How does cannabis know when the moon is out
Control your outdoor greenhouse grow with light deprivation

Fooling Mother Nature: reaping extra cannabis harvests outdoors

Fooling Mother Nature: reaping extra harvests outdoors

By: Paul Josephs


Cannabis responds to increasing night length by beginning to flower. To initiate flowering, indoor growers can simply set the timer to 12 hours on and 12 hours off. Growers can do the same outdoors by blocking the sunlight for part of the day, allowing for more than one harvest during a normal growing season. Sometimes it is nice to fool Mother Nature.


Forget 12 and 12

Indoor growers use the 12 hours on and 12 hours off cycle for three main reasons. One, it’s easy to set on a timer. Two, all Cannabis plants will respond to this light cycle by flowering.  And three, it is cheaper to run only 12 hours of light than, say, 14 hours.


Cannabis is an obligate photoperiodic plant. In simple terms that signifies that both Cannabis indica and sativa plants will not flower unless the period of darkness they are exposed to exceeds a certain amount of time. This amount of time is called the critical night length. Exposure to bright light during the night cycle for a duration of time will interrupt the hormonal flowering response to darkness. If the interruption occurs regularly, and the plant is already flowering, the plant will cease flowering and begin to grow vegetatively again.


The critical night length varies somewhat for different Cannabis strains. In general, the further north or south from the equator the parentage originates from, the shorter the night period will need to be to initiate the flowering process. As a broad guideline, Cannabis indica landraces are found in northern latitudes, and Cannabis sativa is found in tropical latitudes closer to the equator.


If Cannabis plants required the 12 hours of darkness that indoor growers use to induce flowering, plants would not respond until the autumnal equinox, which takes place every year around September 22nd north of the equator and March 20th for southern latitudes.


Realistically, most growers are thinking about harvesting some cultivars around that time, and most finish by late October in the north and late June for our mates south of the equator. Obviously, the plants had experienced their critical night length threshold well before this date. If the threshold is 14 hours, that night length occurs on August 12th in Boston this year.  The Farmer’s Almanac is filled with good data for the outdoor gardener for determining dates, times, moon phases, and much more! Since it varies by strain, growers can’t be sure of the exact threshold without experimenting under lights or from past observation outdoors, but 10 hours of darkness is a good starting place.


Auto plants

With auto-flowering strains multiple harvests can be achieved outdoors without artificially altering the length of daylight conditions. Auto-flowering strains possess genetic traces of Cannabis ruderalis and flower regardless of day length. This article is going to focus on strains that do not have this auto-flowering characteristic.



Since there are many ways to keep the plants out of the sun for part of the day to allow early flowering, let’s examine and compare them.



If the plants are in containers and are also located near a house or structure that is light-proof, they can be moved in and out daily. Obviously, this has a couple of drawbacks. Not only will moving plants twice a day put stress on the grower, his back and his schedule, the plants’ ability to grow evenly may be affected due to the stress of being moved. On the positive side, the house or structure may well be warmer than the outdoors at night, which allows the grower to extend the normal frost-free period on both ends of the season. The ability to move the plants is beneficial for protection from freak cold snaps and damaging storms.



Rather than moving the plants into a dark space, one can create a darkened space of sorts to place over the plants. This could be as simple as putting bags over the plants, or as complex as creating a shored-up excavation that can be covered with boards and black plastic drop cloths. A MacGyver type tinkerer could devise a method like a roll-up garage door that automatically covers and uncovers the excavation on a timer. This concept could fit well with growers using a greenhouse or sun room, by covering the glass or plastic with dark cloths. Borrowing an idea from gardeners, using wood or  PVC pipes to fashion tall row-cover frames over the plants which could hold a light-proof cover every day.


Considerations for covering techniques relate to temperature, ventilation, and logistics.

Covering plants with black plastic will cause the temperature to rapidly rise around the plants, quite possibly to lethal levels. A reflective covering will be safer, but not very stealthy if the grower is concerned about theft. Making provisions for allowing air exchange is important to allow for cooling, plant growth, and pathogen control. Like moving, the logistics of ensuring the plants are covered in time to allow an adequate night length every day can be tricky but at least covering techniques can be automatized with a little more of an investment and ingenuity.


A case study

Dirti lives near Denver, Colorado. He knows that he has an average of 156 days to work with without a damaging frost — from May 1st to October 4th. He planted some seeds in late February and once May 1st arrived he placed them outside. Dirti checked the Farmer’s Almanac and knows that on May 1st daylight is almost exactly 14 hours and day length will increasing continually until June 21st. He doesn’t know what the critical night period is for the strain he’s growing so he brings them into his unused and darkened garage after 13 hours of daylight. Later that night Dirti brings them back out. The routine continues and on June 30th, he brings them in for harvest and puts out the next batch of vegetative-stage plants. On August 29th, he harvests them and puts out his third batch of plants to flower. This last crop is dicey for his region, as the last frost-free day is expected to be October 4th. But Dirti is growing a mostly Afghan strain, and he knows they can handle mild frosts with ease. Since they go into the garage at night, he doesn’t really have to be concerned. Despite some really windy days and one big snowstorm, Dirti harvests this last batch on October 28th. The first batch had a pretty good yield, the middle one was the biggest, and the third was the smallest, but it turned purple from the colder temperatures.


Why bother?

It does seem daunting to be able to do all this and still have a life, but it also affords the opportunity for up to three harvests in a temperate climate where only one is normally possible. Needless to say, in the case study presented above, the combined total value of the crops far exceeded the value of just one crop. It is possible to do this and with the creativity that many Cannabis growers possess, there are many ways to accomplish this that have not been covered here. Put on your genius MacGyver cap and get creative – has the lack of a pipe ever stopped a creative smoker before?


What Constitutes Daylight?


Cannabis needs an uninterrupted night period of a certain length to begin and continue flowering, usually 10 or more hours. What isn’t clear is at what point does the plant respond with light-reactive hormones that indicates when the night has begun and ended. For example, the sky lightens gradually for some time before sunrise and darkens gradually after sunset. Additionally, the full moon reflects quite a bit of light onto the earth at night. That the light needs to be of a certain intensity and duration to be able to alter the hormonal balance related to flowering in Cannabis is clear, but the exact value of that intensity and duration remains a mystery. This would be a great research topic which would benefit Cannabis cultivators of all varieties.