Flushing is the practice of halting fertilization before harvest and irrigating plants with plain water instead. The goal is to remove excess salts from the growing medium and the plant.[1]
Growers typically flush for two reasons: First, nutrient deprivation is believed to trigger secondary metabolite production as a stress response, indirectly increasing the quality (i.e., color, fragrance, flavor and potency) of the end product. Second, reducing the plant’s nutrient content may degrade chlorophyll, which some believe contributes to the harshness or burning sensation sometimes experienced when smoking cannabis.[2]
Despite how common flushing is, scientific support for the practice remains limited. In this blog post, we examine the theory behind flushing, review what the research says and discuss whether growers should include it in their cultivation regimen.
Examining the claims
It is true that nutrient deprivation can promote secondary metabolite production. Studies have shown that inducing macronutrient (nitrogen, phosphorus and potassium) deficiencies can increase cannabinoid levels—specifically, tetrahydrocannabinol—while other research has found that nitrogen deprivation may boost cannabidiol production.[3]
Cannabis is a “high nutrient feeder,” with evidence suggesting that nutrient deficiencies can appear within weeks of fertilization being withheld,[4] so it is reasonable to assume that flushing could trigger nutrient deprivation. When deprived of nutrients, plants are forced to rely on internal reserves stored in roots and older leaves rather than fresh uptake via the xylem stream.
It is also true that nutrient deficiencies can degrade chlorophyll. One study found that total chlorophyll levels decreased in the leaves of nitrogen-, iron- and manganese-deficient bok choy plants,[5] while another showed that magnesium deficiency caused chlorophyll degradation in rice plants.[6] Whether this degradation actually improves user experience, however, has not been scientifically proven.
Cannabis is a “high nutrient feeder,” with evidence suggesting that nutrient deficiencies can appear within weeks of fertilization being withheld,[4] so it is reasonable to assume that flushing could trigger nutrient deprivation. When deprived of nutrients, plants are forced to rely on internal reserves stored in roots and older leaves rather than fresh uptake via the xylem stream.
It is also true that nutrient deficiencies can degrade chlorophyll. One study found that total chlorophyll levels decreased in the leaves of nitrogen-, iron- and manganese-deficient bok choy plants,[5] while another showed that magnesium deficiency caused chlorophyll degradation in rice plants.[6] Whether this degradation actually improves user experience, however, has not been scientifically proven.
Studies on flushing
Research on flushing in cannabis cultivation is limited, but two studies offer some insights into its effectiveness.
One study examined the effects of flushing versus non-flushing on five medical cannabis cultivars.[7] Four cultivars were flushed two weeks before harvest, while one—due to an unexpectedly long maturation phase—underwent a month-long flush.
Researchers found that flushing had minimal effects on chemical profiles, with cultivar-dependent responses:
- For the four cultivars flushed for two weeks, two showed no change in cannabinoid concentrations, while two exhibited slight increases.
- The cultivar that flushed for a month experienced a decrease in cannabinoid content.
Flushing also had little effect on terpenoid profiles, with only one terpenoid showing a significant increase. Changes in nutrient concentrations varied by cultivar, and flushing did not negatively impact inflorescence yield quantity or quality.
Another study was conducted by The Cannabis Research Coalition in partnership with Clemson University’s Flowering Physiology Laboratory.[8] Cannabis plants were flushed for 1, 2, 3 or 4 weeks prior to harvest, starting with a reduction of growing substrate EC to less than 0.2 mS/cm using drip irrigation, followed by plain water irrigation. Researchers then measured the effects of each flushing duration on nutrient concentrations, cannabinoid concentrations and bud yield.
The study found flushing affected:
- Dried bud weight: Plants flushed for 1 week had similar dried bud weights to the unflushed control group, but bud weights decreased as flushing duration increased.
- Cannabinoid concentrations: These increased in one variety with longer flushing durations, but researchers suggest this was due to yield dilution—higher concentrations resulting from lower overall tissue mass, not increased cannabinoid synthesis.
- Nutrient accumulation: Macronutrient concentrations decreased as flushing duration increased. However, sulfur concentrations rose, which researchers noted might contribute desirable odors and flavors to the final product.
The study found flushing affected:
- Dried bud weight: Plants flushed for 1 week had similar dried bud weights to the unflushed control group, but bud weights decreased as flushing duration increased.
- Cannabinoid concentrations: These increased in one variety with longer flushing durations, but researchers suggest this was due to yield dilution—higher concentrations resulting from lower overall tissue mass, not increased cannabinoid synthesis.
- Nutrient accumulation: Macronutrient concentrations decreased as flushing duration increased. However, sulfur concentrations rose, which researchers noted might contribute desirable odors and flavors to the final product.
Should cannabis growers flush?
In the study with the five medical cannabis cultivars, researchers found no damage to yield quantity or quality caused by flushing and concluded that its effects on secondary metabolism are more likely to be beneficial than harmful. Based on these results, they recommend that growers incorporate flushing for no more than two weeks, noting that it also provides the added benefit of reducing agricultural input costs.[9]
However, in the study examining different flushing durations, researchers concluded that whether or not to flush depends on the grower’s goals:
- For growers cultivating cannabis for extraction, they advised against flushing for two weeks or longer due to the potential reduction in yield.
- For growers producing dried flowers, they suggested that flushing may be beneficial because of the resulting denser, more compact buds.[10]
Based on the available evidence, flushing for less than two weeks may help improve cannabis quality without significantly compromising yield, while also reducing fertilizer expenses.
How to flush cannabis
Flushing starts with proper timing. Our grow consultants recommend beginning 10–14 days before harvest, ideally when trichomes start shifting from clear to milky. Since flowering times vary by strain, always refer to strain-specific guidelines when planning the flush.
Next, use high-quality water, preferably reverse osmosis or distilled water rather than tap water, which can contain unwanted minerals and chemicals. Adjust the pH to between 6.0 and 7.0 to maintain plant health. Irrigate with plain water until approximately 20% runs off, helping flush out residual salts.
During flushing, expect some physiological changes, such as slowed growth or yellowing leaves, due to nutrient depletion.
Conclusion
Whether to flush ultimately comes down to grower preference. Growers may want to experiment to determine whether flushing improves the final product or enhances specific desired qualities. Even if flushing does not boost yield or quality, it offers the potential for input savings, which can help improve the bottom line.
Emerald Harvest Team
[1] Saloner, Avia, Yael Sade, and Nirit Bernstein. 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. https://doi.org/10.1016/j.indcrop.2024.119157.
[2] Ibid.
[3] Ibid.
[4] Llewellyn, David, Scott Golem, A. Maxwell P. Jones, and Youbin Zheng. 2023. “Foliar Symptomology, Nutrient Content, Yield, and Secondary Metabolite Variability of Cannabis Grown Hydroponically with Different Single-Element Nutrient Deficiencies.” Plants 12 (3): 422. https://doi.org/10.3390/plants12030422.
[5] Veazie, Patrick, Paul Cockson, Josh Henry, Penelope Perkins-Veazie, and Brian Whipker. 2020. “Characterization of Nutrient Disorders and Impacts on Chlorophyll and Anthocyanin Concentration of Brassica rapa var. Chinensis 10 (10): 461. https://doi.org/10.3390/agriculture10100461.
[6] Peng, Yu Yang, Li Li Liao, Sheng Liu, et. al. 2019. “Magnesium Deficiency Triggers SGR-Mediated Chlorophyll Degradation for Magnesium Remobilization.” Plant Physiology 181 (1): 262-275. https://doi.org/ 10.1104/pp.19.00610.
[7] Ibid.
[8] Alden, Michael, Jim Faust, and Allison Justice. 2023. “New Experiment in Flushing.” Cannabis Research Center & Coalition, July 15. https://www.cannabisrc.org/post/new-experiment-in-flushing.
[9] Saloner, Avia, Yael Sade, and Nirit Bernstein. 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. https://doi.org/10.1016/j.indcrop.2024.119157.
[10] Alden, Michael, Jim Faust, and Allison Justice. 2023. “New Experiment in Flushing.” Cannabis Research Center & Coalition, July 15. https://www.cannabisrc.org/post/new-experiment-in-flushing.
