How do I prune and deadhead rhododendrons and azaleas?

These shrubs need less intervention than almost anything else in a garden, and most of the harm done to them is done with secateurs. The two jobs worth doing are deadheading, which genuinely improves the following year, and occasional shaping done at the right moment.

What you'll need

• Gloves
• Secateurs
• Loppers and a pruning saw for renovation (optional)
• Ericaceous mulch (optional)

Step by step

1. Deadhead as soon as the flowers go over.Hold the base of the spent truss between finger and thumb and snap it sideways. It breaks off cleanly at a natural joint. This puts the plant's energy into growth and next year's buds rather than seed.
2. Protect the growth buds beneath the truss.Directly under each flower truss sit two or three small pointed shoots that will become this year's growth. Snapping the truss off sideways leaves them intact; cutting through with secateurs often takes them with it and leaves a gap in the plant.
3. Do any shaping straight after flowering.Rhododendrons and azaleas set next year's buds over the summer, so the window is the few weeks after the flowers finish. Remove dead wood, crossing branches and anything spoiling the shape.
4. Clip evergreen azaleas lightly after flowering if you want a tighter shape.They take a light shear well and can be kept as low domes. Do it once, immediately after flowering, and leave them alone for the rest of the year.
5. Treat deciduous azaleas gently.They resent hard cutting and are slow to reshoot from old wood. Restrict yourself to deadheading, removing dead wood and taking out the occasional old stem at the base.
6. Renovate a large rhododendron over two or three years if it has outgrown its space.Rhododendrons break from old wood reliably, which most evergreens do not. Cut a third to a half of the stems back to 300 to 600mm in late winter or straight after flowering, and do the rest over the following years. Expect two or three years without flowers on the cut portions.
7. Feed and mulch after any hard cut.Ericaceous fertiliser and a mulch of composted bark, leafmould or ericaceous compost. These are shallow-rooted plants and the mulch matters more than the feed.
8. Water in summer, particularly a young or a wall-side plant.The buds for next spring form in mid to late summer and drought then reduces flowering the following year. Rhododendrons are shallow-rooted and dry out faster than their size suggests.
9. Do not lime the soil or feed with a general fertiliser containing lime.These are lime-hating plants. On alkaline soil they yellow between the veins and decline, and the practical answer is a large container of ericaceous compost rather than an attempt to acidify a bed.

Tips

• Deadheading a large mature rhododendron is a long job and the plant will survive without it. Prioritise young plants and anything that flowered heavily.
• Rhododendron ponticum is invasive in the UK, spreads readily and is subject to control measures in some areas. Do not plant it, and think carefully before letting a self-sown seedling grow on.
• Yellowing leaves with green veins on a rhododendron is usually lime-induced chlorosis, which is a soil pH problem rather than a feeding one.
• Rhododendrons are surprisingly easy to move, even when large, because the root system is a shallow dense plate. A late-autumn move with plenty of help and plenty of water afterwards usually succeeds.

Common mistakes

• Cutting the spent truss off with secateurs — It is very easy to remove the small growth buds sitting immediately below it, leaving a permanent bare stub. Snapping sideways with finger and thumb avoids this.
• Pruning in autumn or winter — The fat flower buds for the spring are already on the plant and easily visible. Cutting them off costs the display.
• Planting on chalky or limy soil and trying to correct it — Acidifying a naturally alkaline soil is a losing battle. A large pot of ericaceous compost gives a healthy plant with far less effort.

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