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Pruning Deciduous Bonsai Trees In Autumn

Pruning deciduous bonsai trees in autumn – is it a good time to do it?  Like many bonsai fanatics, I love a good project to work on. I’m always looking for good sized nursery trees with great bonsai potential. I’m writing this in late September, having just picked up a few Japanese Maples, about three feet or a metre tall. Due to their larger size, they have pretty thick trunks already, saving me years of growing on. I really want to work on them right now!

Japanese Maple Acer Palmatum project bonsai tree raw material at All Things Bonsai Sheffield Yorkshire

Having had done the bonsai-thing for a good many years now, I’m well aware that I’ll need to do some heavy pruning in order to reduce the height of these trees down dramatically. Its a wood saw job essentially. I know that the trees are going to be ok with this – Japanese Maples back-bud really well. But is it a good idea to heavily prune deciduous trees in autumn? Does it even matter what time of year you prune them? Can you damage them by pruning deciduous bonsai trees by pruning them in autumn?

Japanese Maple Acer Palmatum project bonsai tree raw material at All Things Bonsai Sheffield Yorkshire

When to Prune

I normally say not to prune deciduous bonsai trees in autumn. two main reasons:

A – Pruning trees spurs them into budding and putting out new growth. In late spring and summer this is fine but with winter approaching, the new growth can be damaged by cold weather.  The new growth needs time to lignify – to become woody. Green shoots get frost damaged in winter basically.

B:-If you make cuts in autumn, the tree doesn’t heal in time for cold winter and can be damaged. It makes them more prone to die-back.

But then I say that winter is a better time to prune. The tree is fully dormant, so will not bud until spring. The sap has already retreated down into the trunk and roots, so the tree will not bleed sap. Surely this goes against point B? Thinking about it, yes it does. Winter protection in a greenhouse or similar would be wise for trees pruned in winter.

So in a nutshell, I normally do major pruning in mid winter and summer. Not so much in early spring, as the tree has just pumped its energy into new foliage and it needs a period of growing in order to recoup this energy, so if you just chop lots of growth off in early spring, you weaken the tree. Late spring is fine, as the tree has had time.

I’ve just had a look at this article from The University of Idaho, which says not to prune autumn as its a big time of year for fungal spores which can infect the tree through the cuts. This is a problem for big mature trees, though perhaps not so much of a problem for bonsai, as we can more easily seal the cuts with wound sealant or apply a fungicide if we get problems. I acquired a project Japanese Maple with Coral Spot recently which was living some deadwood present on the tree. A blast with fungicide seems to have got rid of it easily and the tree is very happy.

This article from Barcham’s mentions sulk-mode, which I think is a brilliant term for describing the sluggish growth than can happen with unhappy trees. It agrees with much of what I’ve been reading and also has some info on pruning evergreen trees.

I think I’m going to continue with my routine of hard pruning in midsummer and midwinter. It is probably wise to provide winter protect for trees that have been hard pruned in winter. Important to say that general, lighter pruning takes place throughout the growing season, from when new spring growth has harden off until late summer.