Rinsing Hydro Growing Medium

When you use Rockwool slabs as your substrate, you may encounter some difficulty maintaining consistent TDS and pH levels for your root systems. This is because the Rockwool will hold onto excess waste salts and acids. In this article you will learn why, when and how to rinse your rockwool marijuana growing medium. Same goes for coco, mapito and other hydro growing mediums. If you grow marijuana on soil, read the article How to rinse your soil.

In this article we will discuss:

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Why rinse

Why rinse marijuana hydro

Rinsing the slabs from time to time will ensure that you maintain healthy roots. If the pH becomes too low, the roots can be damaged. Even worse, if the pH in the Rockwool drops down as low as 4.5, the Rockwool itself can be chemically affected. At this level, the Rockwool will release alkaline substances with a pH value somewhere between 7-14. This can be far too strong, and will almost certainly burn the roots of your plant. If the roots are burned, your plant won’t be able to absorb the nutrients it needs, and it won’t be able to grow.

It’s also possible for the pH to be too high in your Rockwool slab. Tap water usually has a pH somewhere between 7-8, depending on your location. In either case, you need to monitor the TDS and pH values of your growth medium. If it rises too high or too low, you need to take action. Makw sure to buy rockwool that is prepared for growing marijuana, like these from Cultilene.

How to rinse

How to rinse marijuana hydro medium

To start, you’ll need just a few simple tools. A meter for pH, a meter for TDS, a syringe, and a measuring cup. Make sure that the syringe and measuring cup are large enough for watering purposes. You can get all of the meters and measuring equipment from a local gardening shop.

The first thing you’ll do is draw up water from various points in the Rockwool and measure the pH and TDS. This isn’t a foolproof method, but it’s a good first step that will often highlight any issues you are having. You should notice right away if any of the measurements are way off. Measure each syringe of nutrient solution separately, then mix it all together to see what the totals are. This will give you a good generalized idea of how nutrients are flowing through the root systems. If you do notice any issues, just adjust the ratio of nutrients you’re adding into the nutrient solution. Remember that your plant needs to be getting plenty of water. This keeps the plant hydrated and also flushes out some of the dissolved solids that can collect in the Rockwool substrate.

A major downside to this technique is how time-consuming it is, especially if you have a large growing operation. Another method is to simply choose a number of Rockwool slabs as your ‘reference’ slabs. Just monitoring these slabs will help you keep track of the general pH and TDS of all your plants, assuming you are feeding them and watering them all equally.

You’ll want to rinse your slabs when the pH has changed by at least 0.5 from the pH of the nutrient solution you’re using. The same deviation holds true for TDS values. Rinse the slabs until the value of the slab is the same as the value of the water you’re rinsing with. This should be somewhere around pH 5.5 and 250 TDS. Use pH up or pH down to adjust pH levels in your water (available here)

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Citric Acid

Citric acid rinse marijuana hydro

Looking at the process of photosynthesis from a chemical perspective, you will see that citric acid is one of the major triggers for photosynthesis. Citric acid is also manufactured as an end product of the process, which makes it cyclical (read wiki in citric acid here). There’s a lot of complex chemistry that goes into all of this, but at the end of the day, it means you can use citric acid in the place of phosphoric acid.

Growing and gardening shops will often stock a special variety of citric acid which some growers like to use as a booster during the flowering stage of cannabis growth. You can use this to help increase the plant’s absorption of nutrients right before you harvest the buds. Before harvest, you’ll want to stop adding nutrients to the Rockwool, but you want to make sure you keep watering the plant.

Right before you rinse, you’ll want to stop adding nutrients to the slabs: that way your plant will absorb the last of the nutrients that are being stored in the Rockwool. As long as you keep the pH of the water between 5.5 and 5.9, the plant will use up the extra reserves before being harvested. This will optimize the final potency of your yield and offer you the highest quality harvest.

Thanks for reading. Please leave comments or questions below and don’t forget to download my free grow bible.


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Robert Bergman


Robert Bergman is an Amsterdam-based marijuana grow expert who has years of experience from small grows to massive operations ... See profile

12 comments on “Rinsing Hydro Growing Medium”

  1. Speaking of Citric Acid, according to my college book “Ed Rosenthal’s Marijuana Grower’s Handbook” p-149, citric acid can also be used at the ratio of 1-gram to 75-gallons of water to remove Chloramine from the water as well. So it would seem that using citric acid is just a win-win situation all around.

  2. certainly like your web-site but you have to take a look at the spelling on seaervl of your posts. A number of them are rife with spelling issues and I in finding it very bothersome to inform the truth then again I will surely come back again.VA:F [1.9.17_1161]please wait…

  3. […] pH too low. If the pH level is 5 or beneath that, then it is clear that certain elements, especially aluminum are shifting to the point of where they are becoming toxic to your plant. You can stop this from happening by making sure the nutrient liquids stay at a pH of 5.5 or higher. Always take a measurement of the pH of your nutrient water and make the necessary changes prior to feeding it to your plant. One way to raise the pH is by mixing caustic potash into the water, which you can find at a local garden center. More info in the article How to rinse your hydro growing medium […]

  4. You download free guides, by clicking on the link. Provide your email, and we send you a link. A PDF, I believe. Happy Growing! 🙂
    Join our support forum for faster and more detailed grow info! 🙂

  5. Could someone tell me how to download the plant care guide. What is the pound density of the rockwool. Is the brand name “roxul”. Love your store and all your info.

  6. Love it . lol I’m tring to read all your blogs. I’m a new grower and I’m trying my best to grow the best. I’m now starting to understand. I’m now building a green house, a bigger one. One thing I was wondering, I know that the sun has all the lite colours. Could I help mother nature by covering my green house with coloured poly.