For years I’ve been impressed with the bold root growth in boxes with collected yamadori, in 100% pumice. Trees as distinct as Pine and Maple have predictably strong root growth.
Once I recall trying to stick a chopstick into the pumice of a Vine Maple I’d collected only the year before and failed, the roots were so dense. Since then I’ve often wondered about pumice and its potential as the sole soil component.
Two years ago, out of a curiosity I could no longer control, I put two Japanese Maples in pumice in a modest-sized bonsai pot, thinking, here’s a tree that likes water. If it struggles in 100% pumice, there’s a ceiling.
A Japanese Maple 2 years after potting in 100% pumice. The tree was grown under 40% shadecloth and grew as vigorously as any in my usual 50/50 akadama / pumice deciduous mix. It had no burned leaf tips or any sign of water stress.
We sifted the pumice to 1/8”. Fairly small.
Last year we repotted one maple, and this spring I repotted the second one. This isn’t an impressive data base, but what I found was intriguing enough to share.
Worried that the top of the pumice would dry out excessively fast, I added a thicker top dressing of sphagnum moss than I usually do. Strong, bold root growth happened here, which fuse well and which could create good nebari. So it wasn’t 100% pumice, more like 95% pumice. Next time I’ll try a thinner layer of moss on top to see if there’s any difference.
Underneath was promising for bonsai. The fine pumice grew fine roots. One thing I shared in Bonsai Heresy was that if a plant has fine ramifying roots, they like it there.
Roots need three things: oxygen, water, and nutrition (covered in chapter 5 of Bonsai Heresy). If they don’t find those, they stop growing. Given the fine root growth in the small pumice—about what I’ve seen in akadama—these roots were happy.
This is the sort of root system that has good longevity in a pot, without becoming woody. I expect with a larger pumice size we’d see more large roots in this lower zone.
It may bear noting that I’m using local pumice from Oregon or Washington. This is lighter with more pore space than the pumice from Japan or in other parts of the U.S. In some situations it floats when watered, though not as much as perlite.
Pumice can be as expensive as akadama in many places. I want to use it because it’s cheap here in the Pacific Northwest, so clearly I am invested in seeing it do well. Which is hardly a strong position even for citizen science. So—take this “soil share” with a hefty grain of salt…