cannabis flushing guide
A cannabis flushing guide covering when to start, how much water to use, how to read runoff, and how to tell when the flush is finished.
Cannabis Flushing With Plain Water
Flushing means giving cannabis plants plain water and no nutrient solution for the last 7 to 14 days before harvest. The goal is to lower salt levels in the growing medium and in plant tissue. Published trial data on smoke quality after flushing is thin. Growers report yield loss when the flush runs long.
What Flushing Changes
Nutrient salts sit in the medium and in plant tissue. When you stop fertigation, tissue nutrient levels fall, but the fall is slow. Runoff electrical conductivity (EC) drops first because the medium holds less salt. Chlorophyll breakdown happens in drying and curing, not during the flush. Water alone does not remove chlorophyll from buds.
Timing by Medium
These ranges are common practice, not tested standards.
Cannabis Flush vs No Flush: A Buyer's Guide to Taste
- Peat and soil mixes: 10 to 14 days
- Coco coir: 7 to 10 days
- Hydroponics and deep water culture: 5 to 7 days
Flush Method for Containers
- Set water pH to 6.0 to 6.5 for peat and soil mixes. Set 5.5 to 6.0 for coco and hydro.
- Pour water equal to 2 to 3 times the pot volume through the medium.
- Measure EC and pH of the runoff.
- Repeat when runoff EC is within 0.2 to 0.4 mS/cm of your source water EC.
- Let pots dry between flush events. Dense soil holds water and root rot risk rises with daily soaking.
What Flushing Does Not Fix
- It does not remove all nutrients from buds. Tissue nutrient levels fall but do not reach zero.
- It does not replace a slow dry at 60F and 60% relative humidity and a cure of 2 or more weeks.
- It does not repair bud damage from mold, mites, or heat stress.
Common Errors
- Flushing too early on fast strains and autoflowers. Cut 4 to 5 days, not 14.
- Ignoring source water. Tap water at 0.6 mS/cm EC cannot show a clean flush reading.
- Watering to runoff every day in late flower. Roots need oxygen in the last two weeks.
- Judging the flush by leaf color alone. Yellow fans come from nitrogen withdrawal and from senescence, and the two look the same.
Check runoff EC and pot weight before you extend a flush. If EC is near source water and pots stay heavy, stop watering to runoff.