The Earth takes about 365.26 days—that is, one year—to complete one orbit around the sun. The axis on which the Earth spins is at an angle of 23.5 degrees relative to the sun. Due to this tilt, the two hemispheres will, depending on the time of year, have either a longer-than-average or shorter-than-average exposure to the sun per day. This difference in sunlight results in the seasons. When the northern hemisphere is pointed closer the sun, the southern hemisphere is pointed further away (and vice versa), the dates for the equinoxes and solstices are reversed in relation to each other. They do not always happen on the exact same date every year due to leap year adjustments.

Exuinox and Cannabis Gardens

The reason all of this is important to cannabis gardens is because cannabis uses a process known as photoperiodism to determine when to flower. Specifically, flowering in cannabis is triggered by long daily dark periods. This makes cannabis a short-day (or long-night) plant. Artificially lit gardens take advantage of this by denying the plants long dark periods to keep them growing until they have reached the desired size. Limiting the dark period to six hours or less is a common method, although any schedule that avoids a long continuous dark period should prevent flowering.

Cannabis Light Schedules

Light schedules intended to induce growth simulate the light around the time of the summer solstice. Summer occurs in a hemisphere when it is tilted toward the sun, resulting in longer days, shorter nights, and generally warmer weather. The summer solstice, also known as midsummer, is the longest day of the year. It occurs near June 21 in the northern hemisphere and near December 21 in the southern hemisphere. The months around the summer solstice are the only months that do not have long enough nights to trigger flowering in cannabis. However, if flowering during these months is desired, outdoor plants can be covered with opaque sheeting to lengthen the dark period. This allows for flowering to start earlier than it would naturally.

(More on photoperiods and cannabis in Manipulating Flowering in the Grow Room Using Photoperiods.)

Fall is a time of transition. The days continue to get shorter and the nights longer until they are the same length at the autumnal equinox. The equinox occurs when the Earth’s tilt points perpendicular to the sun, removing the preference of one hemisphere over the other. The autumnal equinox is near September 22 in the north and March 20 in the south. Light schedules intended to induce flowering simulate the light around the time of the autumnal equinox, with 12 hours of light and 12 of dark.

Winter months occur when a hemisphere is pointed away from the sun. The days get shorter (and the nights longer) until the longest night of the year at the winter solstice, which is around December 21 north of the equator and around June 21 south of it. The long nights of winter are more than long enough to initiate flowering in cannabis, but plants grown in the summer are harvested well before the winter solstice. Winter doesn’t have to mean no growth, however. With the aid of supplemental lighting or indoor gardens, some outdoor gardeners start plants in the late winter. These plants can either be hardened off and set out to flower for a spring harvest or kept in growth until being planted outdoors as large starts in the summer.

Spring is much like fall in that it is the other midpoint between the two extremes. The Earth’s axis is again perpendicular, though on the other side this time. The short winter days get longer until day and night are again equal at the vernal equinox. The vernal equinox is close to March 20 for the top half of the globe and close to September 22 for the bottom half. It again has lighting close to 12 lit and 12 unlit hours a day. 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 generally need to be a couple 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. Then, before they can flower, the nights become too short for flowering. If timed right, it even encourages just a couple of flowers (but hopefully not many more than just a couple) to develop to aid in sexing. Flowering can also be prevented with as little as an hour of supplemental lighting during the middle of the night to break up the long dark period into two shorter periods.

This progression then starts again. The shortest night is the summer solstice followed by nights that get progressively longer until they are even with the day in the autumnal (fall) equinox and continue to get to longer until longest night at the winter solstice. Then the nights get shorter until they are again even with the day at the vernal equinox, and then continue to get shorter yet until the summer solstice, and the cycle continues.

In nature, the long days and short nights of summer allow the cannabis plant to devote energy to shoot and leaf growth before the shorter days and longer nights of fall induce flowering. Flowering can be triggered at the pleasure of the gardener by giving them long dark periods to simulate the lighting near the autumnal equinox. By understanding and manipulating these principles, many different possible schedules and plans can be developed to best suit gardeners’ needs and their available resources.

(Looking for more information? Try This Highly Recommended Cannabis Lighting Schedule.)