Cannabis Flushing Guide: When and How to Flush Before Harvest
A practical cannabis flushing guide covering what flushing does, when to start by growing medium, how to flush, and common mistakes to avoid.
Cannabis Nutrient Flush Before Harvest
A nutrient flush is plain water fed in place of nutrient solution near the end of flower. Start 10 to 14 days before harvest in soil. Start 5 to 10 days before harvest in coco coir. Start 5 to 7 days before harvest in deep water culture and other hydro systems. The aim is to lower the mineral salt content left in flower tissue at chop time.
cannabis flushing with plain water
What the flush does
Plants take up nitrate, phosphate, potassium and calcium as dissolved ions. When the medium holds no more of those ions, the plant moves mobile nutrients out of older leaves to feed the flowers. Leaves fade from green to yellow. Soluble salt in the flower tissue drops.
best flushing schedule for weed
Data on smoke quality is thin. Peer reviewed trials that compare flushed and unflushed flower for taste, ash color or resin content are few. Most claims about a smoother smoke come from grower reports, not controlled tests.
Timing by growing medium
- Soil: 10 to 14 days. Soil holds salt. A short flush leaves mineral in the pot.
- Coco coir: 5 to 10 days. Coco holds less and drains fast.
- Hydroponics (DWC, NFT, rockwool): 5 to 7 days. Roots sit in the solution.
- Organic super soil: 0 to 7 days. Most inputs release over weeks.
How to flush
- Stop base nutrients, boosters and additives. Keep pH control. Hold water at pH 5.8 to 6.2 for coco and hydro, pH 6.0 to 6.8 for soil.
- Pour water until 10 to 20 percent of the volume runs out the bottom of the pot. This pushes old salt from the medium.
- Repeat when the pot feels light. In hydro, swap the reservoir for plain pH-adjusted water and change it every 2 to 3 days.
- Measure runoff electrical conductivity at each watering. When the reading stops falling, the medium holds little available salt.
Signs the plant is ready
Trichomes on calyxes turn cloudy. Some turn amber. Pistils darken and curl in. Bud growth stops and the flowers swell one last time. Set flush length by the medium and by trichome color, not by the calendar alone.
Risks
Flushing too early cuts yield. The plant loses nitrogen and phosphorus it needs for the final swell. Two weeks in soil is common. Four weeks is not. Yellow leaves on day 3 of a 14 day flush point to a feeding problem, not to a flush that worked.
A flush does not remove heavy metals or pesticide residue from plant tissue. It does not change THC or CBD content in grower side-by-side tests.
Drying and curing
Dry at 60 F and 60 percent relative humidity for 10 to 14 days. Then cure in sealed jars for 2 to 4 weeks. Burp the jars each day for the first week. Chlorophyll breaks down during cure. This step affects taste more than the flush does.