The Flushing Cannabis Myth Exposed
Flushing – running water only with no nutrients for the last 10 -14 days of flower – has long been argued to improve the quality of cannabis. The argument for flushing has long been something to this effect:
“When the nutrients are removed from the soil, it forces your cannabis plant to use up any remaining nutrients still present in the plant. It is a lot like the human body. We take in a lot of food and, what we do not use is turned into fat. In extreme situations where food is scarce, the body relies on this stored fat for energy.
As flushing forces cannabis to use up any remaining nutrients left in the plant, none should remain to taint the use of the harvest bud.” Source: Royal Queen Seeds. https://www.royalqueenseeds.com/blog-how-and-when-to-flush-marijuana-plants-n312
Amazing stuff! Wow!!!
However, the value of flushing tends to be hotly debated by many growers with some stating that the same thing can be achieved through curing the harvest correctly, while others might tell you that flushing means starving the crop of vital nutrients and, as a result, quality and yields suffer. I tend not to take a dogmatic position on this one other than to say, what published scientific research that has been done on flushing, where comprehensive tissue testing was conducted between two flushed crops and a control (not flushed), concluded:
“6.2 FLUSHING NUTRIENTS FROM GROWTH MEDIA
The practice of flushing is a current common industry practice but there is no evidence in published literature of its effectiveness in reducing nutrient concentrations within the bud or even whether or not this is a desirable result. After testing the nutrient concentrations from each treatment from three separate experiments, there were no significant differences in nutrient levels between any treatments within each experiment. This result showed that the intended purpose of flushing to reduce nutrient concentrations within the bud has no effect. “
To summarise this study:
Flushing occurred over the last two weeks of production when the nutrient solution was replaced with water (no nutrients) during routine irrigation events. Another flushing method was to apply an additional 10L of water without any fertilizer to the plant over two irrigation events at the start of the two-week period of water only irrigation.
Three separate crops were given different treatments. The first irrigation treatment was the ‘control’ which was the standard irrigation procedure used in the facility. This treatment has irrigation events every 2 to 3 days. The second irrigation treatment termed ‘mild-stress’ was an irrigation event applied every 2 days, and the third irrigation treatment termed ‘moderate-stress’ was an irrigation event applied every 3 days.
While flushing had no effect on reducing tissue nutrient accumulation between flushed crops and control (a crop that wasn’t flushed), flushing didn’t reduce yields or quality (THC percentage). However, nor did it improve quality with regards to cannabinoid production.
Flushing, as a result of these findings, may help growers to reduce input costs (i.e. running water is cheaper than running water and nutrients).
These findings really didn’t come as any surprise to myself. Cannabis, a plant of a thousand myths, is the only crop on the planet where growers flush believing they can miraculously make stored nutrients in the tissue disappear. This flies in the face of all known plant science. I.e., once nutrients are lodged in the tissue you cannot suddenly force a plant to “cannibalise” these stored nutrients by starving it of further mineral nutrition.
Other than this, from a scientific perspective, it is probably more important that growers do not overfeed their plants if they wish to minimise stored nutrients in the tissue. The need for flushing is quite probably an extension of the fact that most people are over-feeding their plants. Beyond this, growers who claim that cannabis tastes better if flushed perhaps miss the point that taste is highly subjective and each person’s taste buds are unique. In other words, whose taste buds does it taste better to?
A recent blind taste test conducted by RX Technologies, where flushed and unflushed cannabis was handed out amongst “cannabis industry experts” found the study’s participants preferred the taste of unflushed cannabis.
A summary of this study:
- Rx Green Technologies evaluated the effects of different flushing times on chemical profile, flavour, and smoking characteristics in Cannabis flower.
- Flushing periods of 14, 10, 7 and 0 days were imposed on Cherry Diesel.
- No differences were detected between flush treatments for yield, potency, or terpenes.
- Analysis of mineral content of leaves indicated small changes in content of iron and zinc.
- To determine smoking flavour and characteristics, samples of each flushing period were distributed to Cannabis industry experts in a blind taste test. Each participant received one sample of each flushing time without knowledge of its treatment.
- Participants were asked about their personal Cannabis flavour preferences before tasting. After tasting, participants were asked to rate the sample for flavour, harshness of smoke, and colour of ash.
- Cured flower from Cherry Diesel was evaluated by a group of industry experts for flavour and smoking characteristics in a blind taste test. Overall, the duration of the flushing period had no impact on flavour, smoothness of smoke, or colour of ash. The seven-day flush period had the highest “bad” rating (21.1%) and the 0-day flush had the highest “great” rating (16.7%). Most panelists rated the flavour of the samples as “okay” or “good”. The ten-day flush had the highest “okay” ratings at 48.6% and the 0-day flush had the highest “good” rating at 47.2%
- Thus, taste test panellists tended to prefer flower flushed for 0 days.
However, in many ways the findings that flushing has little to no effect on stored nutrients in the plant tissue while also having no effect on yields, cannabinoid percentages and terpenes are a win/win for both proponents of flushing and those against flushing. That is:
Q: Do you flush?
A: No, I run water only for 2-weeks to save on nutrient inputs.
Q: Do you flush?
A: Yes, I do, for two weeks. It makes the product taste “cleaner/better/less chemically” etc.
That’s pretty much my position on things. Either way, it is worth flushing (running water only with no nutrients) in the last 10 -14 days of the crop cycle.
 Jonathan Stemeroff. Irrigation Management Strategies for Medical Cannabis in Controlled Environments. Master of Science Thesis. The University of Guelph. November 2017.
 IMPACT OF DIFFERENT FLUSHING TIMES ON QUALITY AND TASTE IN CANNABIS SATIVA L. Stephanie Wedryk, PhD, Director of R&D at Rx Green Technologies Taylor Wall, Research Operations Lead at Rx Green Technologies Ryan Bennett, Research Associate at Rx Green Technologies