Can I grow Semps in just lava fines? I found a store that sales black lava fines and its consistency was very much like sandy soil.
Hi Tony, I've never seen lava fines, they sound interesting.
The problem with any kind of product that is in that type of format is that what you're getting is the dust from lava rock processing where they're aiming for a certain sized rock for mulch or barbecue lava, so my concern would be that you might get small dust that could clog the pores of the mix.
To be on the safe side, I might screen this again, just to remove the really small pieces (dust) and then I would mix it with other materials as well.
You will have to experiment, especially in view of the fact that it's dark colored. Sempervivum do not appreciate having their roots too hot. It's odd, because they can withstand considerable brightness, but they resent heat. This has to do with their origins in high mountains, where they would have cool roots, even though the intensity of the light is extreme.
I would mulch with something a little lighter in color.
You may find a difference growing in pots and in ground with the addition of this particular ingredient, so by all means test to see if it's better to mix it in. Your Sempervivum will tell you which specific format they prefer.
Best of luck,