A weeping willow can add a metre of growth in a single season. That vigour is what gives the tree its grace — and what turns a modest garden willow into a tangle of whips if you leave it alone for a few years. Prune at the wrong moment and you will be wiping sap off the patio and staring at dead stubs. Prune at the right one and the whole job takes an afternoon.

There is no single magic date. Pruning a willow is a balance between its sap, which starts moving surprisingly early, and the risk of decay finding its way into a wound. Here is how to find that balance through the year.

The two good windows

For most UK gardens, willow pruning fits into one of two slots.

• Mid-January to the end of February. The tree is dormant, the branch structure is easy to read without leaves, and any wound has a full growing season ahead of it. This is the window for real structural work: taking out a large limb, thinning a congested crown, reducing height.
• Mid-August to mid-September. Growth has slowed and sap pressure has dropped, so cuts weep far less. Ideal for light shaping, tidying whips that trail into a path, and removing dead wood before winter.

Everything outside those windows is a compromise. Sometimes a compromise is fine — a storm-broken branch cannot wait until January — but do not plan a big pruning session for March or October.

Why willow sap bleeds, and why it usually doesn't matter

Willow sap sits under pressure, particularly as the buds swell in early spring. Cut a branch in March or April and it can drip steadily for days, staining paving and looking alarming. It is startling, but rarely harmful in itself: a healthy willow has plenty of reserves and will callus over the cut in time.

The real arguments for timing are practical. A wound made in spring sits wet and open while the tree pours its energy into new leaves. And a big weeping willow with a fresh wound in mild, damp autumn weather is exactly what decay fungi are looking for.

When to keep the saw in the shed

• April to June. Peak sap flow, and the canopy is full of nesting birds — active wild bird nests are protected by law in the UK, so leave the tree alone.
• October to December. Wounds stay open through the wettest months, giving decay an easy route in.
• Frosty spells. Frozen wood shatters rather than cuts cleanly. Wait for a milder day.
• During drought. A stressed willow needs every leaf it has.

A month-by-month guide

• January. The best month for heavy work. Service your tools, then take out dead, damaged and crossing branches while you can still see the skeleton of the tree.
• February. Finish structural pruning early in the month. In a mild year sap starts to rise in late February, so treat the final week as a deadline rather than an invitation.
• March. Stop. Remove only a broken or dangerously hanging branch, and make the smallest cut that does the job.
• April. Leave it alone. This is when bleeding is at its worst.
• May and June. Hands off, apart from a single whip in the way of a path — and even that is better left until August.
• July. Light thinning is possible late in the month if growth has slowed. Small cuts only.
• August. The second window opens in mid-month. Thin the canopy, shorten trailing whips, remove dead wood.
• September. Finish by mid-September. A good moment to tidy up after summer growth.
• October and November. Storm damage only.
• December. Plan, sharpen, order new blades. Prune nothing.

Making a clean cut

Sharp tools do more for a willow than any technique. Use bypass secateurs and loppers rather than anvil types, which crush the wood, and a pruning saw for anything thicker than your thumb. Wipe the blades with methylated spirits between trees, or straight after cutting anything that looks diseased.

For a branch too thick for loppers, use the three-cut method:

1. Make a shallow cut underneath the branch, a hand's width out from the trunk, cutting about a third of the way through.
2. Cut from above, slightly further out, until the branch falls. The bark tears only as far as your first cut.
3. Remove the remaining stub with a clean cut just outside the branch collar — the slight swelling where the branch meets the trunk.

On thin shoots, cut about 5mm above a bud, angling the blade slightly away from it so the bud is not damaged and water runs off the wound. Two habits matter more than anything else: never leave a stub, and never cut flush against the trunk. The collar holds the tree's own defence tissue; a stub will simply die back and, on a willow, hold moisture against the wood.

Skip wound paint. It traps damp underneath and does the tree no favours.

What to remove, in order

Prune for the tree first, then for the shape you want. Most weeping willows need far less cutting than their owners expect.

• Photo: Erik Mclean / Pexels