known as a short-day plant.

To understand this better, let’s look at the science behind photoperiodism, or the reaction of organisms to the length of day or night. There are two main types of plants: long-day plants and short-day plants.

When these names were developed, it was originally thought that the amount of light a plant receives determined when it flowered, thus the terms “short-day” and “long-day.” However, scientists have since shown that the amount of darkness a plant receives actually determines when a plant will flower.

Long-day plants flower when the night length falls below their critical photoperiod (aka, when they receive more than 12 hours of light). Long-day plants include carnations, barley, lettuce, irises and much more.

Short-day plants flower when the night length exceeds their critical photoperiod (aka, when they receive more than 12 hours of darkness). Short-day plants include cannabis, poinsettias, chrysanthemums, soybeans and much more.

This critical importance of long nights is why cannabis plants can have such a detrimental reaction to light during the night cycle of flowering. The light interrupts the night cycle and throws off the plant’s internal clock that tells it how much darkness if has received, as little as 10 seconds of light during the night cycle can impact the flowering ability of a short-day plant.

Alternatively, to keep a Mother plant in the veg state, just ensure it does not receive more than 12 hours of darkness, as this will trigger a move to flower. This is why Mother plants are typically kept on a similar light schedule to veg plants.

Understanding the biology of a plant is critical to successful cultivation and maximized yields. The short-day designation of cannabis is just the beginning of a fascinating biological system that controls everything about the plant. Take the time to see what you can learn and your yields and quality may just increase accordingly.

Topics: Cannabis Basics, cannabis cultvation, featured, growing indoors, lighting

4 responses to “Forcing Cannabis to Flower”

  1. Rodrigo Ayala says:

    I have four , 3 week old plants. It’s January 28th 2019, and all my plants are flowering!… How do I keep them alive until October? I have them in pots, but they’re Outdoors. I planned on transferring them to the ground, in early March. I know how to force them to vegetate, but is it too early or do I need to give them artificial Light?

    • jamie says:

      Your plants are flowering because they are outside and thus receiving less than 12 hours of light a day. The cannabis plant must get at least 13 hours of light a day to stay in vegetative growth. In winter time when the days are short, that means artificial lighting.

  2. Terrell says:

    I switched back to 12 hours of light six days ago, on plants that are about 7 or 8 weeks old and are about 30 inches tall. I see no indication that they are beginning to produce flowers yet. How long does it usually take for pistils or buds to start being visibile after cutting back the lights?

    • jamie says:

      It can take up to two weeks to see visible signs of flowering. You don’t have anything to worry about as long as you’re sure the plants are in total darkness during their dark photoperiod.

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