Flowering is controlled by the plant’s natural response to light, referred to as photoperiodism. This complex process of interacting factors plays a pivotal role in plant reproduction, fruit and grain production, and so forth. Indeed, the entire world’s food supply relies on photoperiodism as much as it does on the sun rising every morning. Yet despite its fundamental importance to life on earth, the science behind this phenomenon remains mostly a mystery.
Scientists don’t fully understand why seasonal changes in the length of days induce flowering in plants, but the pieces of the puzzle we do know have helped cannabis growers produce better pot more efficiently for decades. So don’t keep yourself in the dark: This added knowledge will let you understand your plants on a more fundamental level and help you to troubleshoot—or avoid altogether—any issues you may encounter while growing.
When it comes to photoperiodism, cannabis falls into the category of short-day plants (SDPs), since it initiates flowering during the shorter days that come at the end of summer and the beginning of fall. Other plants initiate flowering with the start of longer days in the spring; these are known as long-day plants (LDPs). In either case, a hormone known as florigen induces the flowering response thanks to the plant’s natural circadian rhythm.
Short-day and long-day plants differ in the ways they flower, but the mechanisms of their circadian rhythms have some similarities. LDPs use the transition from night to day as their reference point, scientists believe, but SDPs use the transition from day to night as their trigger.
In either case, the plants are responding to the length of the night, not the day. This means that a cannabis plant’s flowering cycle can be disrupted or even reversed if the plant is exposed to bright light during its dark period. On the other hand, a period of darkness during the day doesn’t affect the plant’s circadian rhythm and will not disrupt its flowering.
Dim light during the dark period, such as that of a full moon at night, won’t disrupt the plant’s flowering stage in small amounts. Cannabis plants are mainly sensitive to red and blue light, and some growers use narrow-band green LEDs to work in their gardens when the grow lights are off.
For SDPs like cannabis, exposure to light on the red and far-red end of the spectrum helps induce flowering because the plant’s phytochrome (a light-sensitive photoreceptor compound) specifically triggers the expression of a gene that participates in flowering. This flowering response to red light explains why many growers use warm-colored high-pressure sodium (HPS) lamps for the flowering stage and cooler-hued metal halide (MH) lamps for vegetative growth.
Flowering happens in accordance with the circadian rhythm of the plant, which marks the time with an internal clock set to a 24-hour schedule, no matter the season of the year or whether the plant experiences constant conditions. That said, plants that use the photoperiod to regulate their flowering, like cannabis, are more sensitive to a disruption in their internal clock if they’re grown under 24 hours of light. Not only does constant light require exorbitant amounts of electricity, but some research shows that plants can absorb only a certain amount of light each day. The daily light integral (DLI) is the totality of the light that a plant receives on a given day, and it’s a useful measurement for growers who want to optimize both their electricity usage and their plants’ growth.
An inexpensive spectrophotometer can give you a decent estimate of your DLI. Some anecdotal evidence points to 25 mol-1m-2day-1 (25 moles of photons per square meter per day) as the “saturation point” for cannabis. After this saturation point is reached, the plant doesn’t require further light except to maintain its light schedule. Greenhouse growers use the DLI to determine if they need supplemental lighting to ensure high yields, but many indoor growers haven’t tapped into the full energy-saving potential of switching to low-wattage lights after reaching this saturation point.