How to flush cannabis plants

How to flush your cannabis plants

Cannabis is one of a few plants, together with the grapevine, to contain a large amount of terpenes, which when blended give rise to new aromas and flavours that swaddle your taste buds like a piece of candy.

In order to achieve this kind of flavours and enjoy the best cannabis, you need to make sure your plants are nutrient-free right before the harvest.

Nutrient-free plant

Nutrient-free plant

If your plants have an excess of nutrients, the buds’ flavour will be spoiled by the presence of salt build-up in the flowers. When you smoke them, they will release harmful particles that, apart from leaving a very unpleasant taste in your mouth, could make the joint spark due to the excess of mineral salts.

What is the purpose of flushing?

Plants receive fertilisers during their whole life in varying amounts, according to the life stage they are at each moment. The feeding curve goes from less to more until it peaks at around week 6-7 of the flowering period, depending on the genetics, and from that moment on, the cannabis plant needs are notably reduced.

When your plant no longer requires fertilisers, you should stop feeding it, because even if it doesn’t need them, it will eventually absorb them and save them in its metabolism for the future.

Plant in need of flushing

Plant in need of flushing

If your plant absorbs an excessive amount of fertilisers during its cultivation, the leaves will present a very intense green colour at harvest time. This means that the buds won’t have the same authentic flavour when compared to the same plant with yellow leaves. In this case, flushing will be extremely necessary to reduce the nutrient load in its metabolism and to improve the buds’ flavour.

How to flush your cannabis plants

Flushing consists in providing the substrate with a great amount of water in order to leach the soil, coco, expanded clay, perlite, or any other type of medium. You must try to get rid of the nutrients that are attached to it.

The amount of water used should be about three times the pot size. There is no exact number of litres, but it will depend on the nutritional schedule you’d given the plant during its whole life cycle.

Roots after flushing

Roots after flushing

If you use an intensive nutritional diet with high EC levels throughout all the cultivation period, you’ll need to flush your plants with more water than if you used a lighter nutrition programme with a lower EC rate. In order to know whether your substrate’s EC levels are high, medium or low, you should water your plants with the corresponding EC level according to its regular nutritional schedule, and then measure the run-off EC level.

To measure this, fill a glass with the drained water that comes out from the bottom of the pot, and measure it with an EC meter. If the run-off readings are higher than those of the water used for the irrigation – for example; irrigation EC: 1.8/runoff EC: 2.2 – the meter is telling you there is an excess of salts in your substrate.

Now that you know your plants have excess nutrients, you need to reduce their concentration by providing the growing medium with large amounts of water. This water should have the lowest possible EC; tap water should have a maximum EC of 0.3-0.4. If your tap water EC is too high, you’ll need to get hold of some distilled or low mineralization water to perform a more effective flushing, or reverse osmosis water.

Flushing by automatic irrigation

Flushing by automatic irrigation

Water the pot with a watering can or an automatic irrigation system, collect the run-off water and check its EC once more. This time, you’ll see that the EC level is much lower than before flushing. If the run-off water has an EC level of 0.5-0.6, you’ve managed to remove almost all of the salts from the medium, and your plant can start using or draining the nutrients without any problem.

How do cannabis plants react to flushing?

Once the substrate is free of salts and fertilisers, the plant will know (through its roots) that there are no nutrients left, and there is a great imbalance between its internal EC and that of the substrate.

Plants need the right balance between both ECs to be able to thrive and feed themselves. As we’ve already mentioned, your plant has an internal EC that should be similar to that of the substrate. In the absence of the right balance, the plant will try to fix it by itself, but, how?

Effects of flushing. Figure A: Nutrients leaving the substrate Figure B: Nutrients in assimilable form, but excessive amount

Effects of flushing. Figure A: Nutrients leaving the substrate Figure B: Nutrients in assimilable form, but excessive amount

If the plant EC is lower than that of the substrate, the EC of the plant metabolism will tend to rise by absorbing the nutrients in the substrate, for example: imagine the Plant EC is 1.5, while the substrate EC is 1.7; the plant will admit more nutrients. On the other hand, if the substrate EC is lower than the plant’s, the plant will drain part of its internal EC into the substrate through the roots, raising the EC of the substrate until both ECs are almost equal.

Therefore, when you provide your substrate with a lot of water, you’re forcing the plant to drain its internal EC in a very radical way, as it goes from having nutritional stability to not having any, and this causes stress that often translates into few male flowers at the end of the cultivation cycle. These male flowers are the result of stress since the plant believes it won’t have enough nutrients to carry on living, and chooses to reproduce by itself in order to produce some offspring, which is the purpose of any plant species: its survival.

How can you avoid flushing?

To avoid having to flush your cannabis plants, you need to bear in mind their nutrition throughout its whole life cycle, trying not to use a big amount of fertilizers, as the plant will visually let you know whether it needs more nutrients or not.

It’s important to understand that the feeding curve goes from less to more and from more to less, which means that when the plant needs fewer nutrients, you’ll have to lower the amount of fertilisers, and gradually increase it; but you also have to know when to decrease it, since feeding your plant until the very end it’s unnecessary and counter-productive.

Nutrient-free plant cultivated with organic fertilizers

Nutrient-free plant cultivated with organic fertilizers

But, how do you know the exact amount of fertilizers needed by your plants? An EC meter makes everything easier, as you can use an EC table as a guide. This way, you’ll know the amount of nutrients you need to feed your plant, and in what concentration according to the week.

EC and pH in flushing

If your plants have an excess of nutrients, they can also be flushed during its cultivation, both during the vegetative and the flowering periods. But in these cases, you’ll need to take into account the pH of the substrate and the plant EC according to the stage of the plants’ life cycle.

During the vegetative stage, the water used for flushing should have a pH of 5.8, and during bloom, around 6.0. Try to always use the lowest possible EC, so both the nutrient draining and plant recovery are faster.

Using a stable pH during flushing helps the substrate to keep its balance after having used a different pH to that of the corresponding week when you detected the over-feeding. And when the plant recovers from a nutrient lockout, it won’t notice the pH change and will continue to feed properly till the end of the cycle.

Roots in perfect condition. Without Trichoderma on the left, and with Trichoderma on the right; a huge difference!

Roots in perfect condition. Without Trichoderma on the left, and with Trichoderma on the right; a huge difference!

Remember that pH is the valve in charge of letting the nutrients get from the soil to the plant through its roots. No matter how much fertilizer you use, if the pH value is wrong, the plants could suffer nutrient deficiencies.

When is a good idea to avoid flushing?

The answer is very simple; providing that your plants have a nice colour, healthy leaves, etc., you won’t need to flush them, unless it’s the end of the plant life cycle.

Some growers flush their plants just before switching to flowering in order to change the type of fertilizer, but in this case, your plant will only lose part of its stored nutrients. This loss will prevent the plant from growing properly during the stretching phase, and will start flowering without the extra nitrogen that needs to create the bud sites from where the flowers will eventually develop.

Plant in need of flushing

Plant in need of flushing

Flushing with organic fertilizers mixed with soil

Bear in mind that using a mix of fertilizers and soil is not the same as using liquid fertilizers every time you water your plants.

If you use liquid nutrients, the flushing process will be as described above. But when using fertilizers mixed with soil, you need to control the doses your plant will need during cultivation.

If you don’t take into account these doses, your plant will be completely green at the time of harvest, and the flavour, even with organic fertilizers, won’t be as good as if the same plant had been yellow and free from stored nutrients.

Why use enzymes in cannabis cultivation?

Enzymes are a product made of molecules that come from proteins. They help to increase the rate of the chemical reactions that occur in the root system, making dead roots become an assimilable fertilizer for the plant, by acting as a catalyst and speeding up the nutrient decomposition and assimilation processes.

Enzymes act as protection for the root and the plant metabolism by preventing pathogens to enter the plant and destroy it. A good example of this is when a plant is attacked by the fungus Botrytis cinerea or gray mould. The fungus’ enzymes try to penetrate the plant to conquer and destroy it, but the plant’s enzymatic reaction provokes a battle at enzyme level in which the fungus and the plant fight each other.

Enzyme structure from the peroxidase family

Enzyme structure from the peroxidase family

Simply put, this process could be described as a war in which the winner is the one with the best skills to conquer the territory, as if it were a Trojan Horse. Modern science is now carrying out studies on enzymes that can destroy even the most powerful and devastating fungi such as botrytis, a fungus that uses the enzyme laccase to attack the pterostilbene, another enzyme that helps to protect the plant.

This fight triggers some toxic elements, namely 3 different types that can kill the plant very quickly, since they enter its metabolism and reduce its solubility in water, so the plant can’t evacuate them. How does the plant react to this invasive attack? The only chance the plant has to survive the attack and the toxins that have entered its metabolism is to encapsulate these toxins, causing an increase of their solubility and evacuating them, so it can survive and carry on with its life.

Enzymes are very important for this reason. A good example is Power Zyme by Hesi, which helps the plant to carry out the natural processes of nutrient decomposition and assimilation by actively intervening in the ion exchange between roots and nutrients. In organic crops, you should use big amounts, unless you replace them by beneficial fungi such as Trichoderma or endomycorrhizae among others, to avoid other dangerous fungi that can kill off the plants.

Evolution of the nutrient load through the leaves during cannabis cultivation

In this image you can see a normal colouring of the leaves, a healthy green tone which means that everything is going well. Continue with the usual diet following a nutrition program according to the week of the plant life cycle. At this stage you need to boost the root growth with phosphorus and a good dose of nitrogen.

Green colour of growing leaves using fertilizers

Green colour of growing leaves using fertilizers

In the second image the plants are evolving gradually, week by week. They show a less bright green colour which indicates they are growing adequately. We continue without any nutrient deficiency.

Green colour in growing plants without deficiencies using fertilizers

Green colour in growing plants without deficiencies using fertilizers

In the image below, the plants have started the bloom phase. They stopped receiving fertilizers for the vegetative phase and have started to get nutrients for the flowering period, with no flushing in between. In the lower part of the plant you can see a slight nitrogen deficit. This suggests that the substrate has low nitrogen availability, and the plants are using their reserves, which is ideal in order to reach the end of the cultivation period with nutrient-free plants.

Flowering plants turning yellow in their lower parts. The nitrogen reserves are being depleted. No more nutrients will be used

Flowering plants turning yellow in their lower parts. The nitrogen reserves are being depleted. No more nutrients will be used

In this image, the plants are in late flowering, around week 6, so they only need 2 more weeks to go. You can now see that the leaves are more yellow than green, or at least they have a less intense colour than during the growing and early flowering periods. As they approach the harvest, they will become even more yellow.

Leaves turning yellow towards the end of the cultivation

Leaves turning yellow towards the end of the cultivation

In the last image, you can see that even the leaves that previously had a yellowish colour, have now fallen or dried up in the plants, which means there are no nutrients left. The plants have now a very pure smell and taste, but the best way of appreciating this flavour is with a good cannabis tasting session using the same genetics, but with a plant cultivated with nutrients till the end, and another plant properly flushed just before harvest.

End of the cultivation cycle with yellow plants nutrient-free and flushed

End of the cultivation cycle with yellow plants nutrient-free and flushed

We hope this article will help you to know when to flush your plants, as well as the reasons for which it is necessary, which usually takes place at the end of the grow cycle.

Happy harvest!

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