The flowering phase is the reproductive stage of the cannabis plant, occurring when the plant matures and environmental conditions signal the end of the growing season. Female plants produce the resinous flowers (buds) during this stage, which contain desirable cannabinoids and terpenes. Successful management of this transition is necessary to maximize the quality and weight of the final harvest. The shift from vegetative growth to flower production requires precise changes to the plant’s environment, nutrient intake, and daily care.
Initiating the Flowering Phase
The primary trigger for flowering in photoperiod cannabis strains is a change in the light cycle, a phenomenon known as photoperiodism. This process is initiated by reducing the daily light period to 12 hours and ensuring 12 hours of uninterrupted darkness. The long, continuous dark period causes a shift in the plant’s internal hormone levels, signaling that winter is approaching and reproduction must begin.
Hormonal messengers like gibberellins and auxins decrease their activity, while ethylene and cytokinins increase to promote flower development. This hormonal change reverses the focus from stem and leaf elongation to the creation of dense flower clusters. Growers must ensure the dark period is absolute, as even a small amount of light can disrupt the process and confuse the plant’s hormonal signals, potentially causing it to revert to the vegetative state or develop male flowers.
For most indoor operations, this process involves switching the light schedule from an 18 hours of light/6 hours of darkness cycle to a strict 12 hours of light/12 hours of darkness cycle. This precise control over the photoperiod is essential for initiating flowering. In contrast, autoflowering varieties are the exception to this rule, as their flowering is triggered by age, not by a change in the photoperiod.
Optimizing the Flowering Environment
Once the light cycle has been switched to 12/12, the atmospheric conditions must be adjusted to support the developing flowers and prevent mold. This environmental management is best achieved by monitoring the Vapor Pressure Deficit (VPD), which represents the difference between the moisture in the air and the maximum moisture the air can hold. A balanced VPD encourages optimal transpiration and nutrient uptake while keeping the flowers dry.
The ideal temperature range during the light cycle is between 75°F and 80°F (24°C to 27°C), with a slight drop of a few degrees during the dark period. Humidity levels must be lowered progressively as the flowers mature to maintain a VPD between 1.2 and 1.4 kilopascals (kPa) during the mid to late flowering stage. This translates to a relative humidity range of about 50% to 60% in early flower, dropping closer to 40% to 50% as the buds swell.
Managing high humidity is necessary because dense flowers are highly susceptible to moisture-related issues like bud rot or powdery mildew. Strong airflow and proper ventilation are required to move air both across and through the plant canopy, removing excess moisture and preventing stagnant air pockets. Constant air exchange, often facilitated by exhaust fans, minimizes disease risk and supports the plant’s metabolic functions.
Adjusting Nutrient Schedules for Bloom
The transition to flowering demands a massive shift in the plant’s nutritional requirements, moving away from the high nitrogen (N) needed for vegetative growth. As the plant focuses on bud production, the demand for phosphorus (P) and potassium (K) increases significantly. This is reflected in the recommended NPK ratios, which change from a nitrogen-dominant formula (e.g., 3:1:2) to a phosphorus and potassium-dominant one.
During the early bloom phase, a ratio such as 1:3:2 (N:P:K) is often employed to support the initial development of flower sites. Phosphorus is important for energy transfer within the plant and the formation of flower structures. As the plant enters the peak flowering stage, the nutrient focus shifts to potassium, which is needed for water regulation, enzyme activation, and achieving flower density, resulting in late-stage ratios closer to 0:3:3.
Growers often introduce specialized bloom boosters during this phase, which are supplements high in phosphorus and potassium to encourage flower size and weight. Beyond the macronutrients, the plant’s need for secondary nutrients like calcium and magnesium remains high and should be monitored carefully. Toward the final week or two before harvest, growers flush the medium with plain, pH-balanced water. This encourages the plant to use up stored nutrients, resulting in a cleaner-tasting final product.
Essential Checks During Flower Development
Once flowering begins, careful monitoring and physical maintenance are required to ensure the health of the harvest. The most time-sensitive check is identifying the sex of the plant by inspecting the pre-flowers at the nodes where the branches meet the main stem. Female plants will display small, teardrop-shaped calyxes with one or two white, hair-like pistils protruding from them.
In contrast, male plants will develop small, smooth, ball-shaped pollen sacs that resemble tiny bunches of grapes. If unpollinated flowers are the goal, any male or hermaphrodite plants showing both male and female structures must be removed from the growing area immediately. Pollination will cause the female plants to produce seeds, diverting energy away from cannabinoid and terpene production.
As the flowers gain significant mass and weight, physical support becomes necessary to prevent the branches from bending or snapping. Techniques like staking, trellising, or using a supportive net help hold the heavy colas upright, ensuring adequate light exposure and airflow. Light defoliation, which involves removing large fan leaves that block light or restrict air movement, can also be performed to improve penetration to lower bud sites.