Nor do I wish to make this a 'how-to' post. That was never the purpose of this blog. Loads of other sites on the interwebs for that, some with people advocating their way or no way. And they probably do it with a lot more brio and conviction than I ever will.
Once in a while, though, I will answer a question or tell you what we do and why. What you do with that information is all up to you.
This one is a result of a remark a friend made, when TOH said we top-dress our newly re-potted trees with chopped sphagnum moss. This protects the top layer from drying out, since this is the layer where the feeding roots tend to be most active. The contention is that, this would encourage the roots to grow into the sphagnum (i.e. up towards the surface) rather than down into the soil (i.e. towards the bottom of the pot).
The only time we've ever had the experience of roots growing towards the top of the soil is when we used to lay down the large cakes of rapeseed fertiliser. This acted like some sort of root magnet but we have long since gone for other forms of fertiliser, since the decomposing rapeseed cakes are a blackbird magnet as well as a root magnet. The blackbirds used to chuck the rapeseed cakes off the bonsai and go for the grubs that would inevitably come with the decomposition factor.
So, going back to the chopped sphagnum layer. What we do is blitz the stuff in a food processor, so that it comes out in relatively fine flakes. We first started out by grating it over a fine-meshed metal sieve, but ditched that for a more high-tech method. 'Cos I'm all about the user friendliness, y'know... :D
A thick-ish layer (about 5 - 8 mm) of this finely-chopped sphagnum is laid over our top dressing of fine Akadama, then tamped down and misted well so that the whole lot doesn't go flying off at the initial watering. The newly re-potted bonsai is then given a VERY thorough drenching, then left alone until the layer of sphagnum starts to show signs of drying out. Then it gets drenched again, then only re-watered until the sphagnum does its show-and-tell thing again. And so on ad infinitum for the rest of the life of the bonsai (or until real live moss starts growing over the existing sphagnum layer). Which is basically the same way we water every potted (or not) thing in the garden - accent plants, kusamono, et al.
To illustrate the sphagnum business, this is a 7-lobed Japanese maple (previously featured in this post) that was re-potted at the tail end of this Winter. The top dressing of sphagnum moss is lighter in colour before watering, thus making it easy to identify which trees are in need of a drink:
And after watering (check out the water mark on the bark of the tree), it goes a darker colour (and so does the Akadama; click to enlarge the photo). The moss also clumps together when it's wet:
Before watering, the top layer of akadama just beneath the sphagnum is only very slightly damp while the lower layers of soil deep beneath will still retain more moisture; hence the feeding roots will naturally find their way towards this source of water. Another person I know used to call this 'making the roots work'.
The sphagnum moss layer prevents erosion of the top soil layer, particularly for people like me who use a watering hose. Here is where we haven't bothered to replenish the sphagnum dressing on this exposed-root Trident Maple, which of course is not the most energised of bunnies at the moment:
I suppose, if I were less careful with the watering and kept the soil evenly damp all the time, the roots of our bonsai would probably work their way to the top layer of sphagnum moss. I would probably also have lost lots of trees by now. Still, we have been using this top-dressing method for the past 10 years at least, so there must be some merit to our madness.
Some people say the moss should be taken off after a time - I've never bothered as the birds tend to do that for me, either through boredom, foraging, nest-building or just the desire to cheese me off. If anything, we wind up having to renew the moss layer after a few months as it gets eroded by watering and bird activity. And it is fairly easy to tell if moss has started growing due to over-watering; it has this sort of manky look to it, plus the surface would always be wet to the touch anyway.
A word about the sphagnum-blitzing business, though: it really does tend to get blown away by the slightest breeze once it's finely chopped. And watering - either with a hose or by dunking - is another quick way to lose the lot when it's freshly laid. Hence our utter reliance on a good nozzle that has a 'mist' feature. Never say you haven't been warned...
BTW, did anyone notice the gratuitous pun embedded discreetly up there? Energised? Bunnies? Yes? Yes?
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