Is Rockwool Safe or Harmful?


Last Updated on October 31, 2021 by Phil

Rockwool is one of the most popular hydroponic growing media used for growing crops today. That’s because it offers many different benefits that make it easier and more convenient to grow plants in.

Additionally, there are so many varieties which you can choose from for the kind of application you’re doing. Rockwool also comes in many shapes and sizes.

Thus, it is heavily used for growing hydroponic vegetables, fruits and herbs.

Unfortunately, one of the lesser discussed things about rockwool is the health and environmental problems it brings.

That’s what I’ll go through in detail below. This way, you know what you’re getting into when using this substrate.

And, if you do decide to use it, know how to use it properly as well as protect yourself and your loved ones from potential long term health issues.

What is Rockwool?

Rockwool is also called stonewool. It is made from processing molten basaltic rock and spinning them into fine fibers to form cubes and slabs which are used for insulation as well as in hydroponics.

It is a very popular substrate used in hydroponics, especially in commercial settings, thanks to its lightweight nature and unique structure. The latter allows rockwool it to retain water while still keeping enough air space (18% or more space for oxygen). This balanced combination of air and water allows the roots of plants to grow optimally.

As such, many growers use it for all sorts of crops including fruits and vegetables including strawberries, tomatoes. Rockwool is also often used to grow herbs and even cut flowers.

Additionally, rockwool has the ability to drain moisture in a slow, steady manner. This allows plants and crops to absorb enough water without getting too much of it.

This characteristic also makes it easy to manipulate the grown environment of plants without causing sudden or significant changes in pH and EC.


Why Use Rockwool?


  • Rockwool is a popular inorganic hydroponic substrate because its advantages outweigh its disadvantages.
  • To give you an idea…

Unfortunately, the use of rockwool has its disadvantages as well, which I’ll go into more detail below. Yet despite these drawbacks, it is still widely used as an inorganic hydroponic substrate because many growers believe that the benefits outweigh the risks.

Here are some of the biggest advantages of rockwool as a hydroponic substrate.

  • Lightweight
  • Able to retain moisture well
  • Good aeration
  • Slow steady drainage
  • Easy and convenient to use
  • It is naturally sterile (no susceptibility to disease and pathogens)
  • It does not breakdown over time (so you can keep reusing it to save money)
  • Manufacturers can alter the properties of rockwool providing many different varieties for growers to choose from. There are those for drier root zones or the opposite depending on what application you’re doing.

The first four characteristics make it great for growing plants. The final three properties make is very useful and convenient not to mention economical for growers.


Is Rockwool Biodegradable?

Now that you know why rockwool has overtaken sand and gravel as the most popular inorganic hydroponic substrate, it’s time to take a look at why rockwool is not totally safe and can be harmful.

The biggest issue with rockwool is environmentally.

Since rockwool is made from natural substances from volcanoes, it should be natural, right?

Unfortunately, not.

The problem here lies in how the rock is manufactured and processed. This is done by heating volcanic rock along with chalk to 3,000 to 3,500 degrees Fahrenheit. The result then goes through a spinning process before it is allowed to cooled down.

The final output is what you buy from the garden center or hydroponic store.

So, while the original rock material is natural, the manufacturing process turns it into something non-natural. As a result, it is not biodegradable.

This is why rockwool does not degrade (allowing you to reuse it over and over again).

Sadly, if you throw it away, it will end up in a landfill and never decompose, leaving all of us with environmental issues down the road.




Is Rockwool Reusable?

Rockwool for Hydroponics

Because rockwool is not environmentally sustainable, it is a good idea to keep reusing rockwool over and over.

The worst thing you can do for the environment is to throw the used ones away and but new rockwool cubes or slabs every growing season.

That said, you don’t want to just store it away then reuse next season. Instead you need to sanitize the rockwool before each use.

You can do so by treating it with boiling water, baking at 180 degrees Fahrenheit or using steam. I’ve also seen some small growers just place them in the microwave, which seems to work well too. Doing so ensures that any existing pathogens or disease that may have been left is cleaned out before replanting.

There are also some chemical disinfectants for this purpose. But, I prefer staying away from them. If you do use these produces make sure to rinse the chemicals off completely before replanting.


How to Reuse Rockwool Properly

To make sure that you get the most out of your crop when replanting rockwool, here are a few tips on how to reuse rockwool properly.

  • Switch up the plants each time – Avoid growing the same crop when reusing rockwool as it seems to result in lower quality crop. Instead, changing the type of plant your grow each time will let you start fresh.
  • Remove any roots and dried materials leftovers – Before you reuse it, make sure to clean the rockwool by removing any material that’s left on it. This can include roots, and other leftovers that get stuck. Make sure to use gloves so avoid skin irritation.
  • Always sanitize rockwool before replanting – in addition to cleaning, disinfecting and sanitizing the substrate to get rid of potential pathogens or diseases is important. Always, start over with sterile rockwool. You can steam it, heat it, or use hydrogen peroxide.
  • Use protective gear when handling rockwool – as you’ll see below, rockwool is harmful to health. So, always use gloves, safety goggles and a face mask when working with this substrate.
  • Keep the wrapping – rockwool comes with plastic wrapping foil. You want keep using this even when replanting. The foil keeps light from getting in.
  • Stabilize the pH before each use – rockwool has high pH. This pH is too high to allow plants to absorb nutrients. Thus, it negatively affects plant growth. So, you want to lower the pH to around 5.5 or so before growing. Also, pH will fluctuate along the way. So be ready with some lime or sulfur solution to adjust the pH when this happens.


Is Rockwool Harmful/Dangerous to Health?

Rockwool in Hydroponics

The other concerning thing about rockwool is it poses health hazards as well.

It is listed by the EPA (Environmental Protective Agency as “possibly carcinogenic to humans.”

In addition to being un-environmentally-friendly, it can affect your eyes, lungs and skin as well.

Remember the fibers that were created from the processing earlier? These can get loose and enter your eyes, nose and mouth. Also, the blocks can accumulate quite a bit of dust.

This makes them work similarly like asbestos where the little particles can eventually get into your body can cause harm in the long term.

That said, it is nowhere near as damaging as asbestos. And, the long term negative effects only happen if you use a lot of it.

Nevertheless, it is not something you want to risk your health for, especially if there are other growing media alternatives. Like those below.

The other option is to make sure you’re properly protected when using rockwool. This means using gloves, protective goggles and a face mask or shield. Doing so will prevent any tiny particles from entering your orifices.

Also, don’t forget to properly wash the clothes you use directly after handling. Dust and particles can stick to clothing which makes you a “carrier”.

For example, if you play with your kids after handling rockwool, particles that get into your clothing can end up being inhaled by your children.

Needly to say that storage is also very important since kids and animals can easily get hold of rockwool if left where they can reach it.


Is Rockwool Safe to Breathe?

From the previous section, you already know that rockwool is not safe to breathe.

Rockwool are classified as MMVFs or Man-Made Vitreous Fibers. MMVFs cover a broad group of inorganic fibers created from manufacturing different materials including natural rock and glass.

More importantly, several studies associate MMVF with the incidence of respiratory diseases like chronic bronchitis, asthma, COPD, and emphysema.

As such, high exposure to rockwool dust puts you at risk of chronic respiratory problems. However, it is worth noting that the smaller fibers can dissolve with your body’s fluids. This makes them harmless as they’re destroyed before they negatively affect the body.

Unfortunately, there’s really no way of telling how small the particles are since there’s no way to test that unless you put peoples’ lives at risk during the testing.


It Has a Naturally High pH

Above, I mentioned that you need to stabilize your rockwool before using it. That’s because it has a naturally high pH of about 7 or higher, which is way above those of other growing media.

As such, if you just directly use it out of the package, you’ll notice growing problems happen with your seeds and seedlings.

That’s because the high alkaline nature will affect plant growth as well as how they’re able to absorb nutrients.

Thus, it is very important to balance out the pH levels to make it ideal for the plants’ root zone. You can easily do so by soaking the rockwool in acidic water for about an hour or so to get the pH down.

There are pre-made solutions for this, and you can use sulfur solution as well. Although, one of the easiest ways is to add a few drops of lemon juice to water and test the solution with pH strips. Shoot for pH of between 5.5 to 6.5 for the solution.

As you would expect, this takes time and effort.

The other issue with rockwool’s pH is its instability. That is, it likes to fluctuate even after you’ve balanced out the pH. So, regular monitoring is key when using rockwool as a growing medium.

If you notice any changes, then you’ll need to rebalance the solution to keep the crops happy.

Otherwise, left alone, you may experience slow, stunted growth or no growth since the nutrient absorption is being blocked by the changing pH levels.


Other Hydroponic Media Alternatives

So what should you use if you rockwool is not safe?

Here’s a very comprehensive hydroponic growing media guide I’ve put together that lists down, explains and lists the pros and cons of each of the hydroponic media available.

Some of the popular hydroponic growing media included are coco coir, expanded clay pellets, oasis cubes, perlite, pumice and many more.

Check if out to find the perfect hydroponic medium for you that’s not only economical but also easy to use, low maintenance, safe for your health and environmentally friendly.

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