Flushing cannabis before harvest means giving only water, or a very dilute solution, during the last days of flowering: usually 7–14 days in soil, 5–7 in coco and 3–7 in hydroponics. Controlled trials do not confirm that it improves aroma: in a 2026 trial on rockwool, flushed plants had on average 21.9% fewer terpenes and a trimmed yield about 7% higher. Flushing is mainly useful as a correction, when salts build up in the growing medium during the grow.
In brief
- Pre-harvest flushing is widespread, but its promised benefits are not proven.
- 2026 trial on rockwool: flushing meant fewer terpenes, slightly higher trimmed yield and less waste.
- A corrective flush is needed when runoff EC is much higher than input EC.
- A cautious alternative: taper nutrients gradually over the last 1–2 weeks.
What flushing means and what it promises
“Flush” refers to two different things. A pre-harvest flush replaces nutrients with water for the final days, on the idea that the plant uses up the nutrient reserves in its tissues and the buds become more aromatic and “cleaner”. A corrective flush, on the other hand, is a heavy rinse of the growing medium at any stage, to remove accumulated salts and bring pH and EC back into the right range.
The second has a clear technical basis. The first comes from growers' experience and, as of October 2026, has been measured in only a few controlled trials.
What the 2026 controlled trial shows
In 2026 a trial was published that rockwool maker Grodan ran at CRIC Labs in Montréal, Canada: 48 plants of two F1 varieties in rockwool, grown between October and December 2025. The standard group received nutrients at EC 3.0 until the end; the flushed group was given, for 9 days, a solution stepped down first to EC 1.7 and then to 1.0, so a gradual reduction rather than plain water.
- Terpenes: on average 21.9% lower in the flushed samples; nine of the ten highest concentrations came from normally fed plants.
- Trimmed yield: about 7% higher with flushing, with much less waste after harvest.
- Limits: the authors themselves warn that the result depends on starting EC, light intensity and feeding schedule, and should not be taken as a universal rule.
In practice: if aroma is your priority, there is no evidence that flushing helps, rather the opposite; if you grow in rockwool at high EC, you may gain some yield and save trimming work. For a personal-use grow in soil, the result does not transfer directly.
Corrective flush for salt build-up
Flushing is needed when runoff EC stays well above input EC, as a rough guide by 0.5 mS/cm or more, together with burnt leaf tips, runoff pH out of range or stalled growth with no other obvious cause.
- Measure the pH and EC of the input water and the runoff with a pH/EC meter.
- Prepare 2–3 times the pot volume in water at 18–22 °C with the right pH: 22–33 litres for an 11-litre pot. Adjust pH with a pH adjuster.
- Pour slowly in several passes, at the start of the light period, over a tray or outside the grow tent.
- Measure the runoff EC towards the end: it should get close to the input EC.
- Let it drain, empty the saucer and resume feeding at a reduced dose once the pot has become lighter.
In soil, such a heavy rinse leaves the medium saturated for days: only do it if the pot drains well and do not repeat it soon after. The rules for normal watering are in the guide on how often to water cannabis.
How to flush before harvest
If you decide to do it, you do not need more water than usual: water with the normal volumes and frequency, with 10–20% runoff and the right pH for your growing medium.
| Growing medium | Typical length | Water | Note |
|---|---|---|---|
| Soil | 7–14 days | pH 6.0–7.0 | slow response; soil holds on to nutrients |
| Coco coir | 5–7 days | pH 5.5–6.5, or a very dilute solution | never let it dry out |
| Rockwool and hydroponics | 3–7 days | pH 5.5–6.5 or a gradually falling EC | fast response; check EC daily |
During these days, gradual yellowing of the large leaves is normal. Harvest timing is decided by the trichomes, not by the flushing calendar: see the guides on when to harvest cannabis and how to harvest cannabis.
Alternative: taper the nutrients gradually
A middle way is to lower the dose over the last 1–2 weeks rather than switching abruptly to water, as the flushed group did in the 2026 trial. Many manufacturers' schedules already do this: the BioBizz feeding schedule 2026, for example, cuts Bio·Grow and Acti·Vera in the eighth week of flowering and then has one week of plain water.
With organic nutrients, flushing with water makes less sense: the nutrients are bound to organic matter and released slowly by life in the growing medium, so water washes out little of them. Stopping feeding for the last 1–2 weeks, following the manufacturer's schedule, is usually enough.
Frequently asked questions
How many days before harvest should you flush?
In practice, 7–14 days in soil, 5–7 in coco and 3–7 in rockwool and hydroponics. It is not mandatory: gradually reducing nutrients over the last 1–2 weeks is a sound alternative.
Does flushing really improve bud aroma?
The available trials do not show it: in the 2026 rockwool trial, flushed samples had on average 21.9% fewer terpenes. The result applies to those conditions and is not a general rule.
Do you need to flush with organic nutrients?
Usually not: water carries away little of the nutrients bound to organic matter. Simply stop feeding for the last 1–2 weeks as the manufacturer advises.
How do you flush coco?
Keep watering at the usual frequency, pH 5.5–6.5 with 10–20% runoff, never letting the coco dry out. For a corrective flush, use a very dilute solution, about a quarter of the usual dose, so the coco does not lose calcium and magnesium.
Containers and accessories for preparing water are in the Gro.w.LaB Water and irrigation collection.
Updated on 13 October 2026.