First, Be Sure It Is Actually a Cedar

"Cedar" is a crowded word in British gardening. The true cedars belong to the genus Cedrus — the Atlas cedar, the Lebanon cedar and the deodar — and those are the trees this advice is written for. Western red cedar is really a Thuja, Japanese cedar is a Cryptomeria, and eastern red cedar is a juniper. That matters, because their habits are not the same.

True cedars carry their needles in clusters on short shoots, grow in distinctive whorls of branches, and — this is the crucial bit — they barely regrow from old wood at all. Cut a deodar back into a bare, thick limb and the tree will simply sit there with a permanent hole. Thuja and Cryptomeria are more forgiving of a trim, provided you leave some green behind the cut.

If you prune a true cedar as though it were a leylandii hedge, you will regret it for the next thirty years. Get the identification right before the saw comes out.

Why Cedars Reward Restraint

A cedar does not keep dormant buds tucked away along its older branches, waiting for a reason to grow. New growth comes from the tips and from relatively young shoots. That single fact governs everything: any cut that removes all the live growth from a branch will leave dead wood, and no amount of feeding or patience will change it.

In practice, this means mature cedars need very little. Most well-placed garden cedars go a decade or more with nothing more than the removal of dead, broken or rubbing branches. A light tidy every couple of years beats a drastic cut every ten, both for the tree's appearance and for its structural strength.

Repeated hard pruning into the canopy opens it up to light and wind, and encourages weakly attached shoots that whip about in gales. Better to take out one or two branches properly than to nibble all over the crown.

Choosing the Right Weather and Season

Work in dry conditions. That is the single most useful rule, because damp wood and damp air are what fungal problems need to get a hold.

The best window for live growth is late winter into early spring, roughly late February to early April, just before the tree starts moving. A second good window is mid to late summer, once the year's growth has firmed up. Avoid autumn altogether — wounds stay wet, spores are plentiful, and the tree is winding down rather than sealing cuts. Avoid heavy frost too, when brittle wood splinters rather than cutting cleanly.

Dead wood is different. You can take dead branches out at any time of year, and doing so promptly is always worthwhile. Just pick a dry day and get on with it.

Making the Cut Cleanly

Use sharp bypass secateurs for anything up to about finger thickness, loppers for the next size up, and a pruning saw for limbs thicker than your wrist. Anvil secateurs crush green stems — leave them in the shed for this job.

For any limb you are removing entirely, use the three-cut method. First, make an undercut a hand's width out from the trunk, cutting a third of the way through. Second, cut from above, slightly further out, so the branch drops away without tearing bark down the trunk. Third, remove the stub with a clean cut just outside the branch collar — the slight swelling where branch meets trunk. Never cut flush to the trunk, and never leave a long stub.

• Dead wood: cut back to live growth or to the collar. No dressing required.
• Storm-damaged branches: remove cleanly above a side shoot rather than leaving a ragged break.
• Crossing or rubbing branches: take out the weaker, less well-placed one, usually the inward-growing of the pair.
• Co-dominant leaders: on young trees, remove one to leave a single clear trunk before the union becomes a weak point.
• Low branches over paths: lift them gradually, a couple a year at most.

What Not to Do

• Don't shear a cedar into a tight formal shape unless you intend to repeat it forever. Regular trimming into old wood creates bald patches that never fill in.
• Don't top a cedar. Removing the leader destroys its natural form and produces weak, ugly regrowth.
• Don't cut into thick, bare old wood hoping something will break. It won't.
• Don't remove more than roughly a quarter of the foliage in one season — and far less than that on a mature tree.
• Don't paint wounds with sealant or tar. Cedars seal themselves, and dressing traps moisture.

Aftercare and the Long View

Once the cuts are made, leave them alone. Clear away the prunings, particularly any diseased material, and mulch around the base if the ground is bare, keeping the mulch clear of the trunk. Young cedars benefit from watering in dry spells during their first few years, and from ties that are checked annually and loosened as the trunk thickens.

Walk round your cedar once a year in late winter, when the structure is visible without leaves in the way, and note anything that needs attention. Small corrections made early save difficult decisions later.

If a cedar has been badly cut in the past and now has gaps, resist the urge to force it back into shape. A shrub planted nearby can soften the bare patch, and many mature cedars carry an asymmetric, weathered character that suits a British garden rather well.

Finally, if the job involves a heavy limb over a greenhouse, a boundary dispute or anything needing a ladder above head height, bring in a qualified arborist. That is not a failure of nerve — it is good husbandry.