Is Flushing Plants Necessary Before The Harvest?

Francis Cassidy November 8, 2019 1 comment

Flushing your cannabis plants before harvest is a crucial step in growing smooth, good-tasting, bud.

Firstly, know that no plants are being flushed down any drains. Flushing plants simply involves watering the plants without adding any nutrients for a period of time prior to harvest. This process allows the plant to use up the available nutrients already present. Flushing these nutrients out of the plant prior to harvest ensures better-tasting, and better-smelling, cannabis.

Flushing plants is only necessary if you have applied nutrients during the growing process however. During the flushing process the plant is only given pH balanced water. Timing is key though. Start too early and the plants starve before you finish. If you start too late, the plant won’t leach all of the excess nutrients, which may result in harsh-tasting cannabis.

flushing plants involves leaving out the nutrients superimposed onto this picture of a seedling

Why is Flushing Plants Important?

Flushing is important on several levels. Firstly, it affects the taste of the plant. When the plant can’t recycle the surplus of nutrients prior to harvest, excess compounds, like salt, will lead to a harsh, bitter-tasting, product.

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Failure to flush plants can also result in other unintended consequences. One example is the tell-tale black ash rather than a peppered burn when smoking.

Most growers and enthusiasts grow cannabis with some form of fertilizer to supplement the growing process. The predominant compounds in these fertilizers are nitrogen, phosphorus, and potassium (NPK).

What Nutrients Should Be Used on Cannabis?

There are three compounds you should be flushing. All three play an important role in plant development.

  • Nitrogen is vital for the production of chlorophyll. Cannabis plants can only obtain it from the soil, not the air.
  • Phosphorous is hard to find in nature, but it’s essential for healthy roots. Flowering cannabis plants, in particular, require extra phosphorous.
  • Potassium helps aid plant metabolism. It helps in the production of essential proteins, and also strengthens the plant’s immune system.

An effective flushing process helps purge excess amounts of the above from the plant before harvest.

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flushing plants would involve waiting until the end of their life cycle to not give nutrients, represented by these changing cannabis leaves

What Do the Naysayers Say?

Some say that to save water, you shouldn’t flush your toilet. But there is another altogether more interesting group that claim you shouldn’t flush your plants either. Here’s what they say:

  1. They often claim that denying plants nutrients at any stage of the grow cycle is harmful. In reality though, your plants will have an excess of certain compounds. Flushing ensures that the plants use up what they have accumulated throughout the grow cycle.
  2. Others claim that plants grown hydroponically will always taste better than plants grown in soil, as soil can’t be effectively flushed. This is correct, but only in part. While it is true that hydroponic systems are easier to flush, soil isn’t hard. It just takes a little longer.
  3. Many argue that withholding nutrients from a plant causes unnecessary stress. And the truth is that while flushing does cause stress, it’s a healthy stress. Think of it as when a human fasts. The metabolic processes within our bodies actually clean up our cells via a process known as autophagy. During this period we have no nutrients, but our bodies burn the excesses within. Another advantage of flushing plants is that it increases the presence of defense compounds — like terpenes.

What to Beware of When Flushing Plants

One area of contention among those in the community is the use of flushing agents. It is a subject of debate whether they make a real difference or not.

Flushing agents contain chelates, and these organic compounds chemically bind with other compounds and help remove them via natural processes. A high-quality flushing agent is designed to provide your plants the support they need during the flushing process. It helps ensure they efficiently flush out the excess nutrients that may lead to poor-tasting product.

hands holding cannabis plant

When to Flush Your Cannabis Plants

The exact timing of your flush will be dependent on the strain (or chemovar) you’re growing. Most experienced growers have established a working knowledge with particular strains and will start the flush based on the week of flower. As a rule of thumb, you can begin to flush two weeks before finishing, although this isn’t always accurate.

Another useful marker is the trichomes. Upon noticing when they change from clear to cloudy, the flush can then begin. Take note that during the flushing process, plants’ leaves can often start to turn yellow. This is a sign that the plants are depleting the stored nutrients.

With cannabis that grows hydroponically, you may find that you require shorter flush times. Many growers find that flushing plants two to five days before harvest is sufficient. While for cannabis grown in a coco medium, the flushing generally begins one week before harvest. Soil, of course, will always have the longest flush times, and it generally extends to one to two weeks.

After harvesting, the crop should be cured in order for the potent medicinal benefits to manifest. Any harsh tasting compounds not already removed by flushing are through a proper curing process. If you choose to medicate via smoking or vaping, then the efforts made during the flushing and curing process may make a surprising difference to taste.

Author avatar

Francis Cassidy
Francis Cassidy is a freelance writer who writes on a variety of topics. With a particular focus on the cannabis industry, he aims to help ensure the smooth reintegration of cannabis back into global culture. When not writing, he's to be found exploring his new base in British Columbia, Canada. You can follow his other works including his photography on his blog

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1 comment

  1. Christopher Kelly Okano

    My name is Chris Okano and I am the owner of Bear Creek Soilless Media Company located in Goldston, NC. I want to start this by asking some questions and giving my readers some common sense answers and some food for thought.
    They say that water is the “universal solvent” ph neutral composed of Hydrogen and Oxygen explosively created and covalently bonded together. Without water plants will not live, they will wither and die and they will be incapable of receiving soluble food and essential minerals. So water is crucially important. Did you know that of the amount of water absorbed from a plant’s roots 90 percent is transpired through pores in a plant’s stems and leaves? Only 1 percent of the water absorbed by the roots of plants is used by the plant. 90 percent of the water used by the plant makes of the plant weight. Soil is composed of 45% mineral matter, 5 % organic matter, 25 % water and 25% porous air space. Plants absorb water, soluble nutrients, and soluble minerals from tiny root hairs during the process of geotropism by the migration of the root through the soil as it searches for available capillary water that surrounds soil particles and that is held in pore spaces in the soil. This is the water plants use and that dissolves plant food and minerals. So what does all this have to do with flushing plant pots? Why is plant pot flushing necessary? It is necessary if you do not regularly water your plant, or if you do not water them properly by reason that the lack of proper allows plant food to build up in the soils to a dangerous level or toxic level causing curling of leaves and other symptoms. In the case of organic fertilizers, those are broken down by microorganisms; after the breakdown, organic nutrients are made available in water-soluble form. If there is inadequate watering then organic broken down fertilizer begins to overbuild in the soil requiring flushing as it can become detrimental to the plant. I found a good article written by “American Agriculture,” on when to flush in the third paragraph.
    Additive enhancement products such as “Bloombastic,” recommend flushing prior to use of the product.
    “CannaCon,” has written another pretty good article on why and when to flush.
    Finally, Advanced nutrients sell two or more products designed to enhance bud taste as a flush.
    If you are using soilless media especially coir understand coir may have a great deal of salt that should be washed out. Mixing peat moss and coir together equally makes a nice mix but it has no nutrients, organic fertilizers or essential minerals. The value of soilless media is that it is a blank canvas so to speak allowing the horticulturist to engineer a soilless growing mix with all the plant needs that are weedless and that can be very balanced. Soilless media is an expensive way to go but in my opinion one of the best ways to grow. I like to call it the solid form of hydroponics because you can control everything in it.
    Do marijuana seeds need any nutrition when they are germinated? No, and if you do ad anything to a germination mix add phosphorus this is the most essential element for germination for the first two weeks.
    Seeds possess “endosperm” it is the natural food for the new radical to grow in for the first two weeks of life. No other nutrition is necessary. Endomychorizal inoculant is another very essential organic microorganism to add in any soil or soilless media. Water is the most important plant food element by far.