I messed around with top dressings for my trees this spring. In previous years the top layer was the same Boon’s mix that was throughout. I felt it was time to improve on that…
Warning! The following is a pedantic discussion of substrate and strata!
After many years of trying, I gave up on attempting to introduce live moss or moss spores on the top of my substrate. If moss grows there naturally, I let it spread, but I am no longer actively trying to propagate it on all my trees. I have mossed up a couple bonsai for temporary show the past few years at the regional agricultural fair using collected clumps, but that is it.
In an attempt to elevate my game, this spring I added milled sphagnum tossed with basic Boon’s mix in the top layer of my soil. My observations lead me to believe that many enthusiasts don’t need the extra step of sphagnum! Milled sphagnum is bulky, unsightly, unstable, and it makes it almost impossible to know what is actually happening with moisture in your smaller shohin and mame pots. It is an essential medium for establishing root growth in dug up yamadori and also as an antibiotic, but as a surface moisture retention system it is dysfunctional. Sphagnum obviously does nothing for root ramification when mixed into soil, either.
Deciding to eschew the sphagnum for my remaining trees, I opted to use straight pumice, fujisuna, or kiryu, on top of my bonsai. Fujisuna is a black lava available in 1/8" size particles. I had thought that the black might allow the pot and tree to speak more prominently and without any distraction, but instead the black made everything look inorganic, bleak, and gothic. Additionally, it is difficult to gauge water needs based on the darker appearance. There also might be a risk that black will be too hot for some trees left in the sun. So, I am slowly switching the fujisuna out on all the trees I used it on.
Aesthetically speaking and as an indicator of dryness, the white of pumice is just as bad if not worse than black. If you can get small grain Hyuga or Ezo pumice, that might be better, but I’ve never worked with those specific Japanese varieties before.
Lastly, I tried kiryuzuna. “Kiryu”, as it sold, is a smallish sharp river (sand). It is similar in color and price to Akadama. Both Akadama and Kiryu can be used as visual clues to the dryness of a pot. However, Akadama breaks down with repeat watering and it needs compounding pressure to keep from deteriorating prematurely. Kiryu does not break down and does not need to be compacted. That makes it perfect as the loose amendment on the top layer! It is such a small grain and pleasing appearance that even mame can be topped off with Kiryu.
The color variations of kiryu when your bonsai are wet, to damp, to dry, are very easy to discern. I will attach photos to show what I mean.
While this substrate component works well for me, I also operate out of my home and keep bonsai of every size from 2" mame pots to 24" yamadori pots.
If all your trees are all one size, or are on an automatic watering system, or you water on a schedule (shame on you) maybe kiryu is an unnecessary expense. Personally, I keep 100 trees and I like easy-to-spot advance warnings of when I need to water each bonsai.
Here’s what it looks like:
Saturated [just watered]
Moist [no need to water]
Drying [water soon]
Fully Dry [time to water immediately]
If you are concerned about deeper pots retaining more moisture at the bottom, don’t be. First, the lateral roots near the surface are what should be the focus on protecting and promoting at all times anyway. Deep roots in deep pots are fine to run a little moist as long as you are using inorganic substrates like Boon’s or Kanuma. If you doubt me, read Walter Pall’s article on the myth of overwatering.
I think Walter might overstate things, but the idea is if you have free-draining substrate, it’s ok to be a little moist. Unless you are sitting your pot in a deep drip tray or suiban, there should be no standing water to rot or boil roots (on brutally hot days). Rot or “trench foot” in bonsai occurs most frequently in pots with gobs of organic soil, no drain holes, or during the dormant period. The latter is more often in Western species like Ponderosa pine. While I keep bonsai on the wet side in the late spring, all summer, and early fall; during the rest of the year I limit water to all my trees for protection of the roots and foliage reduction. That means keeping my trees covered from rain and snow in the off-season and only watering them manually when needed. I am excited to have the kiryu for this coming winter to more effectively manage my moisture on a tree-by-tree basis. I lost one old Rosemary due to thirst last winter. I would not make that mistake again with a barometer like kiryu.
As mentioned, Kiryu costs about the same as Akadama - which isn’t cheap! But, as a top dressing, you will be surprised at how far a small bag will take you.
All of this might be basic information for some people who have been at bonsai longer or better than I have. However, it is a new revelation for me and one I felt compelled to share.
No offense to proponents of sphagnum like Ryan, but he is keeping a lot more trees than I am - the majority of which are oversized. Those factors may mean that milled sphagnum mixed into top dressings are more effective. Not for me, though. Make mine Kiryu. Straight up!