Flushing Water Practice During Flowering

Flushing is the planned use of low-PPM water to clear excess salts from the root zone during flowering. The objective is simple: stabilize uptake, prevent harshness, and protect aroma and resin quality. Flushing works only when pH, irrigation volume, and drainage are controlled.

When and why to flush

Use a corrective flush any time runoff PPM rises far above the feed trend or plants show tip burn and dark, rigid leaves. Use a pre-harvest flush in the final 7 to 14 days, adjusted to cultivar and substrate. Corrective flush resets the medium after drift. Pre-harvest flush tapers salts while maintaining color and turgor.

Targets and water choice

For coco and rockwool, bring runoff down to roughly 200 to 400 PPM above source water, then hold steady. For soil-blend media, moderate toward feed plus or minus 100 to 200 PPM. Keep pH 5.8 to 6.2 for coco and rockwool, and 6.2 to 6.6 for soil. Reverse osmosis or low-mineral water is preferred. If using tap, pre-condition pH. In coco, include a light calcium and magnesium background, about 20 to 40 PPM Ca and 10 to 20 PPM Mg, to avoid cation imbalance during the flush period.

Step-by-step corrective flush

Measure feed and runoff baseline: volume, pH, PPM.
Prepare 2–3x pot volume of low-PPM water at target pH.
Apply in pulses. Let each pulse drain fully.
Stop when runoff PPM meets the target band.
Resume a lighter feed next irrigation at 80–90% of prior PPM.
Recheck runoff on the next cycle and fine-tune.

Coco binds calcium and magnesium. Without a small Ca/Mg background, leaves can pale and curl even while EC falls. Rockwool responds quickly, so EC can drop too fast; shorten pulses if fade accelerates. Soil carries more buffer. Use gentler volumes and a longer taper to avoid shock. In all media, full drainage and air exchange are mandatory. Over-flushing strips nutrients and costs yield. Watch leaf color and runoff trend rather than calendar days. pH drift reduces uptake even at perfect EC, so meter calibration must be routine. Standing water invites root issues; confirm trays empty fast and fans keep the lower canopy dry. Late-stage stress reduces aroma and density; keep climate stable and avoid last-minute recipe swings.

Solar Farm Thailand schedules flushing by stage and substrate under a GACP framework. Rooms target runoff bands tied to water source and media. Teams verify with meters each session and adjust within the next cycle. Setpoints, readings, and actions link to lot records and COAs. This discipline keeps late-flower quality steady for cannabis wholesale Thailand and cannabis wholesale Bangkok.

Recording each flush so the batch file can prove it

A flush that is not logged did not happen. Each session needs one line per room: source water EC and pH, applied volume, runoff EC and pH, drained volume and operator initials, plus the meter’s conversion scale, since PPM readings on different scales cannot be compared.

If runoff appears quickly and its EC barely moves, water is channelling around a dry core; rewet with smaller pulses before continuing.

Readings only carry weight with the calibration entry beside them: solution used, its expiry and the date checked. Solar Farm Thailand ties flush readings to the lot record and COA, so the calibration entry belongs in the same file, and a batch’s flush history can then be produced beside its third-party COA.

What an auditor or licensed buyer will ask to see

A GACP inspector expects irrigation, and therefore flushing, to be a documented step: is there a written procedure, does it state the runoff target per substrate, who approved it, and do the logs match it? A procedure promising one band while the log shows another is a finding, even when the crop looked fine.

Flush volumes exceed a normal feed, so expect questions on the source water’s test history for microbial counts and heavy metals, plus the pre-conditioning record where tap water is used; licensed importers look at the same panels on the COA. Any missed target, failed calibration or off-schedule flush goes in the deviation file with cause, corrective action and sign-off.

Where a flush goes wrong and how to respond

The first pulse carries the most concentrated pore water out of the medium and reads high against later pulses, so judge the flush on the trend, not a single reading.

Humidity is the other trap: a dense late-flower canopy plus a full flush volume pushes moisture into the room. Confirm dehumidification headroom first and stage the flush row by row; botrytis in the final week undoes everything the flush was meant to protect.

Runoff not falling after the planned volume: check source water, then the meter, then channelling, before applying more water.
Runoff pH drifting up as EC drops: low-mineral water has little buffering, so re-check input pH on every pulse.

Last updated: 22 September 2026