When can I prune a Japanese maple?

Japanese maples are among the shrubs most often spoiled by good intentions. They have a naturally beautiful layered branch structure that takes years to develop and is destroyed in an afternoon by shortening branch tips all over. And they bleed sap prodigiously if cut at the wrong time of year, which weakens the plant and looks alarming.

What you'll need

• Sharp, clean secateurs
• Pruning saw for larger branches
• Gloves
• Garden disinfectant for blades (optional)

Step by step

1. Prune between midsummer and early autumn.July to September is the safe window in most of the UK. The sap is not under pressure, the plant is in full leaf so you can see the shape, and the cuts have time to seal before winter.
2. Never prune in late winter or early spring.Acers, like birches and walnuts, bleed heavily from cuts made when the sap is rising. It is very hard to stop, it weakens the plant, and it stains the bark.
3. Start by deciding whether anything needs to come off at all.An acer allowed to develop naturally is a better plant than a pruned one. Remove dead wood and genuine problems, and resist the urge to shape.
4. Remove dead, damaged and crossing branches.Cut back to a junction or to the main stem. Dead twiggy growth accumulates inside an acer canopy and clearing it improves the appearance considerably without altering the structure.
5. Thin whole branches rather than shortening tips.This is the whole technique. Taking out a complete branch back to its origin keeps the layered, open, tiered look. Shortening every branch end produces a dense blob of twiggy regrowth and destroys the shape permanently.
6. Open the canopy so you can see through it.The traditional aim is that you can see the branch structure and glimpse light through the plant. Remove branches that cross the middle, grow inward, or clutter the tiers.
7. Work in small steps and stand back constantly.Cut one branch, walk round the plant, decide again. Acers are unforgiving and there is no way to put a branch back.
8. Reduce the height only by cutting to a lower side branch.Never cut a leader back to a bare stub. Take it back to a side branch that can become the new leader, and the plant continues to look natural.
9. Keep the tools clean and sharp.Acers are susceptible to coral spot and verticillium wilt, both of which enter through wounds. Clean cuts on healthy wood with disinfected blades are the sensible precaution.

Tips

• Acers in containers can be root pruned and repotted in early spring, which is a different job from branch pruning and is genuinely useful for keeping a potted specimen in scale.
• Crisped brown leaf edges in summer are almost always wind or sun scorch or dry roots, not a pruning issue. Acers want shelter from wind and from hot afternoon sun.
• Coral spot — bright orange pinhead pustules on dead twigs — is common on acers. Cut affected wood well back into healthy tissue and bin it.
• The finest acers in gardens have been pruned least. If in doubt, take nothing off this year.

Common mistakes

• Pruning in February or March with everything else — The sap is rising and the plant bleeds heavily from every cut. It is difficult to stop and it weakens the tree.
• Shortening all the branch tips to reduce size — It destroys the layered structure permanently and produces a dense twiggy mass. Thin whole branches back to their origin instead.
• Cutting a leader back to a bare stub — The plant either dies back from the stub or throws a bunch of upright shoots, neither of which looks like an acer. Cut back to a side branch.

Questions people ask