Before harvesting or trimming cannabis plants, it's vital to ensure that the crop is truly ready for harvest. Commonly, this involves a process known as flushing.
For growers who flush pre-harvest, finishing the plant's flowering cycle in this way helps ensure optimal flavor, aroma, and smoothness in the final product, whether that's live resin or ready-to-smoke flower.
But, even though flushing has been a mainstay in pre-harvest protocols for decades and will continue to be an important part of the grow cycle for many cultivators, there is debate as to whether it's really necessary and beneficial.
What is flushing, and how and when is it used in cannabis cultivation? Below, we answer these questions and break down the pros and cons of flushing, along with a guide on how to flush cannabis plants pre-harvest.
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What is flushing?
Flushing is the process of watering plants with plain, properly pHed water and without added fertilizers. It is commonly used in indoor growing environments, where plants are often grown in individual containers and are given nutrients via synthetic fertilizers — often referred to as “salts" — via water, as opposed to growing environments where nutrients are delivered via amendments and microbiomes in the soil.
While flushing is often thought of as a pre-harvest practice, it is a technique that may also be used during the vegetative phase of growth.
Correctional versus pre-harvest flushing
If nutrient delivery needs to be suddenly changed or if plants are experiencing nutrient lockout — a condition that happens when root systems are no longer able to absorb nutrients — during the vegetative phase, flushing may be used as a sort of “reset" for the roots and medium, by flushing away existing nutrients and restoring pH levels to their optimal range.
As a pre-harvest technique, flushing is the process of removing any unused nutrients from the growing medium through watering with nutrient-free and pH-neutral water for the final one to two weeks of the plant's life.
Why do growers flush cannabis plants pre-harvest?
Proponents of flushing utilize the process as a way of:
- Removing nutrients from the medium. Plants are forced to use up remaining stores of nutrients. Some growers argue that excess nutrients in the flower post-harvest results in a harsher and less flavorful smoking experience.
- Creating temporary stress. Removing the plant's access to nutrients may produce higher cannabinoid and terpene content in the buds as the plant devotes more energy to reproduction as a survival tactic.
- Breaking down increased amounts of chlorophyll. This also impacts the final flavor and smoothness.
Should you flush weed plants?
“This topic is highly debated," said Colin Decker of Sensei Growth Consulting. Despite being a commonplace cultivation technique that many cultivators swear by, it's not universally accepted or practiced.
Ask a room full of growers whether the benefits of flushing are a myth, and you'll get a lot of different answers.
The reason for this is the fact that cannabis plant biology is complex, and, as a grower, there are many different variables you could tweak to impact the plant's development at any given moment in the growth cycle. If we look to science to settle the debate, to date, there hasn't been a lot of research looking at the effects flushing has on the plants or the final product.
One 2017 unpublished and non-peer-reviewed study conducted by Master's student Jonathan Stemeroff at the University of Guelph in Canada took a systematic approach to look at the effects of flushing over the final two weeks of growth and found that the practice does not impact nutrient concentrations within the harvested flower.
Another unpublished and non-peer-reviewed study done in 2020 and conducted by a manufacturer of cannabis nutrients and cultivation products, RX Green Technologies, found no differences in yield, potency, and terpene content when comparing final flower samples from plants flushed for 7, 10, and 14 days versus non-flushed plants. Additionally, the test panelists they invited to a blind sampling of the flushed and non-flushed flower did not detect any differences between them and preferred the non-flushed flower.
“Flushing and watering to run off are general approaches to steering and soil management that in practice have very little commercial utility when looking at end flower quality, including potency, terpene production, ash quality and burn rate, yields and aesthetic," said Eric Culberson, Senior Vice President of Horticulture at multi-state cannabis operator Columbia Care.
According to Culberson, commercial growers should instead focus on developing an understanding of the root zone of plants, both before and after watering, by measuring the electrical conductivity (EC), pH, and moisture content at different stages of growth. “This is where they stand to increase profitability via high-quality flower with repeatable results."
Qualities of flushed vs non-flushed plants
There are still many cultivators who flush their plants pre-harvest and report they have seen, tasted, and smelled the difference flushing can make in their final product.
“The importance of flushing [cannabis] plants is widely recognized by seasoned cannabis growers, who have firsthand experience with the contrast in crop quality between flushed and non-flushed plants," said representatives from 420DC.com.
Among these differences is harsher and more bitter-tasting smoked flower and a general chemical aroma to the bud. “I've even seen it sparkle or crackle as it's being smoked and can cause you to cough and hack with an unnatural off flavor," said Scott Vasterling, founder of California's Humboldt Family Farms.
Flushing may be a helpful technique if plants have been over-fertilized throughout their life cycle, something that often happens with less experienced growers. Many cultivators would agree that under ideal growing conditions appropriate nutrient amounts would be given according to a plant's needs throughout the life cycle — and these needs change through each phase of growth.
“At no other point in the plant's life cycle are we looking to deprive it of food or to stress it out," said Decker. “Sure, it may help with maturation of trichomes, but perhaps the idea of 'tapering' down of food and not 'flushing' food from one's plant is actually the most reasonable and sensible choice to allow both things to occur."
Pros and cons of flushing cannabis plants
Setting aside the ongoing debate as to whether one should or shouldn't flush weed plants, here are a few of the potential pros and cons to consider before trying the technique yourself.
Pros
- May improve aroma and taste: One idea behind flushing is to remove any residual nutrients that may affect the taste or quality of the final product. By removing these nutrients, the plant will use up any stored nutrients in its system, and this can lead to a cleaner, more neutral-tasting product.
- May produce a higher cannabinoid content: Flushing may help increase the THC and general cannabinoid content of the buds, as the plant will use up the stored nitrogen in its system, and cannabinoids will become more concentrated in the buds. However, the increase in cannabinoid content may be small and only be noticed with a laboratory test.
- Saves time and money: By withholding nutrients for the final week or two before harvest, you'll save money on both the nutrients themselves and the time needed to mix and administer the nutrients.
Cons
- May not do much: Flushing is not a universally accepted practice among cannabis growers, and some growers do not believe it is necessary or effective.
- May increase water usage: If flushing with larger quantities of water or on a more frequent schedule than would normally be used to water plants, this technique would require more water.
- May cause plant stress: Flushing can be stressful for the plant which may result in hermaphroditic tendencies, re-vegging, or increases in fan leaf and sugar leaf compared to bud or flower.
- May reduce yields: If done too early, flushing may prevent flowers from reaching their growth potential, so it is important to be mindful of the timing of the flush.
- Somewhat tricky process: Flushing is considered an advanced technique that requires a good understanding of the plant's nutrient needs. If not done properly, it can lead to nutrient deficiencies and negatively impact plant growth.
How to flush weed plants
Flushing cannabis plants is a relatively simple process, though the specifics as to exact timing, the quantity of water used, and other considerations often come down to the type of grow and preference of the grower.
- Mark your calendar
Ideally, flushing begins one to two weeks before the targeted harvest date, which is determined by your plant's strain and growing conditions. Reputable seed and clone sources will outline the flowering time for each strain, and it is usually measured in the number of days or weeks after switching to a 12 hours on/12 hours off light schedule.
Mark your calendar for two weeks before the scheduled harvest so you can check back in at that time to evaluate whether or not your plants are ready to be flushed. - Keep a close eye on bud maturation
Two weeks before harvest is the general rule for flushing, though this timing will vary from crop to crop. Ultimately, the timing of flushing comes down to whether or not your buds have matured enough. “If flushing is to be done, timing is everything," said Decker. “The flowers must be fully formed and only be 'bulking' up at the point at which flushing is introduced."
Flushing too early will deprive the plant of nutrients needed to finish flower development, while flushing too late will not be as effective since plants take up less and less water as they near the end of their life cycle.
Ideally, you'll want to time the end of flushing to coincide with the harvest date, and this can be tricky for newer growers. How do you know when to start flushing? Many cultivators base harvest date on a few visual cues present in mature flowers: the appearance of the trichomes and the color of the pistils.
When buds are ripening and approaching harvest the trichomes will be changing from clear to milky white. “Generally, you should begin flushing your plants when the trichomes on the buds are mostly cloudy, with a few amber-colored trichomes," said Alan Robinson, co-owner of Herbal Aspect. Mostly clear trichomes, on the other hand, are a signal that the buds are still ripening.
At the beginning of the harvest window, pistils will also begin changing from a bright white to a vibrant orange or brown color. “Typically, the pistils at this point are starting to turn red, and the nuggets aren't swelling anymore," said Luke McCann of G's Trees Cultivation Club in Red Bank, New Jersey.
When you see these cues, you can begin flushing. - Water your plant without nutrients
The next step is simple: water your plants without nutrients, adjusting to the appropriate pH for the medium as needed.
For solid mediums where nutrients need to be washed away — like soil, coco, or rock wool — different approaches can be taken. Some growers will continue to water the plant when needed and with the same amount as they normally would. Some will up the amount of water by two or more times. Others may flood the medium, then let it sit for a few minutes to allow the water to dissolve excess nutrients before flooding again to wash them away.
“The [duration] you should flush your plants will depend on a variety of factors, including the strain, the grow medium, and the size of the plant," said Robinson.
The soil will need the longest flush time, ranging from one to two weeks. Soilless mediums like rock wool and coco are generally flushed for about a week, while nutrients in hydroponic grows can be tapered by replacing half the reservoir water with plain water (pH 5.5-6.5) over the course of 5 - 7 days.
Cultivators who are experienced with flushing or who want to take their flushing game to the next level might use an electrical conductivity (EC) meter, a parts per million (PPM) or a total dissolved solids (TDS) meter to track that the nutrient levels in the runoff are decreasing with each watering. Ideally, you want to match the levels in the water you're using. - Monitor the process closely
Without added nutrients, plants will feed on whatever is left in the growing medium and then use up the accessible stores in the fan leaves leading up to harvest.
The bottom and innermost leaves will start to fade to lighter green and then yellow as a sign of nitrogen deficiency. Plants in flush grow more sensitive by the day, so pay close attention to any indicators of problems. Signs of light burn, such as yellowing from the top down and yellowing or bleaching bud sites, are different from normal flush starvation where leaves will yellow from the bottom up and inside out.
Harvest before leaves in and around the flowers start to yellow to protect the appearance of the buds.
FAQs about flushing weed plants
What does flushing mean in cannabis cultivation?
Flushing is a technique that utilizes nutrient-free and pH-appropriate water to flush away excess nutrients available to the plant.
It can be used during the vegetative cycle to reset nutrient levels and/or the pH of the root system when the plant is experiencing deficiencies or nutrient lockout. It is also used pre-harvest to flush away nutrients and force the plant to use its own reserves in an effort to improve the quality of the final flower.
How long should you flush cannabis plants?
Flushing is typically done anywhere from 1 - 2 weeks before harvest, and the timing of the flush is important. Too early and the flowers will not have reached their full growth potential, too late and the plant's decreased water uptake in late-stage bloom will not allow for a sufficient flush.
Flushing should start when the buds are fully formed and have begun ripening and end when most of the plant's leaves — but not its flowers — have turned yellow. The growing medium will also impact flush length. Soil typically requires between one and two weeks, coco and hydro about one week, and in hydroponic grows, reservoir water can be incrementally replaced with plain fresh water for five to seven days.
Can you flush cannabis plants too early?
Yes, flushing cannabis plants before their flowers have finished growing and begun ripening will lead to underdeveloped buds and lower yields.
What are the benefits of flushing cannabis plants?
The reported benefits of flushing cannabis plants include:
- Better aroma and flavor
- High cannabinoid content
- Better smokeability and smoothness
- Saved costs on nutrients, time, and labor
Do you need to flush organic cannabis plants?
Many growers argue that organically grown cannabis does not need to be flushed. This is because the microbiome present in organic soil — whether it be in super soil or cultivated through regenerative farming methods — works in symbiosis with the plant and rarely leads to the excessive nutrient levels that using synthetic fertilizers may encourage. As the plant ages, the microbes in the soil respond to signals from the plant's root system and will naturally taper down the levels of nutrients made available. Further, flushing living soil may negatively impact these microbes, and the soil biodiversity organic methods are used to develop.
Is flushing cannabis really necessary?
There is ongoing debate as to whether flushing cannabis is necessary. On one hand, for many experienced cultivators, flushing is an essential pre-harvest activity that improves the aroma, flavor, and smokeability of the final product.
On the other hand, many cultivators argue that flushing does not actually make any difference in the flower's final mineral, cannabinoid, or terpene content. There is little scientific data to support either argument, though initial studies support that flushed and non-flushed flower are comparable.
Bottom line
Flushing is a relied-upon pre-harvest technique, though there is little evidence to support or undermine its use in both home growing and commercial contexts. More careful study and research are needed to understand the impacts, advantages, and disadvantages of flushing, but it may be a more useful practice for plants that have been over-fertilized to begin with. Overfertilization is most common among novice growers, and when using pre-mixed liquid nutrients, that may not account for the conditions in any particular grow or the needs of any particular cultivar.
The timing and execution of flushing can be tricky to master. If you are new to growing cannabis, it may be best to avoid flushing until you have more experience and knowledge about your plants. But if flushing is a technique that interests you, consider trying your own side-by-side comparison to explore whether flushing makes a difference in your next harvest.
