Cannabis Flushing: Rinsing With Plain Water Before Harvest

Flushing means giving only plain water for the last one to two weeks before harvest. Often so much is poured that it runs out of the bottom of the pot. The aim is a cleaner flower with smoother smoke and white ash. In trials the nutrients in the flower stayed the same. Cannabis flushing often cost yield instead.

Does rinsing make the flower cleaner?

No. Nutrients in the flower dropped in no trial. Only nitrogen in the leaves dropped. That is why the large fan leaves turn yellow. This is leaf senescence, the ageing of the leaves, not cleansing.

  • Do not read yellow leaves at the end as a sign to keep rinsing.
  • Keep feeding to plan until harvest.

Does flushing cost yield?

Often, yes. In peat one cultivar lost, by calculation, about 6 % of its flower per week of rinsing. Another cultivar lost about a fifth in two weeks. In coco one cultivar showed no loss up to 14 days. So it depends on the growing medium, the cultivar and how long you rinse.

  • Best not to flush.
  • If you do, measure the EC of the runoff. It shows how much salt is left in the pot.
  • In peat, rinsing down to below 0.2 mS/cm cost flower.

Does flushing change taste, ash or potency?

Not taste or ash. In a blind tasting the testers did not know which sample was flushed. They found no reliable difference in taste, harshness or ash colour. The content of THC, the main intoxicating compound, also stayed the same. In one cultivar high in CBD, the non-intoxicating compound, content rose slightly. But the flower got lighter. Then the higher value can be a dilution effect.

  • Do not count on flushing for smoother smoke.
  • To find out yourself, flush only one of two groups of the same cultivar and taste blind.
  • Whether other stress shortly before harvest raises potency is covered in the article on plant stress.

How do you save fertiliser at the end without losing flower?

Leave out only the nitrogen, not everything. Without nitrogen, one cultivar lost no yield over the last two weeks. Plain water cost yield in another cultivar. That is a pointer, not a rule, because these were two different cultivars. Without any nitrogen from the start of flowering, the flower shrank to a good quarter.

  • Leave out the nitrogen fertiliser for the last two weeks and keep giving the rest.
  • Test it on part of the crop first. Split cuttings of one mother plant into two groups and compare dry weight.

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