Flushing your medium

December 18, 2018 4 By percysgrowroom

Flushing your medium is the act of pouring water, through your soil or soil like medium to remove excess salts, and nutrient build up, or to prepare for harvest. Things will differ slightly, depending on your grow medium, and choice of nutrients.

If you’re growing cannabis organically, there is no need to flush your plants, as there are no salts to remove. You can learn more about organics in our organics section.

If you have any questions about flushing your cannabis plants, then feel free to ask us anything in our cannabis growers forum

Preparing water for the flush

To make sure you don’t shock the roots of your plants, water should always stand for at least 24 hours at room temperature before feeding. This allows the water to reach a good temperature ( 18c is optimal), and also allows chlorine to evaporate.

This is known as “degassing”. Degassing will also give the microbes in your medium a better chance of survival. Chlorine will kill a lot of the bacteria that are very important to the health of your medium. 

Adding an air stone to your water to help keep a good supply of oxygen is recommended whenever you let water sit. That air will be dispersed through the water and given to the plants roots. It will also keep friendly bacteria in the water alive.

The amount of water you will need, will depend on your pot size. For a good flush, you will need 3x water, as you have medium in your pots. For example:

4 plants in 10L pots. That’s 30L each, 120L for a full flush.  

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Flushing in Coco
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Flushing in Soil

Once the water has been prepared, and has stood to degas, you are ready to begin the flush. 

Flushing in soil

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When it comes to flushing in soil, drainage can be an issue. It is very easy to over water plants when they are grown in soil. To get the best results, after flushing, you should let your soil dry out for a few days before watering or feeding again. 

Use 3 times as much water, as you have medium in your pots. A ten litre pot will need 30L of water for a good flush.

Start by gently pouring water into the pot, wetting the medium throughly. Leave this to sit for a few minutes, to help any salts dissolve into the water. 

After a few minutes, continue to pour water heavily into the medium. The rush of water will push out the dissolved salts, and leave the medium fresh, with a much lower nutritional content. Remove all of the run off immediately, never let your plants sit in run off. 

Once the nutrients have been removed from the medium, your plants will start to eat the foods that have been stored up inside themselves.

After the flush

Depending on the reason of the flush, you may want to introduce new feed to the medium. Do this by feeding the plants straight after the flush, whilst the soil is still wet. Then let the soil dry out for a few days, so the roots get a chance to breathe. 

If you are flushing because harvest time approaches, it is good practise to let the medium dry out for a few days to let the plants breathe. Give one normal watering when the medium is dry, and then let it dry again.

Once it is dry the second time, repeat the flush again, to ensure all nutrients have been removed from the medium. Leave the medium to dry, until the pots are light, before watering again. You should try and flush at least 10-14 days before harvest if you have grown using salt based nutrients.

Flushing in Coco

Prepare your water, 3x water as you have coco in your pots. 10L pots will need 30L of water. 

Pour the water gently into the pots, dampening the medium. Let this sit for a few minutes, as this will allow the salts to dissolve a little more.

After the medium has sat, pour the rest of your water through the medium, and remove all of the run off. 

This will leave you with fresh clean coco, at a stable pH. 

After the flush in coco

Depending on the reason for your flush, you may want to introduce food again. If this is the case, re feed straight away, with a slightly lower EC than your previous feeds. 

How to feed plants in coco

If you are flushing because harvest time approaches, then it is best to flush a second time, around five days to a week after your first flush. 

Continue to water your plants with pH’d water everyday, with at least 10% run off, which is drained away. Do this for five days to a week, until the second flush.

The second time round there is no need to pH the water, as you would prefer to let the plant use up any stored nutrients inside itself, instead of anything left in the medium.

Thanks for reading! You can find more information on anything related to growing in our cannabis grow guides. All of our guides are written by experienced growers, and members of our cannabis growers forum. If you have any questions, sign up. It is free, and we are always happy to help!