Grodan or rockwool challenges

Grow Guides
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For people that have used Grodan products or other types of rock/mineral wool, what challenges have you experienced while using it?

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Just did my first round with it. Check out my log!

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Welcome @ganjagirl! I love rockwool and it’s an amazing medium. But I’ve always had bad luck starting seeds in it.

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@Tygrow78 Thanks for your response! Do you mind me asking what you love so much about rockwool? I’m hearing it’s really great for a more controlled environment and it’s very clean. When you are using it to for seeds, is the problem they just don’t grow or is it more to do with the rockwool breaking down in some way? Appreciate your insights :slight_smile:

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Rockwool is great at holding moisture and my roots have always loved it @ganjagirl. I grow dwc and rockwool is very clean and easy to move around before they get in my lids for the rest of their lives. The moisture retention can be tricky with rockwool so I prefer starting seeds so with root riot plugs because they breath so much better. It’s really hard to over water those plugs. I have 4"×4"×4" rockwool cubes I’m going to Veg some Reg seeds in till I know there sex. I’ll be using rockwool with every grow as long as I’m grow hydro.

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@Tygrow78 Got it - really appreciate you further clarifying! One additional question (I posted this elsewhere in the forums but I’m really interested since you are a hydro grower) - what % of your consumables costs is rockwool annually? So when you look at lighting, water, nutrients (or anything else) is rockwool say between 5 to 10% of your costs? Less or more?

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I grow exclusively in rockwool. Are your questions targeted more at the hobby or commercial scale?

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@devjyarn thanks for responding! I’m actually interested in learning more about both the hobby and commercial scale so would appreciate whatever perspective you bring!

I’ve had a bit more difficulty learning about the commercial side so would love to hear any insights you have on that side if it’s relevant.

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Absolutely! Can you give me a few specific questions that you’re most curious about right now?

In commercial settings rockwool will be one of the most expensive mediums when compared against coco or other soiless options, but it has many benefits that also need to be considered such as the fact it is inert and carries no unwanted pests, and it occupies a lot less space and saves labor (no need for potting up.)

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I would say right around 5% of my costs because I use smaller blocks, and only a few at a time. I should have tagged @devjyarn in my first response. He’s the one with the knowledge that can really answer your specific questions @ganjagirl.

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@devjyarn thanks for your willingness to help! I’m a student and conducting a final project on the industry so am grateful for any help getting up to speed on this fascinating industry!

  1. Do most commercial growers use rockwool? In your mind, what are characteristics of a grower that might lead to them using rockwool?
  2. What percentage of consumables costs go towards rockwool purchases? I actually asked this elsewhere but I’m interested in when you are looking at lighting, nutrients, etc. what percentage of that is going towards growing media?
  3. Are most folks using Grodan? Any other brands people are trying?
  4. Any challenges with using rockwool? I’m providing recommendations on how to improve rockwool products and I’ve heard a bit about disintegration, growing mold, and the sustainability issue but would be curious to hear your thoughts.

Thank you SO much!

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I am glad to help, @ganjagirl. I enjoy the technical side of things!

  1. Given the varied scale of operations and the fact that currently there are legacy growers who are moving into commercial and commercial horticulturalists that are moving into the cannabis industry, I would say that rockwool is not the most commonly used medium. Scale plays a big role in this. There comes a point where a capital savings makes more sense than a labor saving, so things like coco, peat, perlite and other soiless mediums look more attractive. Other operations make decide that inceasing spending on their growing medium might save costs elsewhere (warehousing, long term labor costs associated with transplanting, the threat of pests or diseases coming in to the facility with the medium, etc.). The characteristics of a rockwool grower are someone who wants consistency, efficiency, a medium that is easy to work with (no mess, very lightweight), it is standard/uniform right from the moment it is received (no pots, so bags that are more wet or dry than others). There is also a large body of research around rockwool as a commercial medium, which allows cannabis to tag on to others expertise.

  2. The answer to this question depends on operation specific preferences, but I’ll try to generalize. To bring a plant from clone all the way through to harvest costs about $20. This is all inputs aside from labor, and assuming a 16 to 18 week lifespan. If purchasing in small volumes rockwool could account for a quarter of the non-labor costs. This number drops dramatically as the scale of the operation increases, but will never be as low as other mediums (for example, some operations reuse their perlite medium, sterilizing it between grows, and can nearly eliminate media costs from their expenses at the increase of labor and machine investment costs).

  3. Grodan is working to make themselves the most recognized name in the game, but there are other rockwool options that are marketed more specifically at large scale hydroponic greenhouse crops. Grodan is doing a good job at targeting the cannabis market with fiber structures targeted to retain and drain the right amounts of water for the crop.

  4. Rockwool holds no nutrients and has no buffering capacity, so the plants grown in it will only grow as well as the person growing it will allow. It allows total control over plant growth and inputs, but it also requires total commitment or the plants will not reach their potential. I have not had any issue with disintegration or mold, though algae regularly grows on the surfaces if left exposed to the light. I would need a bit more information about your stance on its “sustainability issue” in order to comment on it correctly. It is compostable and adds good aeration to soil, in my experience.

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@devjyarn thank you so much for these thorough answers - so incredibly helpful! A few follow-up comments and questions:

  1. From what I’m seeing, Grodan seems to be the dominant name when it comes to Cannabis but am aware of other brands in the hydroponics space like Cultilene. When it comes to Grodan for cannabis, are you aware of any issues with the product or with distribution?
  2. Interesting - I actually didn’t realize rockwool was compostable. From my understanding, I thought a pain point for larger growers was how to get rid of rockwool after their grow since it’s often a single use product (I heard some growers paid to get it dumped or would just discard it somewhere on their property). I was thinking from sustainability in a few different ways, first being the actual method of discarding the product at the end of life. Other thoughts were surrounding packaging and overall efficiency of resources compared to other products. If it’s compostable, the first part about the end of the product life isn’t as much of a pain point as I thought.
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