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Ask Erik: RAW! Disposal Nightmares: What To Do With Spent Growing Medium?

Coco coir looks like regular garden mulch, so it’s easier to dispose of discretely. Coco coir looks like regular garden mulch, so it’s easier to dispose of discretely.

Q: Hey, really liking your new RAW! column. You have always told it like it is, but my friends and I are really appreciating your help as a grower with what goes on outside of the grow room too.

This one might be right up your alley:

The only real answer, in my opinion and experience, is to come up with a sustainable growing medium or method

My circle of friends and I grow commercially. Most of us are growing in rockwool, however, some of us are also growing in peat mixes like Sunshine #4 as well. Every four months or so, we have an awful lot of growing medium to dispose of. I am talking about enough to easily fill the back of a pick-up truck with. On top of that, we grow year-round, so the stuff can really stack up.

I don’t feel like advertising to my neighbourhood that I grow, so creating gardens in front of my house with old rockwool or Sunshine #4 mix really isn’t in the cards. I am also proud to say, that we as a group give a hoot about the environment, so disposing of it properly is important to us. On top of that, a few of the guys in our group use some pretty strong PGRs in their waterings to control crop height which adds another dynamic to the situation.

Help! What’s a good way for us to dispose of our spent growing mediums that will still allow us to sleep at night?


First, thanks for the props on the new column. I’m having lots of fun with it, and it’s a great feeling to be able to talk to my fellow growers about all of the important stuff that affects us outside of the grow room.

Disposing of spent growing mediums, especially ones like rockwool which will never decompose, or growing mediums that have become toxified with PGRs (Plant Growth Regulators) can be a real nightmare, akin to trying to dispose of spent uranium, yes?

The only real answer, in my opinion and experience, is to come up with a sustainable growing medium or method, especially if you are a professional grower who must use and typically replace significant quantities of growing medium on a regular basis.

Believe me, I understand why you might not want to reuse growing mediums, as it can complicate management with nutrient applications due to residuals from previous crops, as well as the possibility of having problems due to PGR applications.

  1. For you and your friends who like either rockwool or peat mixes and DON’T use PGRs:
    Switch to coco coir or custom blend your own peat mixes using pumice instead of the shiny white, tell-tale perlite that comes in commercial peat mixes. Good quality coco coir, save for the pH, performs quite similar to rockwool. It is available just like rockwool in loose form, slabs/growbags, and cubes. Coir has the advantage of actually holding a little more air than rockwool, which often translates into faster growth rates. Just make sure to add some chelated calcium and magnesium when using new coir for the first couple of weeks.
    Coir looks like a lot of popular garden mulches if you need to get rid of it. Better yet, it keeps its texture and doesn’t compact like peat or rockwool, so with good management you can re-use it over and over, unlike rockwool.
    If you make your own peat mixes using pumice instead of perlite, you won’t have the tell-tale white shiny bits to worry about. Unlike perlite, pumice will stain and take on the color of the growth medium it is in. This makes it much easier to add to garden beds outdoors without your additions sticking out like a sore thumb. Also consider making a contact with a trusted landscaper who can put your spent PGR free mediums to good use elsewhere.
  2. For you and your friends who like either rockwool or peat mixes and DO use PGRs:
    The only real sound advice I can offer besides the obvious “don’t use PGRs,” is to switch to water culture methods. Try growing in a system like the Under Current or an Aerflo, where there is little to no growing medium whatsoever. This way, the same system is re-used over and over again with either no or very little medium to be replaced or disposed of. Consider PGRs that are good to use for your crop as foliar sprays, rather than root applications. This way you will need to use much less of the PGR to achieve the desired effect, helping to minimize potential harm to the environment.

In the end, sustainable practices like deep raised beds or very large containers/totes with organic nutrients like Iguana Juice, microbials and enzymes are your best bet. With a little practice, you will achieve the same or better yields, often with a noticeable increase in crop quality and growth rates over time. Make sure to flush the medium with lots of H-2, SensiZym, Piranha, Voodoo and Tarantula between crops. I know growers who have re-used their growing mediums this way for years with amazing results, even on a commercial scale.

Cheers, Erik Biksa

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Erik Biksa talks about using Iguana Juice in your hydroponics growing medium.
Last modified on Saturday, 01 September 2012 07:38

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