Mother Nature’s Light Schedule and Growing Cannabis

The term “short day” is somewhat of a misnomer, as non-autoflowering cannabis is affected by photoperiodism and will flower if given daily uninterrupted dark periods of approximately 12 or more hours. It would technically be more correct to call them “long-night” plants, but the term “short day” is a convention set many years ago after the phenomenon was observed (cannabis flowers in the fall), but before the actual mechanism involved was understood (phytochrome conversion in the dark). Indoors, this is implemented by growth lighting schedules not having a 12-hour dark period (0- to 6-hour dark periods being the most common), and flowering schedules including a 12-hour dark period (12 hours of lights on, 12 hours of lights off being the most common). Any lighting schedule can be easily identified as either growth (not having a long dark period) or flowering (having a long continuous dark period).

Solstices and Equinoxes

The axis on which the Earth spins is tilted at an angle of 23.5 degrees relative to the sun. Because of this tilt, created by billions of years of celestial collisions, the two hemispheres will have either a longer than average or shorter than average exposure to the sun per day depending on the time of year. This difference in sunlight exposure results in the seasons; when the Northern Hemisphere is pointed closer to the sun, the Southern Hemisphere is pointed further away (and vice versa), the dates for the solstices (and seasons) are reversed. In the Northern Hemisphere, the winter solstice is in December and the summer solstice is in June. In the Southern Hemisphere it is the other way around. The two equinoxes are also reversed, but it tends to make less difference since they are equal to each other. These events do not always happen on the exact same date every year due to leap year adjustments so there may be a calendar day difference one way or the other for a particular year.

The word equinox literally translates as “equal night,” and the two equinoxes occur on March 20th and either Sept 22nd or 23rd. During an equinox, the day and night are approximately equal, and the Earth’s tilt is perpendicular to the sun, without preference of one hemisphere over the other.

The winter solstice has the longest night of the year when the tilt faces away from the sun, and the summer solstice is the shortest night of the year when the tilt faces toward the sun. In the Northern Hemisphere the summer solstice is on June 20th or 21st, and the winter solstice is on December 21st or 22nd (these are reversed in the Southern Hemisphere).

For the progression of seasons this means that at the start of spring (on the spring equinox) the nights are the same length as the days. The nights will then get progressively shorter until the summer solstice (the shortest night of the year), and then get longer until the fall equinox (12-12 again). Then the nights will continue to get longer until the winter solstice (the longest night of the year), and then get shorter until it is again the spring equinox (12-12 again). This cycle will then repeat annually.

These dates are commonly used to mark the start of each of the seasons. Even though the winter solstice is the day that gets the least amount of light, it isn’t usually the coldest day of the year. The reason for this is the same as the reason that even though noon has the brightest light of the day, the hottest part of the day is usually later, it takes a while for things to heat up or cool down due to thermal inertia.

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Spring Equinox and Cannabis Seeds and Clones

Outdoor cannabis planting is generally done a month or two after the spring equinox. This is to allow the nights to get short enough to not trigger flowering. In mild enough climates, a quick spring harvest can be had by setting out plants grown under short night conditions during the winter early enough for the plants to finish before (or about the same time as) spring planting.

Cannabis plants grown from seed can be put out earlier than those from clones because cannabis seedlings need to be a few weeks old before they can flower regardless of the lighting. This allows for an overlap where seeds can be planted under the last couple weeks of spring flowering lighting, and then before they are mature enough to start flowering the nights become too short for flowering. Since clones are already mature plants (even as cuttings) they will flower if put out under flowering lighting, and therefore have to be put out a little later than the earliest seeds in relation to the equinox.

Fall Equinox and Cannabis Harvest Time

Cannabis normally grows vegetatively through the short nights of summer, and then as the fall equinox in September gets closer, the long nights will trigger flowering at some point (a month or so is common depending on latitude) before the fall equinox and harvested a month or so after (often during the month of October or Croptober as it is sometimes called). The reason that there aren’t universal planting and harvesting dates for cannabis grown outside is that the precise hours of light and dark for a given date can change depending on latitude.

By understanding the natural progression of seasons and cannabis, the dark hours can be artificially manipulated to prevent or induce flowering. Flowering can be prevented with an hour of supplemental lighting during the middle of the night to break up the long dark period into two shorter dark periods. In this way, cannabis (temperature and other factors permitting) can be prevented from flowering during the fall, winter, and spring when it would otherwise flower. On the other hand, by pulling tarps or covering the plants to create a longer dark period, cannabis can be induced to flower during the summer even though that is when it would normally be in growth. Another tactic for a summertime harvest is to grow autoflowering varieties, which will flower at maturity regardless of the lighting schedule.