Short answer. Don't flush before harvest. Pre-harvest flushing is a Myth, and it costs you yield and resin. The last weeks are when the plant packs on most of its final weight and finishes its trichomes and resin; starving it then does far more harm than good. Controlled studies found no gain in flower quality or cannabinoids, and longer nutrient withdrawal tended to cut dry mass and cannabinoid yield [1][2][3]. Feed through to harvest at the strength the stage calls for.
The full evidence, the studies table and the how-to live in one place: Pre-harvest flushing: a myth that costs you yield and resin.
The short version
- Cannabis is a bioaccumulator. What it takes up is built into tissue. Plain water at the end doesn't wash it out of the bud; it strips the leaves (that's the yellowing) while flower nitrogen stays the same [1].
- Taste and ash come from a healthy plant that was never overfed, then slow drying and proper curing.
- The one valid flush treats the medium, not the plant: a root-zone flush to wash out accumulated salts when runoff EC is far above input, or there's salt crust or lockout. Run 2–3× the container volume of pH-adjusted water (soil) or a light solution (coco, rockwool, hydro), check runoff EC, then go straight back to normal feeding. Any stage. See runoff testing.
Evidence
Pre-harvest flushing improves quality: Myth (house position). No benefit and a yield cost with longer withdrawal: Supported [1][2][3]. Details and caveats in the flush debate.
Feeding only plain water for days or weeks before harvest to "clean" the plant; GenetixBay rates it a Myth that harms yield, unlike a root-zone salt flush.A hair-like outgrowth of the plant's skin; in cannabis the glandular types make and store the resin holding cannabinoids and terpenes.A cannabinoid is any compound that binds the body's cannabinoid receptors or shares the typical cannabis terpenophenolic structure; the plant-made ones are phytocannabinoids such as THCA and CBDA.The weight of cannabis after drying (and usually trimming), the basis growers use for yield; it is typically about 20-25% of trimmed wet weight.Running a large volume of low-EC water or mild solution through the medium to wash out accumulated salts; it treats the medium, not the plant, and suits any stage.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.Salt build-up is the accumulation of unused fertiliser and water salts in the growing medium, raising root-zone EC and causing osmotic stress and lockout-type symptoms.Nutrient lockout is when plants show deficiency symptoms although nutrients are present, because pH, salt build-up, ion antagonism or damaged roots stop uptake.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.Rockwool is an inert growing medium spun from melted basalt rock into fibres, sold as plugs, cubes and slabs, that holds water and air in fixed proportions.
