Is Rockwool Harmful? Yes, But…


Rockwool has long been a popular media for growing hydroponic fruits, vegetables and herbs. However, I’m going to make the case against rockwool and argue why you should never use rockwool again because rockwool is harmful.

This post has gotten a lot of attention recently, and as a result is in the process of being updated to include more information. I cite studies and in no way reference any particular company – I am talking about mineral wool as a growing media in this post.

If you want better alternatives to rockwool, please check out my hydroponic media guide.

It’s Not Environmentally Friendly

I believe in environmental sustainability – it’s one of the reasons I’ve chosen to grow hydroponically. Rockwool doesn’t score well on the environmental scale. It’s not a natural material. Manufacturers use combine chalk and rock and then heat them up to around 3,000 degrees Fahrenheit. Next a stream of air is blown through it, resulting in extremely thin fibers of the rocky material. As the strings are blown out, they bunch together and form the material that you see for sale at the local hydroponics store.

Basically, they are taking two materials that are 100% natural (chalk and rock) and turning them into a hybrid material that will remain in that form forever. When you throw away your old rockwool it’s going to sit in a landfill looking just like that for a long, LONG time. If you absolutely insist on using it, try to save your rockwool in between your growing season and reuse it.

It’s Not Healthy To Be Around

Not only is rockwool unfriendly to the environment – it’s also potentially harmful to your health. New blocks can contain a lot of dust and loose fibers that can get in your eyes, mouth, skin and lungs. It’s similar to asbestos in the sense that the little fibers can lodge themselves in your lungs if you’re working with it a lot. It may not be as toxic as asbestos, but why take the risk? Not something that I’m willing to gamble with if I don’t have to – there are plenty of other hydroponic media choices! If you’re using rockwool, you should be using a mask, goggles and gloves when you work with it to protect yourself.

Here is what a 2002 study on man-made mineral fibres found:

The International Agency for Research on Cancer (IARC) has reviewed the carcinogenicity of man-made mineral fibres in October 2002. The IARC Monograph’s working group concluded only the more biopersistent materials remain classified by IARC as “possibly carcinogenic to humans” (Group 2B). These include refractory ceramic fibres, which are used industrially as insulation in high-temperature environments such as blast furnaces, and certain special-purpose glass wools not used as insulating materials.

The important part:

In contrast, the more commonly used vitreous fibre wools, including insulation glass wool, stone wool and slag wool, are considered “not classifiable as to carcinogenicity in humans” (Group 3). – Wikipedia article on mineral wool safety

A note on the IARC study:

Some manufacturers of insulation products have cited this volume while making erroneous claims that “IARC scientists confirm safety of mineral wool insulation”. These claims are just false. The findings in this volume are not a determination of non-carcinogenicity or overall safety.

What this means is that the study was not able to determine if it these mineral wools caused cancer or not. Again, my original point: why bother with it when there are better options?

If you’d like to read the original study, click here.

More Resources on Mineral Wool Safety

It Has a Naturally High pH

If you use rockwool right out of the package, you will likely have a problem once you plant seeds or seedlings in the material. It’s pH is much higher than other media, so it requires treating before it can be safely used with plants. Not only is this annoying and a hassle to deal with, it just slows down your entire process and puts a barrier in front of your growing efforts. Even when you get the pH correct, it can fluctuate much more than other types of growing media. You’ll have to watch the pH levels of your rockwool like a hawk to make sure there’s no nutrient blockage to your roots. The last thing you want is for your media to actually slow down the growth of your plant instead of speed it up.

If you’ve been convinced to try something else out, you might be wondering what options are out there. I’ve put together a hydroponic growing media guide that breaks down the advantages and disadvantages of some of the most popular growing media, but if you need some quick guidelines, here they are:

  • Ability to reuse the material helps the environment, and saves you money
  • No potentially negative health effects
  • Easy to use, little maintenance required

​What do you think? Do you use rockwool?

Header image courtesy of ilovebutter

I’m the founder of Epic Gardening, a website dedicated to teaching 10,000,000 people how to grow plants. I enjoy skateboarding, piano, guitar, business, and experimenting with all kinds of gardening techniques!

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54 thoughts on “Is Rockwool Harmful? Yes, But…

  1. Just because “man is natural”, doesn’t make the things that man makes, natural. I have been toying with the idea of using J*ff* pellets (compressed coco coir things that expand in water) instead of rock wool. Renewable, and I have re-used them when seeds don’t germinate. Worried about algae and fungus? soak the used pellet in vinegar for a bit, then rinse with clean water. I think there is a smaller sized net pot they would fit into perfectly.

  2. I am not an expert on using rock wool for planting or as a planting medium. However, I can tell you that Holland has become one of the largest tomato producers by using this very product. I believe it originally comes from Denmark and is a mix of basalt rock and chalk. You can go to the active lava fields in Hawaii and see the same product, albeit not cmpressed, floating around in the air. I know it is used by the people of Holland, and the tomatoes consumed by them, so it obviously hasn’t had any ill effects thus far.

    Although rockwool was originally made for building insulation, its properties are quite different. You couldn’t grow a plant in rockwool insulation as it doesn’t hold water. Think about it: if rockwool insulation were the same as rockwool planting medium, people would be screaming for a refund on their insulation jobs and hme builders would be sued by all. Not only would it hold water from any leaks or from a lightly sealed home in a humid climate, but it would grow mould, maybe algae, cause the wood trusses and attic walls to warp, among other things. It would also be the bane of homeowners who love ivy growing on their exterior walls. It would happily find the nearest tiny cracks, worm its way inside and happily grow to fill the attic. It would also be attractive to any critters that eat algae, or the plants that might take root in there. Not sure if rats eat plants, but I wouldn’t want to find out by experience!

    It sounds to me like one should use gloves safety goggles and a mask over one’s mouth and nose due to its fibrous dust. Just like you would were you using bonemeal or diatomaceous earth.

    I cannot tell you how many times I have seen friends or family members using diatomaceous earth without anything covering their mouth, let alone their hands or eyes. I’ve even seen many of them use it inside their homes to read them selves of ants or other crawling insects. That is a horrible thing to do. Every time your air conditioner or heater kicks in, every time you are a pet walks by the area quickly you are kicking it up into the air.

    I think if one uses common sense when dealing with a natural product or combination of natural products that have not been shown to be cancer causing, or bad for the environment, we should be fine. I agree with those who say having Rockwall, that looks like an earth product or like dirt, in a landfill, is far better than having piles of plastics .

    If you do use it, it is recommended you recycle it by either reusing the block or similar type rockwool or, reshred it and use it in your compost pile.

    • I definitely wouldn’t expect them to handle it well, although I’m not knowledgeable enough to say it’s super harmful. It’s inert, but their digestive tracts may not handle the fibers well.

  3. Life is harmful. It’s 100% fatal and sexually transmitted. All things are natural, even things that are made by humans, because humans are natural.

  4. “Basically, they are taking two materials that are 100% natural (chalk and rock) and turning them into a hybrid material that will remain in that form forever.” looks like someone skipped chemistry

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