Wiring blackthorn and hornbeam without leaving scars in the bark
Wiring is the part of bonsai people picture first and the part I did worst for longest. The two species that taught me most were blackthorn, which snaps, and hornbeam, which marks if you look at it wrongly, and this entry is mostly about them.
Written 7 December 2025 · looked at again 19 April 2026 · 8 minute read
Why winter is wiring season here
I do most of my wiring on deciduous trees in late autumn and winter, once the leaves have fallen and before the buds begin to swell. With the leaves off, I can see the structure of every branch clearly, and the wire goes on without catching or crushing foliage. The wood is also at its most dormant, so the tree is not thickening quickly and the wire has longer to hold a branch before it starts to bite. On a balcony in south-west London the leaves are usually down by the end of November.
There is a second, quieter window in early summer, once the spring growth has hardened off, when I sometimes wire new shoots that have extended in useful directions. That is riskier. The tree is growing fast, the bark is soft, and wire applied in June can cut in within three or four weeks. I only do it on vigorous trees I can check every few days, and I never do it on hornbeam, for reasons that will become clear further down.
Blackthorn raises a separate question of timing that has little to do with wiring as such. Like other members of the plum and cherry family, it is vulnerable to silver leaf disease, whose spores are most active in the cool wet months, and the general advice is to prune these trees in summer when cuts seal quickly. I therefore wire blackthorn in winter but leave any significant cuts until July, which means planning the branch structure months before I make it.
Thickness, angle and the direction of the turn
The wire needs to be thick enough to hold the branch in its new position, which usually means a wire about a third as thick as the branch being bent. I use anodised aluminium for most work on native trees, as it is soft and forgiving, and a heavier annealed copper only for the occasional thick hawthorn branch that will not hold with anything lighter. Too thin and the branch springs back within weeks. Too thick and you are likely to crush the bark while putting it on.
The wire goes on at about forty-five degrees to the branch, in an even spiral, and it has to be anchored properly first, either around the trunk or by running a single length across two branches of similar thickness so that each anchors the other. The direction of the spiral matters. To bend a branch downwards, the wire should support the outside of the curve as you move it. I got this backwards on my first blackthorn and snapped the branch cleanly in two.
Before any bend, I take the branch in both hands, thumbs on the inside of the intended curve, and move it slowly while listening. Small cracking sounds on hornbeam and maple usually mean fibres are giving, and I stop and let the tree rest. On blackthorn there is often no warning at all. The wood is dense and brittle in winter and goes from bending to broken in a single movement, so I bend it in small stages over several weeks rather than all at once.
On the potting bench
- Wire roughly a third as thick as the branch
- Anchor first, around the trunk or across a pair of branches
- Coil at about forty-five degrees, evenly spaced
- Let the wire support the outside of the bend
- Bend slowly and stop at the first sound
Hornbeam, which marks if you let it
Hornbeam has smooth, thin, grey bark that is one of the most beautiful things about it in winter and one of the most difficult things about it once spring arrives. From about April the tree thickens its branches remarkably quickly, and wire that was loose in February can be biting into the bark within a month. The spiral scar that results can take years to grow out on a hornbeam, and on a smaller branch it may never fully disappear.
My first hornbeam carries a spiral mark on its lowest branch from wire I left on until June of 2021, and every time I look at the tree I see it. Since then I check every wired hornbeam weekly from the first sign of bud break, running a fingernail along each coil to feel whether the bark has started to swell against it. The moment a coil feels tight, the wire on that branch comes off, whether or not the branch has set.
If the branch has not set, I rewire it on a slightly different line so that the new coils do not sit where the old ones were, and check it again in a fortnight. It is more work than leaving the wire on, and it means some hornbeam branches take two or three seasons of wiring to settle. The alternative is a scarred tree, and a scar on hornbeam bark lasts long enough that I would rather accept the extra seasons.
Taking it off again
Wire comes off by cutting, not by unwinding. Unwinding a coil that has been on for a season pulls at the bark all along its length and can snap a branch that has only just set. I use a pair of wire cutters with short, blunt jaws and cut each turn separately along the branch, then lift the short pieces away. It takes a good deal longer, and on a well-wired tree a single evening can fill a small tin with clipped aluminium.
Once the wire is off, I look carefully at where it sat. Faint impressions that fade within a few weeks are normal and do no harm. A raised ridge or a line of torn bark means the wire was on too long, and I note it in the diary against the tree and the date, so that I can bring that tree's check forward the following year. Over several winters that record has become a useful map of which trees thicken earliest on my balcony, which no general guide could have told me.
The deepest pleasure in wiring, for me, arrives a year or two later, when a branch placed by wire has thickened, divided and held its new line entirely on its own. On the blackthorn I snapped in my first year, the replacement branch now sits almost exactly where the broken one should have gone, grown from a bud I chose the following spring and wired lightly for a single winter. It is the most patient thing on the balcony, and it exists entirely because of my mistake.
On the potting bench
- Cut each turn; never unwind
- Check the bark under the wire as it comes off
- Note any marks against the tree and the date
- Bring next year's check forward for trees that marked
Written on a south-west facing balcony in Twickenham, south-west London. What works here may need adjusting for a colder garden or a shadier ledge.