Should I flush before harvest?

By Curtis Olson — retired software developer, home grower, and the builder of the device this site is about. More about me and why I built it. · Last reviewed 2026-10-28

The short answer

If you flush, you are not making a mistake. You are following a convention rather than a result — and it’s worth knowing which of those you’re doing.

Flushing means running plain water through the growing media for the last stretch of flower, on the theory that it strips residual nutrients, drops chlorophyll and gives a smoother smoke. As far as I can find, no published study on cannabis has shown that. One has now tested the chemistry side of it, and that is next.

What has been tested since the handbook

In 2024 a peer-reviewed trial (Saloner, Sade & Bernstein, Industrial Crops and Products) grew five medical cultivars in a controlled environment and split each at the end of flower: one group fed to harvest, the other on distilled water. Flushing changed cannabinoid concentration in about 30% of the samples and terpenoids in about 3%, in both directions depending on the cultivar, with small shifts in a few minerals in two cultivars. Yield and plant function were unchanged. The authors’ conclusion is that flushing neither damages nor meaningfully improves the flower’s chemistry; they recommend it only because it saves fertiliser.

Two things that study did not do. It did not smoke anything: no taste panel, no ash, no smoothness score, which is the claim flushing is actually done for. And five cultivars is five cultivars. The only taste panel on record is a 2019 industry trial by a nutrient company (one cultivar, flushed for 0, 7, 10 or 14 days, twelve plants a group): no significant difference in THC, terpenes or minerals, and blind tasters found no effect of flush length on flavour, smoothness or ash colour, leaning slightly toward the unflushed flower. It was not peer-reviewed and the sponsor sells nutrients, so read it as a data point, not a result.

So the honest position moved a little. It used to be “untested.” It is now “tested once for chemistry, and little happened either way; never tested for the smoke in a journal.”

Where the idea comes from

The reasoning is borrowed from tobacco, where nitrogen oversupply at curing time is a real and documented problem for smoke quality (Sumner & Moore, 2009). The analogy is plausible. Plausible is not the same as tested, and the transfer to cannabis has not been shown: the 2024 trial saw no consistent drop in the flower’s mineral content from flushing, which is the step the analogy needs.

The wider picture, which is the more useful part

Flushing is one of seven late-flower “stress” practices catalogued in the Handbook of Cannabis Production in Controlled Environments (Caplan, Matzneller & Gutierrez), each graded for whether scientific evidence exists. Restated, with the flushing row updated for the 2024 trial, which the handbook predates:

• Controlled drought stress — evidence exists, and in the handbook’s one study it’s positive: higher THC and CBD concentration and yield (Caplan et al., 2019). Four studies since have not reproduced it; see below.
• UV-B during flowering — evidence exists, and it’s negative. Long-term UV-B did not improve yield or cannabinoid concentration indoors (Rodriguez-Morrison et al., 2021).
• Pre-harvest flushing — “no evidence” in the handbook; tested once since (Saloner et al., 2024), little effect either way, chemistry only.
• Low CO₂ at the end of flower — no evidence.
• Low humidity in the last weeks — no evidence.
• Wounding — no evidence.
• Cold, at the root zone or in the air — no evidence.

Seven widely-practised techniques. One with a positive result that later work has not repeated, two tested without the effect they are done for, four with nothing behind them at all.

The trap in that list

Read the grading column on its own and you would conclude UV-B is the best-supported technique here, because it’s one of only three with real studies attached. It’s the opposite. UV-B was properly tested and did not work, and flushing was tested and did nothing much either way.

“There is evidence about this” and “this works” are different statements. Most of the confident advice in this hobby quietly swaps one for the other.

And the one that did work — why I still won’t tell you to do it

Controlled drought in late flower is the single positive result in the table, and I am not going to turn that into advice. Four reasons, and the chapter itself raises the first:

• Its own authors note that for most controlled-stress treatments in cannabis, the mechanisms are still poorly understood and the effects depend on cultivar and on the specifics of the environment.
• It rests on a single study, and one of the chapter’s authors is an author of that study. That doesn’t make it wrong — it’s peer-reviewed work — but “promising” is the right word for it, not “established.” Read in full, it is cleaner than the folklore version in one way and softer in another. The flower was dried two days and cured twelve before analysis, the lab measured residual moisture and corrected every number to bone-dry, so the 12% THCA and 13% CBDA gain is real concentration, not a drier sample. But the dry-weight gain behind the 50–67% yield figures did not pass the authors’ own test (P = 0.06, three plants a group), and the yield is concentration times that weight.
• Four studies since, none of them a repeat. Morgan et al. (2024) droughted floral hemp in a greenhouse over two seasons: moderate drought changed neither yield nor THC and CBD, severe drought cut all three. Lucchetta et al. (2025), two CBD-dominant varieties: CBD fell as stress rose. Pena et al. (2025), one THC-dominant and one CBD-dominant genotype droughted at early flower: the CBD plant lost cannabinoid content, the THC plant held it. Crispim Massuela et al. (2026), two CBD-dominant genotypes indoors, moderate deficit cycled at early, mid and late flower or one severe dry-down: little effect on biomass or cannabinoid yield either way, severe stress harmful, genotype mattered more than water. Every one of these is CBD-dominant or hemp; nobody has re-run the 2019 protocol on a THC-dominant cultivar. So the positive result is neither confirmed nor refuted on its own terms, and the balance of what exists says moderate drought does nothing and severe drought costs you.
• Telling a hobbyist to deliberately stress plants at the most expensive possible moment, on the strength of one self-graded result, is not a trade I’d make with someone else’s grow.

So what should you actually do?

Flush if you like it. It costs you water and a couple of weeks of not feeding, and the one trial that tested it found no penalty. Just don’t expect the smoothness it’s sold on, and don’t let it displace the things that are measured — your environment through flower, and how you dry and store what you cut.

Sources

• Caplan, D., Matzneller, P. & Gutierrez, G. Harvest and Post-Harvest, in Handbook of Cannabis Production in Controlled Environments (CRC Press). DOI 10.1201/9781003150442-10, §10.3 and Table 10.1.
• Saloner, A., Sade, N. & Bernstein, N. (2024). To flush or not to flush: does flushing the growing media affect cannabinoid and terpenoid production in cannabis? Industrial Crops and Products. DOI 10.1016/j.indcrop.2024.119157 — five cultivars, chemistry only, little effect either way.
• Rx Green Technologies (2019) — industry trial, one cultivar, 0/7/10/14-day flush, blind taste panel; no significant differences. Not peer-reviewed; the sponsor sells nutrients.
• Caplan, D., Dixon, M. & Zheng, Y. (2019). Increasing inflorescence dry weight and cannabinoid content in medical cannabis using controlled drought stress. HortScience 54(5):964–969. DOI 10.21273/HORTSCI13510-18 — controlled drought, positive result; open access.
• Morgan, W., Singh, J., Kesheimer, K. & Davis, J. (2024). Severe drought significantly reduces floral hemp yield and cannabinoid content but moderate drought does not. Environmental and Experimental Botany 219:105649. DOI 10.1016/j.envexpbot.2024.105649.
• Lucchetta, I., Cappello Fusaro, M. & Bona, S. (2025). Water stress effects on biomass allocation and secondary metabolism in CBD-dominant Cannabis sativa L. Plants 14(8):1267. DOI 10.3390/plants14081267.
• Pena, M. M. et al. (2025). Genotypic variation in photosynthesis and biomass partitioning underlies agronomic performance and cannabinoid profile in Cannabis sativa under drought. Plants 14(24):3840. DOI 10.3390/plants14243840.
• Crispim Massuela, D. et al. (2026). Less water, same buds: moderate drought as a water-saving strategy for indoor cannabis. Frontiers in Plant Science 17. DOI 10.3389/fpls.2026.1930650.
• Rodriguez-Morrison, V., Llewellyn, D. & Zheng, Y. (2021) — UV-B, negative result.
• Sumner, P. & Moore, J. (2009) — tobacco curing and nitrogen oversupply, the source of the analogy.

More of these at Answers. The device is at croplock.com.

Also read: Is this the fade, or a deficiency? · Can trichome colour tell me when to harvest? · How do I know my flower is dry enough?