Jump to content
Strain Hunters Forum
Guest Guest guest user
Existing user? Sign In  

Sign In

Or sign in with one of these services

Sign Up
Search In
  • More options...
Find results that contain...
Find results in...

Franco's tricks on flushing the medium


This is the fourth article in a series where I present some selected tricks and advanced growing techniques. My name is Franco, and i work at Green House Seed Company, in Amsterdam, since the year 2000. Over the last 10 years I have learned from Arjan countless tricks and advanced techniques, and I developed a few of my own as well. And now it’s time to share some of this knowledge with growers worldwide. If we share the knowledge, everybody wins.

Last month I explained some of the best tricks for vegetative growth. This month, I give you...


Most growers use synthetic feeding to grow cannabis. This is mostly because synthetic feedings are ready-to-absorb, work fast, and are easy-to-dose. Nevertheless, compared to organics, there is one single main disadvantage: salt buildup in the medium. As a consequence, flushing the medium where cannabis grows should be seen as a natural, logical, unavoidable step to maintain a healthy, productive crop. A clean growroom, and clean tools, are just the beginning. It is also important to keep clean the environment where the roots grow, the medium. Roots develop in a dark, wet environment and are prone to absorb fertilizer if certain conditions are met. It does not matter what kind of medium is in use, synthetic fertilizer will cause salts buildup and intoxication of the plants, if proper flushing is not applied. During the crop, fertilizer gets absorbed by the plants in different quantities and concentrations, depending on many factors (temperature, pH, metabolism, mineral composition of the fertilizer). The plants intake feedings, and the leftovers sit in the medium and crystallize into small rocks of salts. This causes an increase of the medium pH and EC, and intoxication of the plants.

Flushing the medium is a tailored procedure, one that varies according to the type of medium.

In earth, salts build up at a slower pace than in hydroponics, but it is also more difficult to wash them away. Once the fertilization program is under way, it will take 3 to 4 weeks for salts to start building up, and another week before it can do any harm to the plants. Therefore, it is advisable to start flushing plants that are growing in earth around week 5-6 of the cycle.

When flushing, it is important to avoid over-watering; this means that the flushing should be integrated in the watering cycles. Before the flush the medium should be fairly dry, and after flushing it is very important to let the medium become dry and light before feeding or watering. The dry-wet-dry-wet cycle in the earth (sponge-effect) is what maintain the plant metabolism at optimal levels. For plants growing in soil, the flush is very important at the end of the crop, during the last 2 weeks before harvest. If properly flushed, plants will produce tastier buds, and the combustion factor of the dried material will improve. If plants are not flushed properly, combustion is slower and the flavor poor, and artificial.

When flushing the medium, it is very important to be organized to avoid water spills in the growroom. The purpose of flushing is to wash the medium, and as a consequence a lot of water flows through the containers down to the ground, and it is very important to drain away excess water from the growroom, so the climate stays good (too much water in the room will cause air humidity to climb fast, creating all sorts of issues). The easiest way of flushing is when working with elevated tables, and proper drainage pipes. Another way of effectively control flushing

In hydroponic mediums (from rock-wool to pebbles to coco fiber) the flushing process is easier to implement, because there is a higher degree of drainage. The texture of hydroponic medium favors flushing, and it is also more necessary than in earth.

When flushing hydroponic medium, it is very important to be exact with the values of the flushing solution. EC, temperature and quantity are very important factors. The EC of the flushing solution must be high enough for the salts to bind with the solution, but not as high as to form more salts. The ideal values are between 0.9 and 1.1.

The temperature must be in the 20-24 degrees Celsius range, to allow salt crystals to dissolve and unbind from the medium. If the temperature of the flushing solution is lower than 20 degrees the salt crystals will not dissolve, and if it is higher than 24 degrees the tiny root-hairs will be damaged.

And finally, the quantity of solution going through the medium should be at least double in volume (for example: 40 liters of flushing solution for a 20 liters container). In my personal experience, flushing with up to 3 times the volume is even more effective, as long as the drainage is quick.

Flushing is best done at the beginning of the light-cycle, so to favor evaporation of excess water and to let the medium begin the drying process faster after flushing.

Once plants are flushed, they tend to get lazy, and slow down growth for a day or two, but this is all part of the game. As soon as the medium dries again, the growth (or ripening) resumes fast, and even increases.

To double check the effectiveness of flushing, it is easy to measure the values inside the medium before and after flushing. First, pour some flushing solution through the medium. Measure the values inside the first liquid draining from the bottom, then apply the complete volume of flushing solution. At the end of drainage, measure again. At this point, the pH and EC readings of the flushing solution should be similar to what is draining out of the container.

In conclusion, flushing should be part of any synthetic feeding schedule, well integrated in the process. Water tanks capacity, and drainage, should be priority considerations when designing or building a good growroom. It can only improve the results of the grow operation.

Franco – Green House Seed Co.

This content is copyright of Green House Seed Co. © Green House Seed Co. All rights reserved. Any redistribution or reproduction of part or all of the contents in any form is prohibited other than the following:You may not, except with our express written permission, distribute or commercially exploit the content. Nor may you transmit it or store it in any other website or other form of electronic retrieval system.

  • Like 3
  Report Article

User Feedback

Since I have been growing , only a year now, I watched a few of Arjan and Francos flowering videos and that is when I first heard of flushing aND i HAVE DONE IT TO MY PLANTS EVER SINCE AND WITH GOOD RESULTS. PEOPLE ALWAYS SAY TO ME THAT MINE TASTES GOOD AND CLEAN which is great as it is important. I`ve got 2 that are ready in just over a weeks time and will flush them now for last time with fresh water only. Works for me.

Share this comment

Link to comment
Share on other sites

Yes , flushing is a must, when my stash is low and i need to buy weed from a dealer, 9 times out of 10 it hasn't been flushed and when you are used to smokin clean weed you can tell the difference straight away. Not Nice ! :-( Peace Y'all _\|/_ Mello Jamie

Share this comment

Link to comment
Share on other sites

what about with a recirculating dwc? Is plain water enough or should I run a flushing solution for a bit before i run straight water?

Share this comment

Link to comment
Share on other sites

Franco you say that the water for the flushing solution needs to be between 20-24 degrees Celsius range and you also say that the EC should be between n 0.9 and 1.1 but you dont mention what you use to create that solution with an EC of .9 to 1.1. I have well water that has 82 ppm and a PH of 9.3(i always ajust me ph to the levels you recommend) what types of minirals should i add to create the proper flusing solution?

Share this comment

Link to comment
Share on other sites

Very simple,

get a bottle of PH-DOWN BLOOM and a bottle of PH-DOWN GROW and one bottle of PH-UP for BLOOM/GROW.

That is all you need along with your standard fertilizer, preferably PowderFeed.


Mix PF into your water bucket until EC level 0.9-1.1 is achieved, THEN adjust PH level as PF lowers the EC you should use it first.

Once you are at the desired values your monthly water games may start ;)

From experience and lazyness:

I tend to carry the ladies to the tub and just use plain water ( EC 0.2 PH 7.8 ) from the tab.

Yes, it takes some 10-20L UNTIL you can read an increase on EC on your EC Meter, the first 10-20L will most likely run through without taking much salt with them. Take your time, let it SOAK for 10min when the first 10-20L have been through and then go for another round UNTIL your EC meter reads top and bottom the same.

Readings where 0.5 EC till about 20L when it fastly jumped to 3.1 and took A LONG time to go down to 0.5. That was a 16L root pouch and "maybe" 50-60L of good tab water.

I drink this tab water and it tastes very good, that is a good good sign !


Share this comment

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Add a comment...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.