Flushing a cannabis plant is a cultivation technique performed just before harvest. It involves stopping nutrient delivery and applying plain water to the growing medium. This process is designed to leach accumulated mineral salts and chemical residues from the substrate, such as soil or coco coir. Flushing forces the plant to metabolize nutrients stored within its own tissues, enhancing the overall quality of the harvested flower for a cleaner final product.
Defining the Purpose and Optimal Timing
Excess mineral salts in the plant’s medium and tissues negatively affect the final product’s quality, often resulting in a harsh taste and smoke. When the flower is consumed, these residual compounds can lead to a less desirable experience, sometimes evidenced by dark or black ash when burned. Flushing mitigates this issue by encouraging the plant to deplete its internal nutrient reserves.
The optimal time to initiate the flush depends heavily on the type of growing medium being used. In soil-based systems, which retain nutrients longer, growers typically begin the process 10 to 14 days before the anticipated harvest date. Media like coco coir require a shorter flush, generally lasting between five and seven days. For hydroponic systems, where nutrient availability is immediately cut off, the flush can be as brief as a few days.
Determining the precise start date also relies on observing the plant’s reproductive structures, specifically the trichomes and pistils. When the tiny, resinous trichome heads begin to turn from clear to a cloudy white color, the plant is entering its final stage of maturity. This visual assessment provides the necessary cue to stop the nutrient regimen and begin the water-only period. Starting the flush too early risks starving the plant during a period of rapid bud development, potentially reducing the final yield.
The Step-by-Step Flushing Procedure
The flushing process begins with the preparation of the water, which must be free of added nutrients and balanced for pH. Most growers use purified water, such as Reverse Osmosis (RO) water, or dechlorinated tap water to ensure no foreign minerals or chemicals are introduced during this phase. The pH of the water should be adjusted to the ideal range for the growing medium, typically between 6.0 and 6.8 for soil and 5.5 to 6.5 for inert media like coco coir.
The physical execution involves applying a large volume of this plain water to the growing container, far exceeding a normal watering session. A general guideline for container-based systems is to use a volume of water equal to at least two to three times the size of the container. For example, a five-gallon pot would receive 10 to 15 gallons of water poured slowly through the medium.
This high volume of water is necessary to dissolve the crystalline salt buildup around the root zone and effectively push the soluble nutrients out of the substrate. The goal is to achieve significant runoff, which is the water that drains out of the bottom of the pot. This collected runoff water is then used to monitor the progress of the flush.
To accurately track nutrient depletion, the electrical conductivity (EC) or parts per million (PPM) of the runoff water must be measured using a specialized meter. EC and PPM readings quantify the concentration of dissolved mineral salts in the solution. The flushing process must be repeated until the measured concentration is drastically reduced.
Indicators of a Successful Flush
The most reliable indicator of a successful flush is the measurement of the runoff water’s EC or PPM. The process is considered complete when the runoff PPM closely approaches the PPM of the plain input water, signaling that the medium is essentially free of residual nutrient salts. A target runoff reading is often cited as being below 50 PPM, or an EC of 0.1 to 0.2, although this can vary based on the initial water source.
Accompanying the measured indicators are distinct visual changes in the plant itself, known as senescence or fade. As the plant is starved of external nutrients, it draws mobile macronutrients, primarily nitrogen, from its older, larger fan leaves to fuel the final development of the flowers. This internal resource migration causes the leaves to turn a uniform yellow color, starting at the bottom of the plant and progressing upward.
This yellowing or fading is a desirable sign during the final week of the flush, confirming the plant is successfully consuming its stored reserves. It is distinct from nutrient burn, which typically presents as brown, crispy leaf tips and margins. Once the runoff measurements are low and the plant exhibits yellowing, the final stage is to allow the medium to dry out slightly before the harvest cut. A drier medium makes the final drying and curing process easier and reduces the risk of mold formation.