No Love For Lava
Andrew Robson who does a lot of traveling and repots more bonsai in various soil media than I do, and he’s developed strong negative opinions about both red and black lava. Here are Andrew’s observations:
As a bonsai professional who travels to work with students across North America and with a large collection of my own, I end up repotting several hundred bonsai each repotting season. One thing I've noticed is the effect of different soils in different locations across North America, and with different skill sets. While the professional bonsai community has come to the consensus that volcanic soils work best in a bonsai pot for optimal root growth, I've observed that lava grows worse roots than pumice and akadama alone.
I don't utilize lava for many reasons, but the first and foremost is the root systems I routinely see in it. Almost without exception, every bonsai I've repotted where lava made up at least 30% of the soil mix has had poor roots. Oftentimes when I take these bonsai out of the container, much of the soil just falls away because there isn't the fine root mass to hold it all together. In my experience, and now the experience of my students and clients, lava isn't creating the root systems that we’re looking for. The same exact bonsai, switched to an akadama and pumice mixture, grows roots that are far superior in a much shorter period of time.
There's a few other reasons I can't stand working with lava. Other than the poor root growth I see in it, it's extremely heavy compared to the alternative (pumice). It can add a lot of weight to a bonsai pot, especially a large bonsai. Lava is also hard and will destroy the sharp edge of bonsai tools with just a few repottings. Plus, it's one more material to source, purchase, sift, and mix up for our bonsai. For all these reasons, I avoid lava with a passion.
Year after year I continue to see the same thing as I repot bonsai across North America: poor roots, heavy trees, and dull tools when lava is over-utilized in the soil mix. Many of the top bonsai gardens in Japan go without lava for some of the most valuable bonsai on the planet, and if those trees can thrive without it ours definitely can as well. I always encourage bonsai practitioners to keep doing what is working well for them even if it's not something I teach or practice. But over the last several years I've moved away from using lava in my own and my student’s bonsai pots, and we couldn't be happier with the results.
—Andrew Robson