Short answer
Flushing means plain, pH-adjusted water for the last 7 to 14 days before harvest. Feed stops. Water only. The claimed result is flower that burns smooth and tastes clean. A controlled trial at the University of Guelph tested that claim. Flushed plants and unflushed plants showed no difference in yield, cannabinoid levels, or mineral content in the dried flower. Flushing is a choice, not a requirement.
Flush vs No Flush Marijuana: Comparing Effects
What flushing removes
Water carries dissolved fertilizer salts out of the growing medium. Runoff takes nitrate, phosphate, potassium, calcium, and magnesium with it. The electrical conductivity of the root zone drops. That matters when salt levels climb high enough to burn root tips and leaf edges.
Flushing does not remove minerals from inside plant tissue. Nitrogen and potassium that sit in the cells stay in the cells. Nothing in plain water pulls them back out.
flushing cannabis in hydroponic system before harvest
What the research says
In the Guelph trial, plants received full-strength nutrient solution or a 7-day or 14-day flush. Dried inflorescences from all three groups held similar levels of nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. THC and CBD concentrations were similar as well. The study found no support for the taste claim.
blackstrap molasses flush weed recipe
Peer-reviewed cannabis nutrition research is thin. Most flushing schedules come from grower forums and nutrient company charts, not from replicated greenhouse or field trials.
Reasons growers still flush
- Runoff EC above 2.5 mS/cm late in flower.
- Brown leaf tips, clawed leaves, or stalled growth from salt stress.
- pH drift in the root zone that blocks nutrient uptake.
- Routine. The feeding chart lists a flush week.
How to flush
- Start 7 to 14 days before the harvest date. That is the end of week 7 or week 8 of flower for most photoperiod strains.
- Mix plain water. Adjust to pH 5.8 to 6.2 for coco and peat mixes. Adjust to pH 6.0 to 6.5 for soil.
- Water until 10 to 20 percent of the volume runs out the bottom of the pot.
- Repeat at the normal watering interval. Do not let the medium dry to the point of wilt.
- Measure runoff EC. It should fall toward the value of the input water.
- Stop when runoff EC is within 0.2 mS/cm of the water you pour in.
- Harvest at the trichome stage you want. Cloudy heads with some amber is a common target.
Growers in living soil or organic beds often skip the flush. Those mediums hold nutrients in organic form, and plain water will not strip them out in two weeks.
Risks
A 14-day flush removes two weeks of phosphorus and potassium. Those two nutrients drive bud weight and density in late flower. Plants that run short can finish with lighter flower and more larf. A nitrogen shortage late in flower also speeds leaf yellowing and can cut yield. Weigh the risk against the benefit you expect.
Dry and cure matter more
Taste and smoothness come from the dry and the cure. Chlorophyll and starches break down in the jar, not in the pot. Target 60 to 65 F and 58 to 62 percent relative humidity for 10 to 14 days of drying. Then cure in sealed jars for 2 to 4 weeks, and burp the jars each day for the first week. Flower that dries too fast smokes harsh no matter what the plant was fed.
Bottom line
Flushing is optional. Use it if your runoff EC is high or your plants show salt stress. Skip it if your medium and feed schedule are in range. The published evidence does not show a taste or potency gain.