How do I prune a hedge properly?

Cutting the batter so light reaches the base, the timing that keeps a hedge dense, and the species that will not regrow from old wood.

Difficulty: beginner
Time: 3 hr
Read: 3 min
Safety: warning

Short answer

Cut the sides sloping inwards towards the top so light reaches the base — a hedge wider at the top always goes bare underneath. Trim little and often for a dense hedge, and check for nesting birds first.

The commonest hedge problem is a bare base, and it is caused by the shape of the cut rather than by anything the plant is doing.

Safety

It is an offence to damage or destroy an active bird's nest — check thoroughly before cutting, and avoid hedge cutting during the main nesting season of March to August where possible. Wear eye protection and gloves. Never use a powered hedge trimmer above shoulder height from a ladder; use a platform or a long-reach trimmer from the ground.

What you'll need

Shears or a hedge trimmer
String line and canes for a straight top
Eye protection and gloves

Step by step

Check for nesting birds.A legal requirement as well as a decency one. Look carefully into the hedge before starting, and avoid the March to August nesting season where you can.
Cut a batter.The sides should slope inwards as they rise, so the base is wider than the top. This lets light reach the bottom and is why formal hedges stay dense to the ground.
Set a string line for the top.Canes at each end with a taut line. A hedge top cut by eye follows the ground rather than the horizon and always looks wrong.
Work from the bottom up on the sides.Cuttings fall away as you go rather than sitting on uncut growth and obscuring the line.
Trim little and often for formal hedges.Two or three light cuts a season keeps box, yew and privet dense. One hard annual cut gives a coarser, more open hedge.
Know which species regrow from old wood.Yew, box, privet, hornbeam and beech will regenerate from bare wood. Conifers such as leylandii, thuja and juniper will not — cut into brown and it stays brown permanently.
Renovate deciduous hedges in stages.For an overgrown beech or hornbeam, cut one side hard one year and the other the following year. It keeps the plant supplied while it regenerates.
Time it by type.Formal evergreens in late spring and again in late summer. Deciduous hedges in winter when dormant. Flowering hedges immediately after flowering.
Feed and mulch after a hard cut.Renovation is demanding. A general fertiliser in spring and a mulch helps the hedge recover and regrow densely.
Clear the base.Remove clippings from the bottom of the hedge. Left there they smother the base growth you are trying to encourage.

Tips

Wider at the bottom than the top. It is the single reason hedges stay green to the ground.
Leylandii will not regrow from brown wood. Cut into it and it stays bare forever.
Check for nesting birds — it is a legal requirement, not just good practice.

Common mistakes

Cutting the sides vertical or wider at the top — The top shades the bottom, so the base loses its leaves and never recovers. A sloping batter is what keeps a hedge furnished to the ground.
Cutting conifers back into brown wood — Leylandii, thuja and most conifers have no dormant buds in old wood, so a cut into brown never regrows. The bare patch is permanent.

If it doesn't work

Hedge bare at the base

Cause: No batter, so the base is shaded — Fix: Cut a taper into the sides so light reaches the bottom. On species that regrow, it will refurnish over a season or two.

Brown patch in a conifer hedge

Cause: Cut into old wood — Fix: It will not regrow. Encourage adjacent growth across, or replace that section.

Hedge top wavy

Cause: Cut by eye — Fix: Set a string line between canes and cut to it.

Hedge getting wider each year

Cause: Cutting back to the previous year's growth rather than into it — Fix: Cut slightly harder each time so it holds its size rather than creeping outward.

Questions people ask

When can I cut a hedge?

It is an offence to disturb nesting birds, so avoid March to August where possible and always check first. Beyond that, formal evergreens suit late spring and late summer, deciduous hedges winter.

Can I cut leylandii back hard?

Not into brown wood — it will not regrow and the bare patch is permanent. You can only trim within the green growth, which is why overgrown leylandii is so often removed rather than reduced.

How do I fix a hedge that is bare at the bottom?

Cut a batter so the sides slope inwards, letting light down to the base. On yew, box, privet, beech and hornbeam it will regrow. On conifers it generally will not.