As cannabis plants enter the final two weeks of the flowering cycle, many growers look for ways to encourage rich autumnal hues—especially deep purples—and increase resin production, all without compromising final growth. Achieving this balance requires carefully timed environmental and nutrient adjustments.
One of the first changes involves the reduction of supplemental CO₂. During peak flowering, growers often maintain high CO₂ levels to support vigorous growth. However, in the final stages, gradually decreasing CO₂ and ultimately returning to natural atmospheric levels (around zero ppm of added CO₂) can help transition the plant from high-energy production to a more natural finish.
Temperature adjustments also play a vital role. Lowering the grow room temperature during the final two weeks mimics natural seasonal changes and can trigger color shifts in the plant. Many growers operate at 80–85°F during flowering for optimal CO₂ uptake and growth, but reducing temperatures incrementally during the final stretch creates conditions similar to outdoor autumn. This process, known as “autumning off,” encourages plants to fade from green to yellows, reds, and purples.
Nutrient management is equally important. Reducing or eliminating nitrogen in the final two weeks helps prevent overly green foliage and supports the natural fade in color. At the same time, introducing carbohydrates during this stage influences the carbon-to-nitrogen ratio in the root zone. This shift draws nitrogen from the leaves into the roots, further enhancing the autumnal transformation without disrupting essential plant functions.
While there are multiple techniques available, the most effective combination for bringing on autumn colors involves cooler temperatures, nitrogen reduction, and the use of genetics already inclined to express purple or red tones. With careful planning, growers can finish their flower with vibrant hues and a resin-rich, frosty appearance that enhances both visual appeal and market value.
