Cultivating cannabis requires careful attention across every growth stage, especially during the final weeks before harvest. This stage involves flushing, a technique where the plant’s diet is manipulated. Flushing means ceasing the application of nutrient-rich solutions and providing the plant with only plain water. This temporary change enhances the quality of the final product.
Defining the Flushing Process
Flushing forces the plant to consume its own stored nutrient reserves. During the growth cycle, cannabis plants absorb and store mineral salts and other nutrients within their tissues, particularly in the leaves and stems. Continued feeding until harvest can leave a residue of these compounds in the finished flower.
The mechanism involves nutrient starvation: removing external sources forces the plant’s metabolism to switch its focus to internal resources during the final days of flowering. This minimizes the concentration of unused salts and minerals within the buds. Reducing these residual salts is believed to result in a smoother smoke and a cleaner flavor profile. The goal is controlled depletion, forcing the plant to metabolize compounds like chlorophyll, which causes a harsh taste if not broken down.
Determining the Optimal Start Time
Initiating the flush requires balancing sufficient time for nutrient depletion with preventing premature yield reduction. Growers use visual cues and calendar planning to pinpoint the optimal start date.
The most precise indicator is monitoring the plant’s trichomes, the glands on the flowers that produce cannabinoids and terpenes. Trichomes change color as they mature, transitioning from clear to cloudy, and finally to amber. Cultivators aim to begin flushing when the trichome heads are predominantly cloudy, indicating peak cannabinoid production, and before too many turn amber. A common approach is starting the flush when 50 to 75 percent of the trichomes have become milky white.
Another visual sign is the condition of the pistils, the hair-like structures emerging from the buds. Pistils start white and upright but darken to an orange or reddish-brown color as the flower matures. The flush is often started when 70 to 80 percent of these pistils have darkened and curled inward. Growers also consult the breeder’s suggested harvest window and subtract the typical flush duration (usually 7 to 14 days) from the anticipated harvest date.
Flushing Techniques and Duration
The practical application of flushing involves saturating the growing medium with a high volume of plain, pH-balanced water. The goal is to wash away accumulated mineral salts from the root zone and the medium itself. For container-based growing, use two to three times the container volume for each watering to ensure maximum runoff and effective salt removal. The required duration depends heavily on the specific growing medium, as different materials retain nutrients differently.
Soil
Soil naturally holds onto nutrients and requires the longest flush, often lasting between 10 and 14 days. This extended period allows the slow-release properties of the soil to dissipate.
Soilless Mediums
Soilless mediums, such as coco coir, hold fewer residual nutrients than soil and require a moderate flush, generally lasting 7 to 10 days. The structure of coco coir allows for faster washout of salts.
Hydroponic Systems
For hydroponic systems, where roots are directly exposed to the nutrient solution, the flush is the shortest, often lasting only 3 to 5 days. The process involves draining the reservoir and refilling it with plain water, instantly cutting off the nutrient supply.
Advanced growers use a PPM (parts per million) meter to measure the nutrient concentration in the runoff water. They aim for the runoff PPM to drop close to the PPM of the plain water source, confirming that stored salts have been physically removed from the medium. This measurable decrease provides confirmation that the flushing process is working effectively.
Identifying the End of the Flush
The visual confirmation that the flush is complete centers on the onset of senescence, or aging, in the plant’s leaves. As the plant exhausts nutrients in the medium, it begins to draw mobile nutrients, primarily nitrogen, from its older, lower fan leaves. This process is evident as the large fan leaves display significant yellowing and fading.
This yellowing signals that the plant has successfully metabolized its internal reserves to support flower development. The depletion of nitrogen, a structural component of chlorophyll, causes the leaves to lose their green color. The final stage is complete when this visible depletion of green color is apparent across the canopy, confirming the plant has finished internal nutrient consumption and is ready for harvest.