Thursday, February 6, 2014

Rock wool for seeds germination

I start many perennial plant seeds which I get from seed exchanges. Over the years I have tried various mediums/substrates for starting seeds. I have mixed my own, using peat moss and sterilized garden soil and vermiculite or Perlite. I have also used peat-based commercial mixes as well as very sandy mixes. Ideally, you would choose the medium best suited to the plant you want to germinate.

For instance, a peat-based mix should do well for plants that like an acidic soil. However, if you start seeds of many different plants, creating a specialized mix for each seed would require a great deal of work. Ideally, you would use a substrate appropriate to most seeds. Last year I tried something new, I used rock wool. The results were mixed.

Seeds from seed exchanges

Horticultural rock wool
Rock wool is a man-made fibre, the result of melted and spun basaltic rock. It is mostly used as an insulating material, but it can also be used to grow plants, mostly in hydroponic systems. I found it relatively expensive. However, you cannot substitute the much cheaper insulating rock wool as it repels water. Horticultural grade rock wool is made to be easily wetted, but to drain well as well.

Out of the package, rock wool has a very high pH (around 8). Soaking it in rain water for half an hour should bring the pH down. Once you have soaked it, you let it drain and then you sow your seed in the wool. I sowed rather thickly as my idea was, after the seeds had sprouted, to take apart the rock wool cubes and move the plants into pots.

Plants sprouting in rock wool

The rock wool did produce better germination rates than usual. Not only seeds germinated well, but they also rooted readily. However, the idea of taking each cube apart and move the small plants into pots did not work out. I tried it, but the survival rate was low.  Even the plants that were left in the wool did not fare so well, as they started to grow.

Plant sprouting in a piece of rock wool in a pot of growing mix

I then tried something else. I took some of the rock wool cubes with the plants in them and, without breaking them apart, "planted" each cube in a pot of potting mix. This seemed to work much better. My plants were still too crowded, but as the roots moved into the mix around and under the rock wool, they did better.

Plant sprouted in rock wool in a pot of growing mix

I am planning to use rock wool once again this year, but differently. I will start my seeds in rock wool cubes (but without crowding them as I did last year) and, right from the start, put each cube in a pot of growing mix. My reasoning is that the seeds will germinate better in rock wool and, without any check, roots will be able to move into the growing mix as they grow. Time will tell if this works better!


  1. That is very interesting, Alain. Never so rockwool around here, maybe in the bigger cities. I use peat free compost and sometimes mix in my own and vermiculite. I think it'll be another few years before they create good formulas. Which autobiography of Leonard Woolf did you read?

  2. Didn't even know such a thing existed. Hope your second experiment with this growing medium is successful!

  3. Glad you explained that insulation doesn't work because I think there's still a chunk of Roxul around here... I won't try it.

    Hang you start seeds in British Columbia and drive them home??? Or are you home again now?

  4. At one time I used garden soil. I guess I'm not that much of a gardener. I don't start plants anymore.

  5. I have several flats of seeds under lights right now. I've never heard of rock wool. I use little coir pellets that expand in water. Once the plants are bigger, I move the entire pellet pot to a plastic Solo cup that has holes I've poked in the bottom with a hot screw driver. It works really well. But I'm curious about your rock wool method, too. Does the high pH affect the rate of germination?

    1. More plants would probably be happy with a neutral pH. However, if you grow a plants that likes it basic (let us say Dianthus) you probably would not have to rinse the rock wool. If you stated an acid lover, you could rinse it in water with a bit of vinegar. Coir pellet must be quite good (and they have a neutral pH if I am not wrong). Last year was the first year I tried rock wool. Germination was better than in potting mix but once the plants started growing, they did not do as well as in potting mix.

  6. This was an interesting article. Where do you find them? Perhaps you don't get them in the US as I never saw any similar thing here. Did you try to grow seeds in coconut fiber/husk (whatever that brown hairy things from coconut is called)? Any experience in success rate?

    1. I think you could find it on the net if not locally. It is available from a local store. There is a funny story connected to it. I am friend with the clerk who sold it to me. He said that it was usually bought in large quantities with hydroponic systems by young men who always paid cash for everything!
      I believe the coconut thing is called coir. I think it would work just as well but here I have just seen it sold in large bricks.


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