Is Lava Good For Bonsai?
This is a legitimate question. My master Shinji Suzuki never used lava (aka scoria), preferring just akadama and pumice.
On coming back from Japan in 2006 I continued using the akadama / pumice mix and have always had strong, fine root growth with dense root masses.
Several lava stories have stuck with me over the years. Joe Harris III reported very high levels of boron in lava at Iseli Nursery in pots with poor root growth. David DeGroot then had root growth issues, and a lab found what they called “toxic levels” of boron traced to the lava in the mix. Mr. Suzuki’s simple akadama / pumice mix has served me well and these stories don’t warm me to lava.
Lava, aka scoria, commonly used in bonsai media
Last year I was in conversation with my former apprentice Andrew Robson who does a lot of traveling and repots more bonsai in various soil media than I do, and he’s developed negative opinions about lava, particularly red. 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.
If you take your bonsai out of the pot after many years and all the soil falls away, or you cut the circling roots and all the soil falls away, it’s probably not a soil mix that is working.
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’s findings are documented in this video (sorry, may only show on a desktop version):
As an aside, Mr. Suzuki used 2/3 akadama, 1/3 pumice (kiryu), similar to what many other Japanese professionals use. (This proportion of akadama is prohibitive for many Westerners. I use 1/3 akadama, 2/3 pumice for conifers, and 1 / 1 for deciduous. Most of this has been an economic decision based on the expense of akadama. I’m exploring 100% pumice as it is cheap where I live, and find very good root growth, though the root structure is different than in akadama. I’ll report in another post about that.)
Lava, however, may create anemic root structures. The source of lava may well be implicated here, some perhaps problematic and others benign. Sometimes it creates good root structures, other times poor. If your source is working for you, use it. If not maybe compare to pumice some year.
Lava can work—a few nurseries in Japan use it in high proportions, but those are the exception. In Indonesia it is often the major soil component. Here in North America I have my doubts all sources are safe. Much of the observations of Andrew come from Texas and the Midwest, and I’m not sure where that lava is sourced from.
That some sources of lava are OK doesn’t move me to be unconcerned. Given the lab results we have and the pure empiricism of Andrew’s findings (and my own observations over the years), this seems worthy of a conversation and more testing.
Please read through the comments, as many have raised points that are worth exploring. I wish to state clearly: if you have a source of lava that is working for you, creating the fine, solid blocks of roots that Andrew’s video shows, then keep using it. This post is meant to raise a flag of warning that lava may not be as predicable as pumice, which is its closest alternative.