To Flush Or Not To Flush – The Grower’s Dilemma

flush your plants

If you’re a cannabis grower, you have probably heard that before harvesting you should always flush. Heck, even I have written about flushing and why it’s important to creating a smooth hit and a tasty smoke…it turns out, that I may be wrong about all of that!


Of course, when I first started learning about growing cannabis, this was the Lore of the Land and I guess I simply accepted it as truth as did thousands of home growers in the past. However, recently in a Reddit AMA, Ed Rosenthal and Dr. Robb – two legendary names in growing cannabis – were asked their thoughts about flushing.

This is what they had to say;

Dr. Robb;

Ed and I have very similar feelings about flushing. A lot of people will claim that you need to flush so you don’t have a harsh smoke. Well, that hypothesis is easily tested in the industry right now. A plant that is grown in a 1- or 2-gallon rockwool or coco coir container can easily be flushed at the end of the grow cycle. Well, what about cultivators growing in huge 200-gallon containers or in the ground in Mendo or Humboldt counties in the Emerald Triangle? They could never fully flush their growing media, but their flower is just as smooth as any indoor grown flower (as long as they are both cured properly).

Science is the best way to get these answers and knowledge is power. Scientia potentia est!

Edit: Ed and I both feel that since the science is showing little to no difference in yield, potency, and general quality of flower that has been flushed vs. flower that has not been flushed, might as well flush. That way you are saving money on nutrients and not detrimentally affecting you quality and quantity of the flower. – REDDIT AMA

One Redditor interjected and pointed out that there are many unreliable variables in the answer of Dr. Robb. The good doctor quickly followed up;

A study has been done.

It pretty much says the same thing as we mentioned in the earlier comment, but here’s the summary from the report:

-Rx Green Technologies evaluated the effects of different flushing times on chemical profile, flavor, 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.

-Taste test panelists tended to prefer flower flushed for 0 days.

In other words, there is not really a noticeable difference between the two, and when it comes to Rosenthal and Dr. Robb – they both agree that “if there is no difference, then flushing is okay because it reduces the need for using nutrients”.  Meaning that flushing doesn’t have an impact on the bud itself, but could be a more cost efficient strategy since you’re saving on nutrients.

For small scale growers, this might not be a significant cost, but as you scale up your operations – the savings would compound overtime. There is an incentive to flush, it’s just not what you thought it was.

Some Redditors also pointed out this study which concluded;

Three treatments; control (irrigation events every 1-2 days), mild-stress (irrigation events every 2 days), and moderate-stress (irrigation events every 3 days) were tested. The effects of flushing were also investigated to determine whether it had the intended effect of reducing nutrient concentrations within the dried bud. Through the use of psychrometers, water status (cWP) thresholds were correlated with humidity (cVPD) thresholds and reduced irrigation frequency resulting in water use reductions up to 45.7% which had negligible impacts on yield and cannabinoid profile. Flushing was found to be ineffective in removing any significant amount of nutrient from the bud.

Now that we’ve nipped the Flushing debate in the bud, let’s take a look at some other important questions answered;

Question: What tips do you have for drying/curing cannabis?

Drying and curing is the most important step in all of production. You can be the best cultivator on the planet, but your flower will be worthless if it doesn’t have a good nose on it. As soon as you harvest your crop, you immediately switch from terpene production mode to terpene retention mode. Some of the monoterpenes in cannabis will start volatizing at temperatures as low as 70F, so it is important to get your crop into a controlled environment as fast as possible after harvest. For a conventional dry space, you will want uniform air movement, ~55% relative humidity, and 60F for the environmentals, but getting it in a cool, dark environment as fast as possible is key.

Ed says: “I am a big fan of a drying product made by CryoCure. They do a freeze dry method and their terpene retention is better than anything I’ve ever seen. The taste is great, but most importantly the smoke is smooth. CryoCure’s method was based on observations from my earlier version of the Cannabis Grower’s Handbook using a standard freezer with a slight vacuum to freeze dry it. Their new technology is much more sophisticated now, though.” – Doctor Robsenthall (Rosenthal + Robb)

Question: What are your thoughts about modern strains? Are they really more potent then what we were able to grow in the past? Are there any old strains or terp profiles that you miss?

Ed says: “Most of the old strains (cultivars) are still available. So any cultivator who misses the old strains can find them available on the internet. I miss seeing the NL5 Haze available, but that’s mostly due to it’s a 12-week plant.”

Dr. Robb says: “A lot of the cannabis breeders are focused on THC production in their new breeds. THC is a 21-carbon plant, so if the genetics of a new breed push THC production, there will be less carbon available for other cannabinoids and terpenes. Modern cultivars may have higher THC content, but that’s usually at the expense of other desirable products in the plant. The carbon budget is dictated by the plant’s genetics, so if the genetics are pushing THC production, less carbon is available for everything else.”

Question: Can you let these Reddit guys know that shwazzing is ok and isn’t plant torture?

Schwazzing is absolutely ok. There are a couple of reasons why it works (carbon allocation, increased light penetration through the canopy, etc), but the one aspect where schwazzing will benefit the plant that we want to mention is that cannabinoid and terpene production is closely related genetically to respond to environmental stress. Giving a little bit of stress without decreasing photosynthesis too drastically will benefit the plant. Perfect example of how tough love can benefit a plant other than cannabis is date palms. Date palms are like cannabis in that they have separate male and female plants. Ancient date palm farmers wanted to have both the males and females flower at the same time to increase pollination and therefore their fruit yields. They learned that if they physically whipped the trunks of the palms that they could induce the plants to flower at the same time and more heavily.

Question: What is your take on the HPS/LED argument? Is one arguably better than the other?

Ed says: “The most important thing to consider when working with lights is the DLI (Daily Light Integral). This is how much light the plants receive over a 24-hour period. The main factor to consider in terms of lights is their efficiency in producing the most amount of light per watt used. HPS lamps used to be considered the most efficient lamp for that reason; however, some LEDs produce more light per watt than HPS lamps now. The efficiency is usually listed as the number of joules. The higher the number, the higher the light per watt. LEDs vary considerably, but the efficiency is the primary factor when choosing the lamp for your grow. There are caveats: some lamps such as Hortilux and California Light produce light that is outside of the PAR range, but are known to increase growth with increased far-red in the spectrum that is being shown to drive photosynthesis as well. Also, UV which stimulates THC production. We have an easy to read table in the Cannabis Grower’s Handbook of LED lamps with their efficiencies.”

Dr. Robb says: “We just ran a trial on some lamps comparing CMH and HPS as well. The gGRO CMH lamp out performed lamps such as the Nanolux CMH and the Luxx HPS lamp. A big part of that is because of the efficiency of the gGRO lamp. I was pretty blown away with how well it performed.”

Question: What is the best way to focus on maximizing taste and smell of the final product?

Ed says: “It’s important to harvest at the right time to optimize the terpene content in the flowers. That should be done in early maturation, when the glands on the trichomes are just starting to change color. Having said that, we are looking at doing a study to scientifically determine when that time actually is, but traditionally that is when you harvest.”

Dr. Robb says: “When you harvest at the optimal time, it is most important to get the crop into a cool, dark dry space as fast as possible. If you can smell the plant that means that those are terpenes that have volatized off the flower and are no longer retained in it. Terpene retention is important because some of the monoterpenes can volatize at temperatures as low as 70F. Your dry space should 60F or so.”

The Sticky Bottom Line:

If you want to explore the rest of the answers, there’s a lot of gold in there. I simply fished out some of the most important questions, but there’s a lot of comments from Redditors that also provides some serious insight. I hope this help clarify the whole Flushing debacle.







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