Phrases like photoperiod, photoperiodic, or photoperiodism refer to plants that take their developmental cues from the amount of light they receive. Lots of different plants are photoperiod sensitive, including cotton, rice, lettuce, potato, soybean, and radish plants.
Cannabis is also a photoperiod plant believed to have originated in central Asia, and then spread to Africa, Europe, and, eventually, the Americas. It evolved to thrive in a variety of different climates, geographies, and growing seasons but, generally speaking, all photoperiod cannabis plants respond to changes in the length of the day/night cycle in order to guide their growth and development.
As an annual plant, cannabis completes its entire life cycle in a single growing season. When summer turns to fall, and daylight hours shorten, that's the cannabis plant's cue to begin flowering so it can produce seeds and reproduce before winter.
Photoperiod cannabis plants go through their most active growth phase (called the vegetative stage) when receiving at least 12 hours of light per day. When light dips to or below 12 hours per day, this generally triggers the plants to stop the active growth phase and begin flowering.
Sativa versus indica genetics
Historically, cannabis evolved as a photoperiod plant that adapted to different climates and growing seasons of varying lengths depending on geography.
Sativas originated in tropical climates where they became accustomed to lots of sunshine and long growing seasons. They are relatively less sensitive to changes in daylight hours since days and nights near the equator tend to be the same length, and as a result their flowering periods can be quite long.
Indicas are native to more northern latitudes, where growing seasons tend to be shorter and winter comes on more aggressively. Indicas tend to respond a lot more quickly to dips in daylight hours, and their flowering cycles tend to be shorter.
Regardless of genetics, climate, or the speed at which it matures, photoperiod cannabis will move through its life cycle based on its exposure to light.
Photoperiod versus autoflower cannabis
Autoflower varieties, on the other hand, do not need light cues to move through their life cycles. With advances in cannabis breeding and the development of new cannabis strains, today there are plants that complete their vegetative phase and move into their flowering phase automatically (hence the name autoflower).
Autoflower varieties were developed by breeding cannabis strains with genetics from Cannabis ruderalis, a member of the cannabis family that evolved in more extreme northern climates with very short growing seasons. These plants do not rely on cues from daylight to begin flowering, and evolved to move into flowering based on the plant's maturity instead. Breeders harnessed this quality from ruderalis by crossing it with existing sativa and indica varieties to create autoflower strains.
Aside from automatically flowering, these varieties also tend to have the general hardiness and durability of their northern grandparents, with relatively short life cycles. Some autoflowers will be ready for harvest within eight weeks after planting.
How long do photoperiod plants take to grow?
The life cycle of any plant will depend mostly on its genetics. As discussed, sativa-dominant varieties tend to have longer growing seasons while indicas will be quicker and hybrids somewhere in between.
Generally speaking, you can plan for the vegetative stage for photoperiod plants to last anywhere from three to 16 weeks, and flowering to take an additional from eight to 12 weeks.
Growing photoperiod cannabis indoors
While you can grow cannabis indoors using natural sunlight or outdoors in a garden, if you're using a grow tent or other enclosed space indoors with artificial lighting, you'll need to be mindful of lighting schedules.
Indoor growers often use timers to keep their plants on a tight routine, and while lighting needs can vary quite a lot from strain to strain, indoor plants generally go on a 16/8 light/dark schedule during vegetation and a 12/12 schedule during the flowering stage.
Growing photoperiod cannabis outdoors
When growing photoperiod cannabis outdoors, you'll need a good understanding of the light cycles where you are and how they change throughout the growing season. This will help guide your timing for seed germination, planting, and harvest.
Look at historical data (an almanac is a great tool for this) or use online sunrise-sunset calendars to figure out when you'll have the longest possible amount of sunlight so that your plants will be supported during their vegetative stage, then work backward to figure out when to start seedlings or acquire clones.
You'll also need to select a variety that will do well in your particular microclimate, because long days are not the only thing your photoperiod plants need to thrive. The type of soil you have available, the temperature, humidity, and amount of rainfall are other factors to consider when selecting a strain to grow. If you have a very short growing season, an autoflower variety might be a better option for you.
Beyond just the cycle of light and night, the effects of global warming are having increasing effects on outdoor cannabis farms. California, Oregon, and Colorado have all seen cannabis crops lost to wildfires, while a catastrophic hail storm and early freeze took out 20,000 ready-to-harvest plants at Los Suenos Farms in Colorado, and Hurricane Laura hurt hemp farmers in Louisiana. It's an added challenge outdoor growers need to be prepared for.
Since photoperiod plants rely on undisturbed hours of darkness to flower, light pollution is a major concern. Even something as simple as nearby street lights leaking into your grow space or light from your phone's screen can mess things up. Darkness is not to be taken lightly when it comes to photoperiod plants.
If photoperiod plants don't get the darkness they need, it can cause them to revert back to the vegetative phase, display hermaphroditic qualities (also known as “herming”), produce inferior yields, or fail to flower altogether.
If growing indoors, black out windows or move your plant to a closet or other completely dark place overnight. In an enclosed space like a grow tent, put your lights on a timer to keep them on a regular schedule. Outdoors, consider a black-out tarp to cover plants overnight — but make sure to get them uncovered in time for sunrise.
The bottom line
Photoperiod cannabis varieties have evolved to mature according to the light cycles of the growing season. While in their active growth stages, they enjoy long days and plenty of light, but when provided with shorter days and longer nights, they begin their final life cycle and produce flowers.
Cannabis is one of many photoperiod plants. Whether you choose to grow indoors or outdoors, paying keen attention to your plants' light needs is a crucial step in being a successful cannabis plant parent.