When and How Hard Should You Prune Buddleia?

Buddleia davidii — the butterfly bush — is one of the most forgiving shrubs in the garden to prune. It is nearly impossible to kill by cutting too hard, and the harder you cut it each year, the larger and more impressive the flower spikes will be in summer. An unpruned buddleia quickly becomes a large, leggy, woody shrub with small flower spikes at the top of bare branches; a well-pruned one produces great arching stems covered in long fragrant plumes that are genuinely spectacular from July to September.

When to Prune Buddleia davidii

Prune in late winter or early spring — March is the traditional time in most of the UK and northern Europe. Pruning too early in autumn or winter risks stimulating growth that is then damaged by hard frost, particularly in colder gardens. Waiting until spring means the new growth that emerges after pruning gets straight into the improving weather. In mild coastal gardens or sheltered urban sites, you can prune a month earlier than colder, exposed gardens in the same region.

How Hard to Cut

Cut all the main stems back to about 30–60 cm from ground level, leaving a low, stubby framework. Cut each stem back to just above a pair of healthy buds or a side shoot. This looks alarming if you have never done it before, but buddleia breaks from old wood very readily and will produce strong new growth within a few weeks. After several years of annual hard pruning, the framework becomes woody and knobbled. If it is getting too large, you can cut back to virtually ground level every few years for a complete reset.

Deadheading Through Summer

Once buddleia starts flowering in July, deadheading the spent flower spikes — cutting them back to the next pair of buds or leaves below — will stimulate a second and sometimes third flush of flowers through August and September. This also prevents the plant from self-seeding prolifically; buddleia is a prolific self-seeder and can become a weed problem in some areas if allowed to set seed freely. Even if you do not deadhead for a second flush, removing the seed heads in late summer is worthwhile to prevent unwanted seedlings.

Other Buddleia Species

Not all buddleias are treated the same as B. davidii. Buddleia globosa, the orange ball buddleia, flowers in early summer on growth made the previous year. Pruning it hard in spring removes all the flower buds. Instead, prune B. globosa lightly after flowering. B. alternifolia, with its arching violet racemes in June, also flowers on previous year's wood and should be pruned immediately after flowering by cutting out older flowered stems to ground level and retaining the new stems for next year.