How To Properly Ripen and Flush Cannabis

By Dan Vaillancourt
Published: October 19, 2022 | Last updated: October 19, 2022 11:01:49
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.

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.


grower examining pistils

Knowing When to Flush

The stigma is the hair part of the cannabis flower, otherwise known as a pistil. Its main purpose is to collect pollen from the male plant when released and it grows out from the calyx which is what makes up cannabis flowers as we know them. 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 (or pistils) will be white or soft yellow to start and over time they will turn a shade of red depending on strain. When your plants’ pistils darken in color (generally red) an overall amount of 60-75 percent of the plants’ coverage, it is time to start the flushing phase for two to three weeks depending on strain.



  1. Read your plants’ hairs (pistils), not the calendars.
  2. Initiate the flushing phase when 60-75 percent of the pistils have turned orange or red.

macro image of cannabis pistils and trichomes


The Flushing Phase

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!).

The calyxes will swell and harden, your pistols will mostly all turn red, and the leaves will start to cannibalize themselves, turning yellow. 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 as every strain and phenotype is slightly different.

This method is more precise than following the recommended growing length for your seed via the breeder. This is because 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.


  1. Flush with reverse osmosis or clean water for a minimum of 14 days
  2. Watch your pistils’ color development

grower inspecting pistils with a loupe

Going a Step Further

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).


  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)

hydroponic cannabis plants

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. 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. This, and the fact that adding pH up or down will add PPM, or acid, to the tank that your flower will take in during the last phase of its life, which we don’t want either.

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.


  1. 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
  2. Do not let plants sit in the runoff; allow proper drainage

cannabis plants in a growroom


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

As soon as you see that your stigmas are at that 60-75 percent mark, you need to either lower your light intensity down 50 percent or turn half of them off. 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. You will also need to cut CO2 off at this point.

Your temperature should also be dropped as well down by about 2-4°F (6-8°C) from your normal. We want to ease the temperature down gradually like with any transition in growing.

CO2 will need to be cut off at this point completely.

All of this will help the plants’ flowers harden up during ripening and develop the most aromas and THC possible.


  1. Cut off CO2 enrichment completely when initiating the flushing phase
  2. Lower light intensity to 50 percent when initiating flush or turn half your lighting off if you can not lower intensity
  3. 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.

ice cubes on top of soil

Cold Water or Ice Treatment

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.


  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

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.


  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

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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