Using Rockwool — What You Should or Shouldn’t Do

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Added 23 January 2022

Rockwool, also known as stone wool, has been a popular growing medium for over four decades and is the most commonly used medium for commercial hydroponic cannabis farming. It is super popular, thanks to its inert properties and water retention capacity.

It is manufactured by melting basalt rocks and spinning the lava — just like cotton candy. This unique process and its abilities give it an edge over other growing mediums. And it's been proven to give you terrific growth results.

However, with anything so good, there are a few things that you must keep in mind. In this article, we explore these details to show you how to use rockwool correctly to grow cannabis. 

Can You Use Rockwool to Grow Cannabis?


Many growers swear that Rockwool is the best medium to grow cannabis because of its ability to retain water, nutrients, and keep the roots as moist as possible.

Indeed, Rockwool offers numerous benefits to help you grow your favorite plant. Primarily, Rockwool prevents harmful pathogens from spreading, facilitates better drainage, and increases seed germination chances.

Benefits of Using Rockwool


Overwatering is a common problem most beginners face. You love your plant so much that you tend to pamper it with lots of water — a situation that's often deadly for the plant. Rockwool is an excellent medium for you if you're constantly killing plants.

First, Rockwool holds just enough water required to make the roots moist and drains the rest quickly. It is similar to fabric pots that hold adequate water while draining the rest immediately.

That said, it is still possible to kill your plants if you saturate the Rockwool with too much water, so irrigate just enough to dampen the cubes rather than soaking it thoroughly.

Germ-free, inorganic medium

An inorganic medium like Rockwool prevents harmful microbes from affecting your plants. In addition, since it is manufactured after being exposed to extreme temperatures, it is sterile and prevents germs from spreading to the plant's roots.

Rockwool can be composted despite being inorganic. It will take a lot of time, but it also adds extra nutrition to the composted soil. To compost Rockwool, you'll have to shred the cubes into many smaller bits, which can take an inordinately long time to decompose.

Perfect for beginners

Beginners often struggle to germinate cannabis seeds due to various reasons. Rockwool makes this process easy by providing three primary elements required for seed germination — moisture, heat, and darkness. Typically, beginners saturate seeds with so much water that they either fail to sprout or die due to dampening. However, it is tough to go wrong for even beginners with Rockwool.

In addition, beginners find it challenging to take care of their seedlings because they are fragile. Seedlings struggle to establish themselves in hard soils that offer no aeration. On the other hand, Rockwool encourages seedlings to establish their roots, thanks to large gaps or pockets within the cubes.

Last but not least, beginners struggle to transplant seedlings from one pot to another and often end up killing them. Typically, the roots are damaged when the grower digs the seedlings out of the soil. However, if you use Rockwool, you can move the seedling directly along with the cube into the soil, so there is no chance of damaging the roots. In addition, Rockwool cubes will not interfere with the growth of the roots even as the plant grows bigger.

Disadvantages of using Rockwool

Like everything else, there are a few disadvantages of using Rockwool. 


Rockwool cubes are alkaline, which means the pH is above 7. Since cannabis plants love acidic soils (below pH 7), you will have to reduce the pH of the Rockwool cubes before using them. To do so, you will have to soak the cubes for at least 24 hours in water containing low pH using pH adjusters. Although the process is relatively easy, it is an extra step compared to growing plants in soil.


Rockwool is made up of extremely tiny fibers that suspend in the air easily. Over time, it can also end up inside your lungs, making it a dangerous medium to use. To prevent this, you can use a mask while dealing with the cubes.

What You Should Do While Using Rockwool

Seedling in Rockwool

When using Rockwool, there are a few things that you must do the right way. If you don’t, your plant’s growth may be affected. Here are those things.

  • Always Pre-soak Your Rockwool

Unlike other growing mediums, Rockwool needs to be pre-soaked before you use it to grow cannabis because it gets shipped with 0% water — it’s completely dry. 

Initial soaking is crucial because it allows the water to penetrate the tiny pores and prepare the cubes for roots. We recommend immersing your brand new Rockwool cubes in a nutrient solution for some time until all the rising bubbles disappear. 

The nutrient solution’s pH should be at 5.5 for the best results. You only need to soak smaller cubes and seed plugs for a few minutes, whereas larger cubes may take longer. 

After that, allow the water to drain completely from the cubes. Doing so would allow sufficient air to enter the larger pores, too, achieving a perfect balance of water and air.

  • Fertilize the Rockwool Cubes Correctly

Rockwool cubes do not lock out any fertilizers; i.e., all the nutrients within the block are available to the cannabis plant at all times. So, if you tune your nutrient solution, you will notice its immediate effects on the plants. 

And if you only use water devoid of nutrients to fertilize the cubes, it will decrease the fertilizer concentration between watering. Additionally, it may cause nutrient-related problems or a slight shock to cannabis. 

Rockwool has terrific water retention capabilities, so it is always recommended to irrigate less often but with a well-balanced nutrient mix instead of plain water. 

  • Always Allow Water to Run-Off

The cannabis plant is thirsty, so it drinks water faster than nutrients. Therefore, the nutrient balance within the Rockwool cubes may get disrupted, causing nutrient problems and pH issues over time. 

So, we recommend giving your plant enough water to flush excess nutrients and refill the cubes with fresh nutrients. The new water will push out the old water and leftover nutrients.

For best results, allow 15% to 30% water run-off to maintain optimal nutrient conditions in the root zone.

  • Recycle or Reuse Old Rockwool

Many growers assume that they can’t reuse Rockwool because it’s toxic. However, you can recycle it and reuse it many times before it’s completely spoiled. You can also reuse it at the end of the plant’s life cycle to grow different plants or shred it and add the remains to your compost. 

However, avoid reusing Rockwool to start new plants and treat it with enzymes before reusing it to purify any rotten root materials.

  • Handle It Carefully

Rockwool can irritate your eyes, skin, and lungs, and too much exposure to it can cause various skin and lung problems. This is because of its physical makeup and not the chemicals involved. 

Plus, the Environmental Protection Agency classifies Rockwool as a Group 2B material — it may be carcinogenic to humans. Therefore, always use dust masks, goggles, and gloves when handling Rockwool. 

Additionally, Rockwool is not natural or biodegradable unless you shred it into tiny bits, as mentioned before. So try reusing it as much as you can. 

What You Shouldn’t Do While Using Rockwool

Rockwool cubes

On the flip side, there are a few things that you must avoid doing for the best growth results of your plant. Here they are.

  • Don’t Overwater Rockwool Plants

When you water your plant, the runoff should never exceed 30%. Overwatering is bad for the plant, and it can cause algae growth on the surface of Rockwool. 

If you are worried your plant may experience drought, don’t worry — the cubes can hold enough water for your plant for a long time before causing water deficiencies. 

  • Never Squeeze Rockwool

Many growers suggest squeezing Rockwool cubes to remove excess water, but did you know that it can damage the cubes?

The fibers within the cubes are beneficial for water and air retention. However, the air pockets may never be restored when you crush the fibers. This would lead to no aeration within the cubes, turning them into a soggy mess.

  • Don’t Over Stack Rockwool

You need to understand your Rockwool cubes’ structure and porosity to determine how tall the container can be. The height must be enough to accommodate the growing roots, but not so tall that water can’t travel through it efficiently.

For instance, if you are feeding water from the bottom of the cubes, it can only reach a certain height before gravity overpowers it. Thus, only the lower portions of the root zone would be hydrated, and the higher zones would be dry.

If you must stack Rockwool cubes, we recommend feeding the water from the top and aiming for 30% runoff from the bottom.

  • Don’t Obsess Over Rockwool

Cannabis growers have an emotional attachment to the plant and want to give it the best environment to grow (we understand that!), but you shouldn’t obsess over it.

Rockwool is popular partly because it is so easy to work with. In addition, it has tremendous water retention capacity and holds fertilizer long enough for the plant to absorb it. As a result, it can go a long time without much attention.

Summary: Using Rockwool — What You Should or Shouldn’t Do

Rockwool is the go-to choice for thousands of cannabis growers, including commercial ones. Still, many misconceptions surround the use of it. You shouldn’t fall for that. Instead, use this guide on using Rockwool the right way to grow fantastic cannabis buds. 


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