Should I flush before harvest?

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, that has never been shown in a published study on cannabis.

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 appears never to have been checked.

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:

• Controlled drought stress — evidence exists, and it’s positive. Increased THC and CBD concentration and yield (Caplan et al., 2019).
• 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.
• 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, one tested and failed, five 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 two with real studies attached. It’s the opposite. UV-B is the one thing on the list that has been properly tested and did not work.

“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. Three 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.”
• 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 nothing in the literature suggests a 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.
• Caplan, D. et al. (2019) — controlled drought, positive result.
• 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.