The flowering phase is the reproductive stage in the cannabis plant’s life cycle, marking the transition from producing foliage and stems to developing the resin-rich buds sought by consumers. Timing this transition is important for growers because it directly determines the final yield and the concentration of cannabinoids like THC and CBD. Understanding when and how this switch occurs is crucial for a successful harvest.
The Primary Trigger Light Cycles
The majority of cannabis strains are photoperiod-dependent, meaning their shift from the vegetative phase to the flowering phase is regulated by the duration of light and, more importantly, the length of uninterrupted darkness. This biological mechanism is known as photoperiodism, the plant’s physiological response to the changing ratio of light to dark hours. Cannabis is classified as a “short-day” plant, requiring a period of darkness that exceeds a specific critical threshold to initiate flowering. This ensures the plant only begins its reproductive cycle as the long days of summer are ending.
For most photoperiod strains, this critical threshold is met when the plant receives 12 hours of uninterrupted darkness paired with 12 hours of light, commonly referred to as the 12/12 schedule. It is the continuous, unbroken stretch of darkness that sends the signal to the plant’s internal clock. If the dark period is interrupted by even a small amount of light, the plant’s flowering hormones may not be produced correctly, which can cause the plant to revert back to vegetative growth or express hermaphroditism.
Applying the Schedule Indoor and Out
The timing of flowering is dictated by whether the plant is grown outdoors under natural sunlight or indoors under controlled conditions. Outdoors, the plant’s life cycle is synchronized with the seasons, with the vegetative phase occurring during the long days of late spring and early summer. The plant naturally begins the transition into flowering once the summer solstice has passed and the days begin to shorten.
In the Northern Hemisphere, outdoor cannabis plants typically show signs of flowering in late summer, usually around August, as the nights become long enough. The exact timing can vary based on the specific strain’s genetics and the latitude of the grow location. Growers then anticipate a harvest time around October, ensuring the buds are ready before the onset of harsh winter weather.
Indoor cultivation provides the grower with complete control over the timing, allowing them to initiate the flowering stage whenever the plant has reached the desired size. The grower manually triggers the switch by changing the light timer from a long-day vegetative cycle (such as 18 hours of light and 6 hours of darkness) to the 12/12 light-to-dark schedule. This switch mimics the natural change in seasons, signaling to the plant that it is time to bloom. This control allows for multiple harvest cycles per year.
The Exception Autoflowering Varieties
Not all cannabis varieties rely on the changing photoperiod to begin their reproductive cycle. Autoflowering plants are an exception, containing genetics from Cannabis ruderalis, a subspecies native to regions with short growing seasons. These plants have evolved to flower based on age and size, rather than the light-dark cycle.
This built-in genetic timer means autoflowering plants automatically switch from the vegetative phase to the flowering phase after a fixed amount of time, typically three to four weeks after germination. Because they are not dependent on light cycles, they can be grown under a consistent light schedule, such as 18 hours of light, throughout their entire life. This characteristic makes them a popular choice for new growers seeking a faster, more predictable cultivation process.
Expected Duration and Physical Signs
Once the flowering stage has been triggered, the plant does not immediately stop growing. It first enters a period called the “flowering stretch,” often doubling in height during the first three weeks. The first visible signs of the transition are the emergence of pre-flowers at the plant’s nodes, which are the junctions where branches meet the main stem. These pre-flowers quickly develop into small white hairs, called pistils, which indicate the plant is female and has successfully entered the flowering phase.
The full flowering process, from the first signs to harvest, generally lasts between 8 to 12 weeks for most photoperiod strains. Sativa-dominant strains often require longer, sometimes up to 14 weeks, while Indica-dominant strains tend to finish faster, closer to the 8-week mark.
As the process continues, the small clusters of pistils swell to form dense buds, and the plant surface becomes covered in sticky, resinous glands called trichomes. The final weeks are marked by the pistils changing color from white to an amber or brown hue, signaling that the flowers are reaching maturity.