Flushing cannabis plants prior to harvest is always been a very controversial topic to many growers but flushing for other reasons is well-mastered practice.

So is flushing really necessary and when to perform it? And are unflushed autoflowers harsher and will not burn or burn with black ash?

Before flushing any plant we need to know what this flushing really accomplishes and how to do it properly!

What is flushing?

Flushing basically is the action when you pure a large amount of water through the growing medium, but it can also refer to the time when the plant is fed only with pure water for some period of time.

Why flush your plants?

Flushing basically gets rid of excess chemicals and salt build-up in the growing medium either before harvest to get tastier buds or when you have added too much nutrients during the growth cycle.

In the wild cannabis plants will eventually “flush” themselves when autumn comes and they are almost done flowering – they start using their own nutrients to swell up the buds and mature the seeds that are in them. Of course, they can’t flush the chemicals away from the ground but the rain helps a little and the plant just stops absorbing when it is almost ready for harvest.

So if in nature autoflowering cannabis plants flush themselves, why do we need to flush them if we are growing them indoors in a controlled environment?

There are multiple explanations why we need to flush autoflowers but they all accomplish the same thing – get rid of dissolved minerals from the growing medium:

  1. The First and the most important aspect of flushing is to get rid of unwanted chemicals both in the growing medium and also in the plant itself before the harvest. If we want the smoke to be smooth and tasteful then we need to get rid of chemicals that are in the plant’s leaves and flushing the plant before harvest forces it to look for food in those leaves. And then your day-neutral cannabis plant will use up these unwanted chemicals like Chlorophyll, magnesium, Nitrogen, and phosphorus. Flushing is most useful if we grow our plants with chemical fertilizers because water binds together with these chemicals and easily flushes them away. The rest is up to the plant itself. But even if you’re growing organically flushing is also necessary. There can still be some salts or chemicals in the ground and they most defiantly are in the plant so flushing again proves useful and we will get healthier and tastier smoke.
  2. Second reason for flushing is to get rid of excess nutrients and chemicals in the growing medium when the plant is still growing. Usually, inexperienced growers feed the autoflower cannabis plant with far more nutrients than it can absorb and they build up in the ground and a good flush can flush that excess supply away. Usually this phenomenonn is called a “Nutrient Lockout” and it is basically a process where you add too much nutrients to the soil or other growing medium but your autoflower plant will not absorb all of them. So in the end what happens is that these excess nutrients build up and can lead to various problems starting from chemical burn to nutrient imbalances.
  3. And the third reason where you need to use flushing is to prepare the soil for seedlings. You need to flush at least 2 to 3 times the amount of pot with water so that every last piece of dry soil is wet and ready to nurture your autoflower roots.

When cannabis is flushed indoors or flushes itself outdoors at first we can see the big fan leaves to start curl and yellow, and later also the smaller leaves start doing that, but that is all-natural and the plants are just using up the food that is stored in those leaves.

How to?

When we want to just get rid of the excess salts or nutrients (total dissolved solids) in the ground we need to flush 2 or 3 times the volume of the pot through the growing medium and let it leach out at the bottom with all those unnecessary elements and salts.

When the plant is flowering and it will soon be ready for harvest then we need to flush both the chemicals and nutrients in the ground and also let the plant use up its own salts, chemicals and nutrients (like excess chlorophyll for example) – this is sometimes called a preventative flush.

We need to time this process just right because too if we flush too fast then the plant uses up all the stored nutrients before the harvest date and then it just don’t have any more food to continue growing – And this can lead to lower harvest. So The flush must be done 5 to 10 days from the harvest and again it has to be 2 to 3 times the pot volume of PH adjusted water.

When the first initial flush is done in those remaining days till the harvest you need to feed your autoflower plant only with pure PH adjusted water so your marijuana plants will use up it’s stored remaining nutrients and your harvest will be healthier and amazingly tasty.

Does flushing differs when growing in Soil VS soilless growing mediums?

The easy answer is that it does not matter that much as you can and need to flush both when growing in soil and also soilless mediums and the process is the same!

A little bit more complicated answer is that when growing in soilless mediums or “dead soil” then the process is a little bit easier than when growing your autoflowering cannabis plants in soil. This is due to the fact that when the soil is alive it is changing and using up various chemicals itself and in the process changing the chemical makup of the nutrients that you feed your plant. But when you grow in Coco or other soilless mediums then there are no chemical processes happening and you have much more control over the growing and also over the flushing process.

One more useful thing about flushing is that by doing it you can fully wet your soil and there won’t be any more dry spots remaining in the middle of your growing medium that could lead to dead roots or less growing medium where roots can expand. This is usually done before planting autoflowering cannabis seeds and is usually a standard procedure but you can also do it periodically during the growing cycle to be sure that all of your plant’s roots are getting moisture and not drying out.


  1. terry mckibbins on

    Finally clear and understandable info. on how why and when to flush thank you!

  2. Ken tchell on

    I cant believe you claim that Chlorophyl, magnesium, Nitrogen and phosphorus are unwanted chemicals! the Nitrogen, phosphorus and magnesium are absolutely essential for plant growth and metabolisim! The Chlorophyl is of course a major part of the plants metabolisim/catabolisim and necessary for the plant to survive!! It is for inaccurate, non-scientific explanatios like this and also just plain common sense that I do not believe in flushing! Its a waste of time and doesnt have any basis in science or plant physiology!

    • I do not state that they are completely unwanted, but if they are in too high concentration they can have damaging effects and that is when you would flush them.

    • Michael C on

      The plant holds those chemicals in its leaves. The idea is to time your flush to the point where you leave the plant no other choice but to feed on itself. Thus minimizing those chemicals in your final product. At that time the chemicals ARE unwanted. By eliminating them at the proper time you achieve a smoother, cleaner, much better tasting smoke. During the plants growth and development, those chemicals are crucial, but not in your harvested weed.

      • Kevin Hawkswood on

        Not just chemicals left in your plant . You don’t need synthetic chemicals in your lungs . Flushing is a must !

  3. this ken guy, f****** idiot thinking he knows what hes talking about. the nutrients were discussed as being negative in both the smoking and over fertilizing sense. f*** me some people.

    • banga j: totally agree.
      keyboard warriors everywhere!
      Great grow guide- very clear – very informative.

  4. I am in my last week of life cycle. Yesterday I started to use ripen instead of flushing them. Today I saw I have a problem with spider mites. Im afraid that that they will suck out the life of my beautiful buds, so I stated to think to either cute them and flush them.

    I don´t know if just cutting them without flush is good. I´ve used all so far Bio Grow and Bio Bloom (I stopped 2 weeks ago) so I don´t know if there is anything to be flushed. Im little confused because I saw there is a lot different opinions on this matter. If I grow in soil and use organic nuts do I need to flush or is flush more for hydroponics.

    • If you have a spider mite problem I suggest you harvest your plants as soon as possible and don’t worry about flushing, as it is not that important. Those tiny creatures will otherwise ruin your crop!

    • Michael C on

      I agree. If you wait to harvest your buds will be so infested they won’t be worth keeping. Unless you like smoking insects, and that’s if there’s anything left to harvest.
      If you’re growing outside, look into companion plants to help keep insects away. Dill is an excellent companion plant to repel spider mites and many othe insects. Some not only repel insects but also provide an assortment of other benefits, such as more vigorous growth, better water retention, flavor enhancement and diversify micro- ecosystems increasing bio – availability of nutrients.

  5. Juwanaman on

    It’s 2016 and this guide is still informative and relative. Guess who just flushed?

  6. You ever notice when someone is wrong, they get even louder and more idiotic? Finding out that you’re wrong about something and learning how to correct it should be a good thing. But, some folks can’t handle the truth, as Jack Nicholson famously said. Thank you for some info on flushing my girls.

  7. If i grow my plant in a little apartment and i can’t pour 2 to 3 times the pot volume of PH adjusted water to the plant at once. is it ok if i just water the plant like i normally do with adjusted water for a longer time? is that

    • Of course!

      The flushing process is not mandatory and in most times you won’t feel the difference in the end product.

  8. Hi, how can u tell if its a salt build up, are there any visual signs to look out for? Tia

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