When and how do I prune a buddleia?

Buddleja davidii is the plant that teaches the new-wood rule better than anything else in a British garden. It flowers in high summer on growth made that season, so everything standing in February is expendable, and cutting it hard turns a leggy four-metre shrub with small flowers into a compact one with far larger ones. The decision tree that governs this applies to every summer-flowering shrub, and it is worth learning once.

What you'll need

• Loppers
• Secateurs
• Pruning saw for old stems
• Gloves
• Eye protection

Step by step

1. Prune in late winter or very early spring.February to March in most of the UK, before growth starts but after the worst frosts. Pruning in autumn leaves the plant with soft cut ends through the winter and removes the seedheads that feed birds.
2. Cut back to a low permanent framework.Take every stem down to about 600mm from the ground, cutting just above a strong pair of buds. On an established plant, cut to a point just above where you cut last year, so a knuckled framework builds up over the years.
3. Do not be timid.It looks like vandalism and it is correct. A buddleia pruned lightly gets taller and leggier every year with smaller flowers at the top; one cut hard produces vigorous new stems and large panicles at eye level.
4. Take out the oldest and weakest stems entirely.On an old plant, remove one or two of the thickest, most congested stems right at the base each year to renew the framework. Buddleia wood becomes brittle and hollow with age.
5. Give it a second, lighter cut in early summer if you want to delay flowering.Shortening some stems in late May or early June produces a later, longer flowering season. Useful in a mixed border where you want the display staggered.
6. Deadhead through the summer.Cutting each spent flower panicle back to a pair of leaves prompts side shoots and a second flush, and it stops the plant seeding itself around. Buddleia seeds prolifically and colonises walls, gutters and waste ground readily.
7. Know which buddleias are exceptions.Buddleja alternifolia flowers in early summer on the previous year's wood and is pruned after flowering, not in winter. Buddleja globosa, the orange ball tree, also flowers on old wood and needs only light pruning after flowering. Cutting either of these hard in February removes the whole display.
8. Leave a few seedheads if you can bear it.Overwintering birds use them. The compromise is to deadhead through the flowering season and leave the last flush standing until the February cut.

Tips

• Buddleia is invasive in the UK and self-seeds aggressively into masonry, railway ground and river shingle. Deadheading is a genuine environmental measure, not just tidiness, and it is worth removing seedlings from walls promptly.
• A buddleia growing out of a wall or a chimney should be killed off rather than pruned. The roots do serious structural damage and the plant will regrow from any stump left behind.
• Compact modern varieties bred to stay under two metres are worth choosing for a small garden; they still want the same hard annual cut.
• This same hard late-winter cut applies to caryopteris, perovskia, hardy fuchsia, lavatera and late-flowering spiraeas, all for the same reason.

Common mistakes

• Pruning it lightly to keep some height — The plant simply builds height on top of height, and within a few years all the flowering is out of reach on a top-heavy, brittle framework.
• Cutting Buddleja alternifolia in February — It flowers on last year's wood and the whole display is removed. It is pruned straight after flowering instead.
• Pruning in autumn — Frost damages the fresh cut ends, the plant looks like nothing all winter, and you lose the seedheads that feed birds.

Questions people ask