Controlled trials so far have not shown that a pre-harvest flush improves taste, smoothness or ash colour, and a 2025 peer-reviewed study found that longer flushes cost flower weight. Flushing still has a place, but as a fix for salt build-up or a feeding mistake, not as a ritual before every harvest.
1. What Is Flushing?
Flushing means stopping all fertilizer and watering with plain water only during the final 1-2 weeks before harvest. The claimed purpose is to make the plant use up its stored nutrients, producing “cleaner” flowers, white ash and smoother smoke.
The practice spread by word of mouth among growers long before anyone tested it.
2. Why Do Growers Believe in Flushing?
- White ash is read as a sign of “clean” flower
- Smoother, less harsh smoke is expected
- It saves fertilizer in the last weeks
- It is community tradition, passed from grower to grower
3. What the Research Shows (2017-2025)
The evidence comes in three kinds, and they do not carry equal weight:
- A company trial: Rx Green Technologies (2020), a maker of cannabis nutrients and coco substrate, tested flushing in its own facility and published the results as a company research report. It is useful data, but it is not peer-reviewed.
- Peer-reviewed studies: Alden & Faust (2025) in HortScience, and Saloner, Sade & Bernstein (2024) in Industrial Crops and Products, which examined whether flushing the growing media changes cannabinoid and terpenoid production.
- A university thesis: Stemeroff (2017, University of Guelph) tested irrigation strategies for medical cannabis and measured nutrients in dried buds after flushing.
The company trial found no taste benefit. The HortScience study found that flushing lowers flower minerals but also lowers yield; it did not test smoking quality. So the studies differ less on benefits than on whether flushing costs anything.
4. Rx Green Technologies Trial (2020)
- Compared 4 flush durations: 0, 7, 10 and 14 days on Cherry Diesel grown in coco coir, 12 plants per treatment
- Results: no significant differences in flower weight, THC or terpene content. Mineral content of the flower showed no significant change overall; after 14 days nitrogen was 6.7% lower, and iron and zinc were slightly higher
- Blind taste test: flush length had no significant effect on flavour, smoothness or ash colour. Panellists tended to prefer the 0-day (unflushed) flower, and ash was mostly grey or black in every group
- Plants flushed for 10 or 14 days had visibly more yellow, dry fan leaves before harvest
5. HortScience Study (2025)
- Peer-reviewed; two high-CBD cultivars, ‘THM Jack’ and ‘Southern OG’, grown in a controlled environment and flushed for 0 up to 3 or 4 weeks before harvest
- Results: flower dry mass fell as the flush got longer, while trim dry mass did not change. Nitrogen, phosphorus and potassium in the flower also fell in both cultivars
- CBD and THC concentrations did not change in ‘THM Jack’. In ‘Southern OG’ CBD concentration rose, but not enough to make up for the lost flower weight, so CBD yield per square metre fell
- Conclusion: flushing does change the flower (lower mineral content, lower yield), and whether that trade-off is worth it depends on what the grower is trying to achieve
6. Does Flushing Remove Substances from Flowers?
Not in the way the myth claims. Plain water in the root zone does not rinse compounds out of the flowers. What happens instead is that, once feeding stops, the plant keeps growing on its reserves and moves mobile nutrients such as nitrogen, phosphorus and potassium out of older leaves, which is why the fan leaves turn yellow.
The results differ by study. The HortScience study measured lower nitrogen, phosphorus and potassium in flushed flowers, but the price was less flower. The Rx Green trial found no significant change overall. The Guelph thesis found flushing “ineffective in removing any significant amount of nutrient from the bud.” In none of them has lower mineral content been shown to make smoke smoother or ash whiter.
7. What Actually Determines Quality?
Growers’ experience points to harvest timing and post-harvest handling rather than to the last two weeks of feeding. The figures below are common practice, not results from controlled trials:
- Slow drying: 10-14 days in a cool, moderately humid room; a widely used rule of thumb is about 15°C and 60% RH
- Proper curing: sealed glass jars for 2-8 weeks, which growers credit with a smoother smoke
- Harvest timing: judged by trichome colour, which moves from clear to cloudy to amber. Peer-reviewed sources describe this progression but give no fixed percentage of amber heads to harvest at
- Genetics: the variety sets the ceiling for aroma, potency and structure
- Ash colour is a poor test: in the Rx Green blind test, ash was mostly grey or black whatever the flush length
8. When Is Flushing Still Useful?
- Salt build-up / overfeeding: runoff EC keeps rising well above the EC you feed. Some growers treat a runoff EC around 3.0 as a warning sign; this is a rule of thumb, not a research threshold
- Nutrient lockout: pH drift in the root zone
- Wrong fertilizer dosage: flush and restart at the correct strength
- Leaching containers: rinsing excess salts out of the growing media with clear water is standard horticultural practice
In these cases, flushing is troubleshooting, not standard procedure.
9. Conclusion
- Flushing has not been shown to improve taste, smoothness or ash colour
- Longer flushes can reduce flower weight and flower mineral content (HortScience 2025); in the company trial, flushes of up to 14 days made no significant difference
- Flushing is useful for troubleshooting salt build-up and feeding mistakes
- Focus on slow drying, proper curing and correct harvest timing
- Experiment: grow 2 plants of the same strain, flush 1 and don’t flush the other, dry and cure both the same way, then compare them blind
References
- Wedryk S, Wall T & Bennett R (2020). Impact of Different Flushing Times on Quality and Taste in Cannabis sativa L. Rx Green Technologies, company research report (not peer-reviewed). rxgreentechnologies.com
- Alden MJ & Faust JE (2025). Exploring the Legacy Practice of Flushing in Controlled-environment Production of High-CBD Cannabis (Cannabis sativa). HortScience 60(10):1818-1825. doi:10.21273/HORTSCI18752-25
- Saloner A, Sade Y & 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 220:119157. doi:10.1016/j.indcrop.2024.119157
- Stemeroff J (2017). Irrigation Management Strategies for Medical Cannabis in Controlled Environments. MSc thesis, University of Guelph. A later erratum concerns the irrigation and water-potential data, not the flushing results.
- University of Maryland Extension. Fertilizer Toxicity or High Soluble Salts in Indoor Plants. extension.umd.edu
Disclaimer: The information in this article is for educational and reference purposes only. Cannabis cultivation must comply with applicable laws in your country. In the Kingdom of Thailand, comply with the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568 and the Ministerial Regulation on Controlled Herbs (No. 2) B.E. 2569.