how to flush cannabis in soil before harvest

How and When to Flush Marijuana Plants

Flushing the marijuana plants before harvest can make all the difference in the best bud, or horrible hash. This small task is simple and super easy to do. Just add water! Be very careful, as the timing of the flush can play a critical role in this process.

The what, how, and why of flushing cannabis plants.

  • 1. What is flushing?
  • 2. The best time to flush your cannabis
  • 3. Prevent nutrient lockout before it becomes an issue
  • 4. Enzymes to the rescue
  • 5. How to properly flush cannabis
  • 6. How to flush hydroponic plants
  • 7. The outcome of flushing your cannabis
  • 8. When to avoid flushing your plants
  • 1. What is flushing?
  • 2. The best time to flush your cannabis
  • 3. Prevent nutrient lockout before it becomes an issue
  • 4. Enzymes to the rescue
  • 5. How to properly flush cannabis
  • 6. How to flush hydroponic plants
  • 7. The outcome of flushing your cannabis
  • 8. When to avoid flushing your plants

You have finally finished your grow, ending up with a beautiful plant covered in tremendous bud, now dried, cured and ready to go – yet something is not quite right. You can hardly get the stuff to burn, and when you take a hit, it feels like a mule kicked your lungs as you end up coughing for your life! The taste is harsh and disappointing. If this is a situation you have experienced, chances are your plants were not flushed properly before harvest.

This less than pleasant smoke is caused by nutrients and minerals used during growth still be present in the plant, altering the way it burns. Flushing removes these remaining nutrients, improving the quality of the experience. Fortunately, flushing your cannabis is an effortless and easy task, and will have you producing smooth and delightful bud in no time.


The act of flushing a plant is using plain water to actively remove any nutrients in the soil. A large amount of water is passed through the soil and drained away on a regular basis. Any minerals and nutrients present in the soil are washed away over time by the water, leaving the soil clean.

Why would you want to strip away all the minerals from the soil? Isn’t this hurting your harvest? It actually helps your harvest a significant way. When the nutrients are removed from the soil, it forces your cannabis plant to use up any remaining nutrients still present in the plant. It is a lot like the human body. We take in a lot of food and, what we do not use is turned into fat. In extreme situations where food is scarce, the body relies on this stored fat for energy.

As flushing forces cannabis to use up any remaining nutrients left in the plant, none should remain to taint the use of the harvest bud. However, if done to early, it can leave your plant unhealthy, so timing is key.


Flushing is usually commenced two weeks before harvest. If the plant has an eight-week flowering period, the flushing will need to take place six weeks after the start of the flowering stage. It is best to take a close look at the trichomes on your plant to assess when your cannabis is likely to be ready for harvest. If the tiny trichomes are just beginning to turn from clear, to a cloudy and milky colour, this could be a good indication that the plant can begin flushing. It should be timed so that the majority of trichomes will have fully changed to the desired colour for harvest after two weeks – this gets easier with experience, so stick with it!

Flushing can also be a good way to reset the soil while a plant is in the vegetative state. Sometime, growers will accidently overfeed their cannabis, causing the tips of the leaves to begin changing colour and shriveling. This is called “nutrient burn”. Flushing the soil can remove the excess nutrients, helping avert the problem. However, it is a drastic measure at this phase of growth, so be sure that the problem is nutrient burn, and not something else first.

Flushing isn’t exclusive to harvest time. The technique can also be used in the vegetative phase to remove excess nutrients from the soil. Sure, nutrients keep your plant healthy and ensure optimal yields, but too much can cause nutrient lockout—a state where plants can’t access nutrients.

Nutrient lockout can be caused by both salt buildup and incorrect pH levels. It can be solved by flushing affected plants with plain water. The fluids push nutrients out of the soil and wash away the buildup, allowing roots to once again freely uptake nutrients.


Nutrient lockout can be an inconvenience at best and devastating at worst. As the old saying goes, prevention is better than a cure. It’s best to take steps to avoid nutrient lockout as opposed to tackling it later down the line.

Preventing nutrient lockout can be accomplished via routine flushing. By flushing your plants once before flowering begins and once halfway through flowering, you’ll minimise the chances of nutrient buildup.


After flushing to counter nutrient lockout or before harvest, you might still notice your plants are dark green in appearance—a sign of excess nutrients. In this case, some growers elect to add enzyme-rich formulas to the soil.

If you don’t remember anything from biology class, enzymes are proteins that catalyse reactions. They help to flush out the soil by breaking down starches, carbohydrates, and nutrients. There’s a variety of products on the market that contain effective enzyme formulas.

If water isn’t doing it for your plants, these small proteins will make lingering nutrients budge!


Flushing your cannabis plants is a straight forward process. Whenever you would normally feed, you flush instead. Untreated tap water is all you need to use for flushing, just be sure to make sure the pH is at a safe level for cannabis. Most well water contains a healthy pH level and will not need treatment, but if it is necessary for you to add treatment to adjust the pH of your flushing water, feel free to do so. The pH adjustments will be the only thing you will need to be concerned about.

Flood the soil with as much fresh water as it can hold. Leave this for a few minutes to allow all of the nutrients to be picked up, the flood the soil again to flush it all away from the plant. If you are indoors using pots, notice the colour of the water that is draining from the bottom of the pot. It will be stained and look dirty. This is where a TDS (total dissolved solids) meter will come in handy. If you were to collect and measure the TDS of the “drained off” water, it would be around the measurement of 1300ppm, which is pretty high. It is important to keep flushing the plant until this number drops to a level of 50ppm, or at least until it is close to matching the TDS of the fresh water that you are using to flush with. The colour of the draining water will lighten up and appear to be cleaner. You want to get as much of the dissolved minerals away from the plant as you can.


Flushing hydroponic plants is much easier than removing nutrients from a soil medium. Hydro growers can simply drain their system and replace the water with plain pH-balanced water instead.

Flushing hydroponic plants is also a much shorter process. Once the water supply has been switched out, hydro plants won’t have access to any external nutrients. Because of this, you’ll only need to flush plants for two days.


After the bud is harvested, take the extra time to cure the bud to its highest potential. A proper cure will cut back even further on that harsh edge, removing aspects like excess chlorophyll. You will be amazed at the difference this small effort can do to your product. All of your hard work will show in that first inhale of that silky smooth smoke that hits the back of your throat as soft as honey. This is nature at its smoothest. You can increase the quality of your cannabis by just adding water!


The only time it’s recommended to avoid flushing is when you’re growing in an amended organic soil or super soil. This medium is carefully developed over time to harbour beneficial microorganisms such as bacteria and fungi. This delicate biodiversity can be washed away and damaged by flushing.

Then again, the lack of flushing shouldn’t be an issue as no synthetic or external nutrients are added to this medium. Instead, plants rely on microorganisms to break down organic matter and deliver it to the roots.

The HI-98107 pHep pH tester provides fast and accurate pH readings. The easy-to-use device is designed for non-technical users, and can help both novice and advanced growers measure water pH.

HI-98107 pHep pH tester provides fast and accurate pH readings. The easy-to-use device is designed for non-technical users, and can help both novice and advanced growers measure water pH.

Flushing cannabis before harvest can makes the difference between a smooth or horrifically harsh smoke. Here is how to do it.

How and When to Flush Cannabis

When it comes to flushing cannabis – timing is crucial! By flushing your cannabis plants before harvest you can avoid a harsh and unsatisfying end-product.

If cannabis plants are not properly flushed, the minerals and nutrients used during the cultivation process are still present. Flushing removes all nutrients and improves the overall quality of your bud. Flushing cannabis is easy! Here’s how:

How to Flush Cannabis

1. Test the pH level of the water (tap is acceptable) to ensure it’s in the appropriate range for cannabis plants (between 6.0 and 6.8 for soil grown).

2. Water the plants at the time you would normally feed them without giving them any nutrients or supplements. Don’t water your plants more than usual to prevent deficiencies.

3. Repeat 15 minutes later.

4. To make sure the flush was successful, use a TDS reader to measure the total dissolved solids and find out how pure the water runoff is.

It’s ideal that the water draining out of the pots is as close as possible to the TDS (total dissolved solids) reading of the neutral water you’re flushing with.

5. Keep an eye out for signs of too much yellowing. The pre-harvest flush may cause a plant to lose its color quickly. It’s normal to see some yellowing, but it’s critical to harvest before the leaves on the buds have yellowed.

Once all the leaves have turned yellow your plant is at the end of maturity and buds will start to deteriorate.

6. After flushing, your plants will be a lot lighter in color and are ready to be harvested.

Flushing cannabis is used to remove buildup from the roots and soil of a plant not. It’s been recommended to flush at the beginning or the end of a day when plants can be misted (this lowers their transpiration so that they do not over hydrate).

When to Flush Cannabis

Finding the best time to start flushing will vary based on how close your plants are to harvest and the knowledge and methods used by the grower.

Generally, flushing cannabis normally takes place two weeks before it is harvested. If the plant has an 8-week flowering period, flushing should start 6-weeks after the beginning of the flowering stage when trichomes begin to form a cloudy white color.

Flushing cannabis removes leftover nutrient buildup from the roots and soil of your plants giving them a fresh start. Flushing effectively allows your plants to absorb any nutrients that are still in the soil.

Flushing cannabis too early and too often will restrict nutrients and restrain the plants from growing and flowering. Flushing too early can also result in yellowing or discolored leaves.

Flushing Cannabis – The 3 Times You Should

1. Pre-harvest flush – Flushing is used during this stage to improve the quality and smoothness of cannabis. A flush during pre-harvest will force plants to use up the nutrients stored within themselves while preventing harshness and removing excess chlorophyll.

If the nutrient reserves are not used or broken down they will negatively affect the quality of cannabis buds. This should be done between a week and 10 days before harvest and repeated three days later.

2. Changes in the nutrient cycle – Cannabis has different nutrient requirements depending on what growth stage it’s in. Cleansing cannabis of old nutrients is a good way to reset soil while a plant is transitioning into a new stage of growth.

Think of this as a preventative flush. It’s not required, but by the time a plant reaches the flowering stage, it has sucked all the nutrients from the soil and could benefit from a fresh start.

3. Nutrient Lockout – Growers that understand how to balance nutrients individually don’t have to worry about flushing as much as those who rely on pre-balanced nutes.

Using pre-balanced nutrients may offer immediate results, however, after the first few doses, your plant is likely getting too much of one nutrient and not enough of the others creating a deficiency. A deficiency in plants is not always due to a lack of nutrients but can be because a plant has too much of a nutrient causing its system to become unbalanced.

By overfeeding a plant’s nutrients, it will absorb what is needed while the access sits in the soil. This leads to a build-up of unnecessary nutrients which is known as Nutrient Lockout.

Generally, when dealing with a serious nute imbalance, the cure is to flush the plants and add a fresh, well-balanced, mild dose of nutrients. It’s important to pay attention to the details; don’t just flush cannabis because of minimal changes.

If you notice a drastic change in your plants but there are no other issues (heat, root rot, etc.), flushing may be a good idea to remove excess build-up and help restore the soils pH balance.

Flushing cannabis effectively removes build-up and allows plants to absorb any nutrients that are still in the soil.