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How To Properly Ripen and Flush Cannabis Prior to Harvest

By Dan Vaillancourt
Published: December 8, 2022
Key Takeaways

You’ve put so much effort into growing a healthy crop of cannabis, and now it’s harvest time. A few extra steps at the end of the cycle can result in good cannabis or great cannabis.

Cannabis takes months to mature and flower, with countless hours and lots of money poured into your investment. Skipping out on a proper ripening phase means ruining your entire crop, and flushing time and money down the drain.

Flushing is much more than just releasing the build up of salts in your growing medium or hydroponics system at the end of the growing cycle. If done properly your flowers will not only be completely cleaned out of any nutrients used during the growth, but also ripened to their full potential. Properly ripened flowers will exhibit a much more profound aroma, look, and taste while the THC and other cannabinoids will have a better, more usable effect on the body. Most importantly, it will burn properly, and you will only be smoking the cannabis, not the nutrients.

If you run a medical growroom (ACMPR), micro-cultivation facility, or licensed producer you owe it to yourself and your customers to implement proper ripening techniques so your product is clean and passes lab inspections

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When To Start a Flush & for How Long

grower examining pistils
If you know that the strain(s) you’re working with takes eight weeks to finish from the start of flower then it’s easy: you just start your flushing phase at week six of the flowering cycle.

If you don’t know when the cultivar you’re working with will finish flowering, then its best to use the red-hair technique.

The Red-Hair Technique

The stigma (or pistils) is the hair-like part of the cannabis flower. It grows out from the calyx and its main purpose is to collect pollen from the male plant when released. The calyxes are designed to protect the plants reproductive system, however, for most growers (other than breeders), you won’t need to protect any seeds.

Your stigmas will be white or soft yellow to start and over time they will turn a shade of red depending on strain. With the red-hair technique we will just watch for the pistil color development on the flowers and start the flushing phase when we have between 50-70 percent overall color development on the pistils of the plant. This will ensure that over the next two weeks of flushing, your finished plants will be in the 75-90 percent red-hair range.

The reason behind this is because we do not want to let the plants continue to grow at 100 percent pistil colorization as the THC can begin to keel over and die during this time. The ideal window for harvesting is between 75-90 percent red hairs with 75 percent being more geared towards the milky trichome preferences and the 90 percent area more for the amber trichome and pronounced terpene profiles.

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Takeaways:

  1. Read your plants’ hairs (pistils), not the calendars.
  2. Initiate the flushing phase when 50-70 percent of the pistils have turned orange or red.
  3. Flush for 14 days regardless of growing style, nutrients, or strain.


The Flushing Phase

grower inspecting pistils with a loupe
Flushing should not be overlooked. It is such a crucial part of the growing phase where it inhibits bulking and ripening of the flowers while packing on THC when done properly. Initiating a flush for 14-28 days when your plant is at the 60-75 percent mark is the general rule, depending on the strain. This will ripen your flowers fully, giving the most robust flavors and highest THC your strain is capable of while packing on weight and cleaning the flowers out of stored nutrients (yuk!).

When you stop feeding the plant, the roots no longer need to assimilate the nutrients which allows them to focus on the terpene production and flower density. During a proper flush your flowers will swell and harden, and the entire crop should start falling over if its above three feet, hopefully on to their trellis or supports! Your plants will also begin to cannibalize themselves by eating up stored nutrients in their leaves — this is what gives those beautiful fall colors of purple, yellow, and orange.

Fourteen days is usually enough for most strains, it is only the sativa-dominant strains that sometimes need longer. Watching your hairs or stigmas is the best way to know when to harvest. This method is more precise than following the recommended growing length for your seed via the breeder. Every phenotype is different and the seed information will be generic or based on average timelines, while every seed will take on different traits, sometimes more of the mom, sometimes more of the dad.

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Ideal Flushing Conditions and Environment

cannabis plants in a growroom
Plain, non-pH adjusted water is a given but what else is involved in an ideal flushing phase?

The growroom design and environment are very important to your success, and controlling it properly will be key to producing craft-quality bud.

You will want to cut carbon dioxide (CO2) off completely during the final two weeks of flower to allow your buds to harden properly and develop the most aromas and THC possible.

Cooler temperatures are also recommended. We want to ease the temperature down gradually like with any transition in growing. The first week of flush should be dropped down 2-4°F (6-8°C) from your normal temperature and the final week should be an additional five-degree drop, sometimes even more if you are trying to pull out the anthocyanins in the cannabis that cause the purple colors.

As soon as you see that your stigmas are at that 60-75 percent mark, lower your light intensity down to 60 percent your first week of flush and then to 40 percent on your final week. This mimics the sun being further from the plants at the end of summer because of the Earth’s axis and allows your buds to harden up properly, becoming much denser. Go the extra mile by turning off every second light to help create density in the flowers and maintain cooler temperatures.


Recommended:
How to Plan Ahead for Your Cannabis Harvest
Identifying and Preventing Mold in Harvested Buds
Tips for Handling Fresh Cannabis at Harvest Time

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Flushing Amounts

The flushing phase is not the same as your normal water feeds. You will want to feed the plants more than normal with water that’s as clean as possible, with zero nutrients and no pH up or down.

Feed your plants two to three times the amount of water as normal—reverse osmosis water is ideal in this stage as well as growth if you can. Do not let your run-off water sit in your trays or tables. If using recirculating hydroponics, flush out your system much more frequently than normal. This will allow the expelled nutrients built up in your root base or medium to wash off. Of course, recirculating hydroponics runoff will reach low PPM quicker than a system with a grow medium, but it is still just as important to flush for the 14–28-day period to allow the plant to go through the ripening process, cannibalizing its leaves, using up stored nutrients, and packing on the THC and weight.

No Need to Adjust pH in Flush

hydroponic cannabis plants
It is not necessary to adjust your pH when flushing, as the only reason to adjust pH is so your plant takes in the right level of nutrients.

The adjustment of alkalinity and acidity in the water from pH up and down adds phosphoric acid, potassium hydroxide, and potassium carbonate when using horticultural pH adjusters. This is done because we need the nutrient solution to reflect the correct pH range for our plants, which in hydroponics for cannabis is 5.5–5.8, so if there are no nutrients in the water, there is no need to adjust the pH. Plus, we want to clean the plants out, not add more things.

Takeaways:

  1. Flush with non pH-adjusted reverse osmosis or clean water for a minimum of 14 days.
  2. Watch your pistils’ color development.
  3. Feed two to three times the regular amount of water in a drain-to-waste system, and flush out two to three times as much in a recirculating hydroponic system during flush.
  4. Do not let plants sit in the runoff; allow proper drainage.
  5. Cut off CO2 enrichment completely when initiating the flushing phase.
  6. Lower light intensity to 50 percent when initiating flush or turn half your lighting off if you can not lower intensity.
  7. Lower temperatures in the room by 2-4°F (6-8°C) the first week of flush.


Advanced Ripening Techniques

Advanced ripening techniques will bring out the anthocyanins in your cannabis. These are the pigments or flavonoids that cause the purple color, changing the colors of your pistils from an orange to a deep red or pink. These techniques can massively increase your THC content or help sugar up the inside of a canopy that did not receive good light.

Cold Water or Ice Treatment

ice cubes on top of soil
This method refers to chilling the grow medium of the plants by either chilling the water of the recirculating hydroponics system, or the feed water to your plants in soil, coconut husk, or stonewool. You can do this by purchasing the large blocks of ice from gas stations generally used for fishing and placing them in your reservoir tank, while setting your water chillers to the lowest setting or adding ice above the plants.

Using this method (along with cooling the room) will bring out lots of colors in the cannabis, as well as THC. THC and other cannabinoids are a defense mechanism for the plant to protect itself from pests (they stick to it). We like it for other reasons, though! Adding more stress like this towards the end of the plants’ life will naturally bring out more of what we love.

Takeaways:

  1. Cool the water of your feed tanks to as cool as possible to bring out colors in the cannabis (anthocyanins)
  2. Use large blocks of ice and water chillers to prolong their cooling period

Armageddon

cannabis plant in darkness
During the final 48 hours of the plant’s life, you can implement what we call “Armageddon” to induce a massive final push of cannabinoids and colors to the plant via light and temperature stress. During this last 48 hours, you will want to keep the rooms completely dark and as cold as possible. This should only be done for 48 hours as more time can cause seeds to begin developing. We have tested this 48-hour method on countless strains and can assure your plants will be just fine.

This technique can be great for sugaring up the inside of a canopy by harvesting all the top flowers, then leaving the inside canopy for two days before you chop it.

Takeaways:

  1. Only implement “Armageddon” for the last 48 hours before harvesting
  2. Make rooms as dark and cold as possible
  3. Be gentle when harvesting cold cannabis as the trichomes are more brittle at this stage


The Best Time to Harvest After Flushing

macro image of cannabis pistils and trichomes
When it is just about time to harvest, you can look a little deeper into things and look directly at the trichomes of the plant with a 5x microscope, a loupe, or a good phone camera. The trichomes are small factories, creating different cannabinoids and terpenes, amongst other things. The head of the trichome will be a clear ball most of its life, though toward the end it will go through a milky white stage, then amber, and finally a darker shade of red as it begins keeling over and dying (we don’t want that!).

I personally prefer to harvest my cannabis when the trichome is a milky white with some starting to turn amber. It will be at high potency at this point with no dying trichomes, without too much of the couch lock that comes with the amber trichomes (and indica strains).

Takeaways:

  1. Use a microscope, loupe, or good camera to see your trichome development
  2. Do not let the trichomes keel over and die (this would be growing past 100 percent red hair)


If you have ever tried to smoke cannabis that didn’t burn or did burn but the ash was peppered or dark, there is a good chance that you had product that the grower skipped a proper flush on. Flushed product running methods like these will come out burning smooth with light grey or white ash. Flushing is not only necessary during and at the end of production but is an integral part of growing craft cannabis.

These ripening techniques and all the methods offered here are the results of more than 23 years in the field. Follow these methods and you’ll grow the best-quality craft cannabis possible.

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Written by Dan Vaillancourt | Owner of 4Trees Cannabis Building Ltd.

Profile Picture of Dan Vaillancourt

Dan has designed and built hundreds of growing facilities as well as grown thousands of plants himself and spent countless hours in both the grow room and consulting. He owns 4Trees Cannabis Building Ltd., a company that designs and constructs growing facilities for both ACMPR as well as recreational (Micro-cultivation, LP). He is proud to be shaping Canada's cannabis industry.

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