The pre-harvest dark period is a widely adopted technique where growers place mature plants in complete darkness for 24 to 72 hours immediately before harvest. This final stage is intended to enhance the quality of the finished product, specifically targeting improvements in flavor and potency. Growers often debate the exact procedures for this final phase, and a common question centers on whether to provide a final irrigation just before the lights go out.
The Purpose of the Pre-Harvest Dark Period
Growers implement a final period of darkness to induce a mild stress response, encouraging the plant to use up stored nutrients and potentially boost secondary metabolite production. During the light cycle, plants focus on primary metabolism, generating energy through starches and sugars. When the light is removed, the plant begins to consume these accumulated energy reserves in the leaves and flowers.
This process is thought to improve the final flavor and smoothness of the dried flower because the reduction of these compounds can lead to a less “harsh” smoke. Furthermore, the light deprivation creates a final environmental stress, which prompts a defensive reaction in the plant. This defense mechanism may involve a final push in the production of resin, which contains cannabinoids and terpenes. Terpene levels are naturally highest at the end of the dark cycle, and extending this period may protect these volatile compounds from degrading.
Irrigation Timing: The Core Decision
The decision to water before the 48-hour dark period requires careful consideration; the recommended approach is to allow the substrate to dry down significantly. The objective is to encourage the plant to deplete its internal moisture reserves during the darkness. A very wet medium will also release excessive moisture into the air during the dark cycle, which poses a substantial risk.
The best practice is to stop the final irrigation 24 to 48 hours before the dark period begins, allowing the container to become noticeably lighter. The medium should be dry enough that the plant is just beginning to show the very first signs of thirst, such as a slight droop in the leaves, but not so dry that it is severely wilting. This “gentle drought” maximizes the stress-induced response without causing severe distress.
Over-watering immediately before darkness is risky because the lack of light stops the rapid transpiration that helps dry out the medium. This can lead to anaerobic conditions in the root zone and elevates the ambient humidity, creating a perfect environment for mold and mildew.
Managing Substrate Moisture and Humidity During Darkness
Environmental control is paramount during the 48-hour dark period, especially because the plant is at its most vulnerable to fungal pathogens. Without light, the temperature in the grow space naturally drops, and this temperature reduction causes a corresponding rise in relative humidity. The plants continue to slowly transpire moisture into the air, further compounding the humidity problem within the enclosed space.
Maintaining continuous air circulation is non-negotiable to prevent pockets of stagnant, moisture-laden air from settling around the dense flowers. Growers should aim to keep the relative humidity (RH) low, ideally in the 40 to 55 percent range, and temperatures cooler than the typical flowering phase, targeting around 60 to 70 degrees Fahrenheit. The combination of a dry medium and controlled atmospheric conditions works to safely maximize the benefits of the darkness while minimizing the risk of losing the entire harvest to mold.
Final Steps Before Harvest
Once the extended dark period is complete, the plants should be harvested immediately to capitalize on the intended effects. There is no need to re-introduce a light cycle, as the goal of the final darkness has been achieved and re-lighting would only risk volatilizing the preserved terpenes.
The plant is ready to be cut down and moved directly into the drying process. The immediate next steps involve carefully cutting the plants, which can be done as whole plants, individual branches, or smaller sections, depending on the grower’s preference and drying setup. After cutting, the flowers are typically trimmed and then hung in a controlled environment to begin the lengthy drying and curing phase. This final transition from a living plant to a marketable product is crucial for preserving the quality, flavor, and potency that the pre-harvest dark period was intended to enhance.