The flowering stage is the reproductive phase where the cannabis plant shifts energy from growing foliage to developing cannabinoid-rich buds. This transition marks a dramatic change in the plant’s nutritional demands compared to the vegetative period. Providing the correct balance of nutrients is directly linked to maximizing the density and potency of the final harvest.
The Essential NPK Shift
Cannabis nutrient requirements are categorized by the NPK ratio: Nitrogen (N), Phosphorus (P), and Potassium (K). While the vegetative stage demands high Nitrogen for rapid growth of leaves and stems, this preference changes significantly during flowering.
Nitrogen must be reduced to prevent excessive leaf growth, which can lead to soft tissue and airy, less dense flowers. High-N vegetative ratios (e.g., 3:1:2) must be replaced with formulas lower in Nitrogen. Early in the cycle, a transition ratio like 1:3:2 (N:P:K) is suitable, shifting to an even lower-N ratio as the plant matures.
Phosphorus (P) and Potassium (K) become the dominant macronutrients. Phosphorus is essential for energy transfer, supporting the formation of new flower sites and the expansion of existing buds. Adequate Phosphorus is also linked to improved resin production, which contains cannabinoids and terpenes.
Potassium is vital for water regulation, enzyme activation, and supporting the overall structure of the plant. High levels are necessary during peak flowering to ensure the development of robust, dense flower clusters. It contributes to cell wall strength, helping the plant support the weight of developing buds and enhancing resistance to environmental stress.
Crucial Secondary and Micro Nutrients
Beyond NPK, flowering cannabis requires secondary and micronutrients for optimal health and bud quality. Calcium (Ca) and Magnesium (Mg) become increasingly important as flowers develop. These are often supplied together as “Cal-Mag,” especially when using filtered water or inert media like coco coir.
Calcium is necessary for building strong cell walls and is immobile, meaning it must be continuously available in the feed. Magnesium is the central atom in the chlorophyll molecule, necessary for photosynthesis and energy production. A Magnesium deficiency often manifests as interveinal chlorosis during flowering, where the veins of older leaves remain green while the tissue turns yellow.
Sulfur (S) plays a role in forming proteins, enzymes, and vitamins, and is involved in producing terpenes, which influence aroma and flavor. Trace elements, or micronutrients, such as Iron (Fe) and Zinc (Zn), are needed in small amounts but are critical for numerous metabolic processes. Iron is used in chlorophyll synthesis, while Zinc is required for growth hormones. Ensuring these supporting elements are available prevents deficiencies that limit flower size and quality, even if the NPK ratio is correct.
Feeding Management and pH Control
Proper feeding management ensures roots can absorb the necessary elements. For growers using soil or inert media, a common practice is the wet/dry cycle. This involves completely saturating the medium and allowing it to dry significantly before the next feeding. This cycle ensures roots receive both nutrients and the oxygen they need.
Controlling the pH of the nutrient solution is fundamental, as it dictates nutrient availability. For plants grown in soil, the ideal pH range for uptake is between 6.0 and 7.0. In soilless media (like coco coir) or hydroponic systems, a slightly more acidic range of 5.5 to 6.5 is preferred to maximize absorption. If the pH drifts outside this window, nutrient lockout can occur, preventing the plant from absorbing elements even if they are present.
The final step in the flowering regimen is “flushing,” which involves stopping all nutrient application. This usually begins one to two weeks before the planned harvest date. During flushing, the plant is fed only pure, pH-balanced water, forcing it to utilize internal nutrient reserves. The purpose is to remove accumulated mineral salts from the plant tissue and growing medium, resulting in a smoother, cleaner, and better-tasting final product.