Repotting week on the balcony: field maple, hornbeam and the swelling bud
Every spring there is a week or two when most of my trees need their roots looked at, and the balcony turns into a potting bench with a sheet of tarpaulin over the decking. This is how I decide when that week starts, and what happens to a field maple or a hornbeam once it comes out of its pot.
Written 6 April 2025 · looked at again 24 August 2025 · 7 minute read
Watching the buds, not the calendar
The standard advice is to repot deciduous trees in early spring, just as the buds begin to swell, and I have found it sound for every native species I grow. At that point the tree is about to push new roots as well as new leaves, so roots cut during repotting are replaced quickly, and there are no leaves yet losing water through a reduced root system. A week or two too early and the tree sits in cold wet soil with fresh wounds. Too late, and the opening leaves demand water that damaged roots cannot supply.
The trouble is that swelling buds do not arrive on a fixed date. On my balcony, which faces south-west and gets the afternoon sun off the brick, the field maples usually start in the last week of February or the first week of March. The hornbeams follow about ten days later. The hawthorns come somewhere in between, and in a cold spring the whole sequence can slip by three weeks. I have stopped writing a repotting date in the diary and instead check the buds every other morning from the middle of February.
What I look for on a field maple is a bud that has lost its tight, dark, pointed look and begun to lengthen and pale slightly at the tip, with the scales just starting to part. On a hornbeam the long pointed buds fatten and turn a lighter brown. The change is small and easy to miss the first time. Once seen, though, it is unmistakable, and it is the signal I trust over any date in any book.
What comes out, and what stays
Not every tree gets repotted every year. Young trees still building roots in a larger training pot might go two or three years between repots, and older ones in their final containers every two years or so. I decide by easing each tree gently out of its pot in late winter, while it is still dormant, and looking at the outside of the root ball. If roots are circling the pot wall thickly, or the soil drains slowly after watering, the tree goes on the list.
When a tree comes out properly, I tease the outer roots loose with a chopstick, working from the top down and from the outside in, until about a third of the old soil has fallen away. Then I cut back the long thick roots, especially any heading straight down, and keep the fine fibrous ones near the trunk. The aim over several years is a flat, radial root system spreading out from the base like the spokes of a wheel, which is what lets a small tree stand steadily in a shallow pot.
Field maple takes root pruning very well and I am fairly bold with it, removing up to a third of the root mass on a healthy tree. Hornbeam I treat more carefully, as it seems to recover a little more slowly on my balcony, and I rarely take more than a quarter. Hawthorn sits between the two. I have never lost a tree to root pruning in the right week, and I have lost one to root pruning in the wrong week, which is a fair summary of how much the timing matters.
On the potting bench
- Roots circling thickly against the pot wall
- Water pooling on the surface for more than a few seconds
- The tree lifting itself out of the pot on its own roots
- Fewer new shoots than last year, with no other obvious cause
Soil for a windy balcony
Soil for bonsai is less about nutrition than about drainage and air. A tree in a shallow pot needs a mix that lets water run straight through and leaves plenty of air between the particles, because roots in waterlogged soil suffocate and rot. I use roughly equal parts of calcined clay granules, pumice and fine composted bark, sieved to remove the dust, and I adjust the proportions slightly by species and by the size of the pot.
The balcony shapes that mix more than any book does. It is exposed and catches the wind that comes up off the river, which dries pots far faster than a sheltered garden would. For that reason I use a little more bark than many growers recommend, since it holds more water, and in the smallest pots I sometimes add back a handful of the finer particles I would normally sieve out. It is a compromise between good drainage and not having to water three times a day in July.
After potting, each tree is tied firmly into its pot with wire threaded up through the drainage holes and over the root ball. On a balcony this matters far more than people expect. An untied tree in fresh loose soil will rock in the first strong wind, and every movement tears the fine new roots it is trying to grow. I learned that the hard way with a hornbeam in 2021 that blew over twice in one week, and it took the whole summer to get its vigour back.
The fortnight afterwards
Once repotted, each tree is watered thoroughly until the water runs clear from the drainage holes and then moved into the most sheltered corner of the balcony, away from both the wind and the strongest afternoon sun. It stays there for about two weeks. I keep it out of hard frost by setting it against the wall of the flat on cold nights, but I do not bring native trees indoors, since the warmth pushes growth before the roots are ready for it.
I do not feed a freshly repotted tree for about a month. New roots are fine and easily burned by fertiliser, and there is enough in fresh soil to keep a tree going until the first flush of leaves has hardened. The only thing I do in that fortnight is test the soil every day with a fingertip and water when the top centimetre has dried, which in March on a sunny balcony can be more often than you might think.
By the time the leaves have fully opened, the trees go back to their places on the shelves and the tarpaulin comes up off the decking. The balcony is swept, the sieves and chopsticks go back in their box, and I write a line for each tree in the diary noting how much root I took and what the old soil looked like. Those lines are the most useful records I keep, because the following February they tell me exactly which trees can wait another year.
On the potting bench
- Water through until it runs clear
- Two weeks sheltered from wind and hard sun
- No feeding for about a month
- Protect from hard frost, but keep the tree outside
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.