Commonly known as the butterfly bush, buddleja boasts deep purple flowers throughout summer and into the autumn. Knowing when to cut buddleja back will help you get the most out of next year's displays.
Butterfly bushes are easy to care for, and they even make our list of the best plants to grow from cuttings.
'Pruning buddleja gives good shape, plentiful flowers and a thriving plant that pollinators will love,' says Morris Hankinson, managing director of Hopes Grove Nurseries.
We've asked garden experts when to cut buddleja back to help you determine the best time to prune.
When to cut buddleja back
Knowing when to cut buddleja back will largely depend on the variety you have, but generally, the beginning of the growing season is the best time to prune.
'You should start pruning buddleja when it is coming into growth in early spring,' says Graham Smith MCIHort, horticultural expert at LBS Horticulture. 'More vigorous varieties can be pruned back hard in early spring, and they will grow back to two or three metres tall in summer.'
However, you can also do some light trimming later in the summer. 'It's a good idea to do a light prune at the end of the summer, too, usually around August,' says Christopher O'Donoghue, co-director at Gardens Revived.
What you'll need
A handy garden trug to catch your clippings while you prune.
How to cut buddleja back in early spring
To help promote healthy growth, it's best to cut buddleja back on a frost-free day in late winter or early spring.
1. Cut it down
Knowing how much to cut back is key to getting buddleja pruning right.
'First, trim back the top growth of the plant to around half its height using a pair of secateurs,' says Graham from LBS Horticulture. 'Then, cut through the thick, woody stems of the butterfly bush around 30cm above the ground, using loppers or a pruning saw. Prune just above a bud or growing shoot where possible, and leave the stems around 60cm long if you want the plant to grow taller.'
2. Remove damaged branches
Next, focus on any dead, damaged or diseased growth.
'Prune away any diseased branches to keep the plant thriving,' says Christopher from Gardens Revived.
Then, use a pair of loppers or a pruning saw to remove dead stubs and branches. 'These should be cut back so that they are flush with the main trunk of the plant to minimise the risk of dieback,' advises Graham from LBS Horticulture.
3. Final thinning
To finish up, you'll want to thin out any congested parts of the plant.
'Thin out any crossing branches to prevent rubbing, and remove twiggy growth that may be emerging from the base of the plant,' advises Graham.
Are there any exceptions?
Not all buddleja varieties benefit from early-spring pruning. 'The Buddleja alternifolia and Buddleja globosa varieties tend to bloom on stems that were grown in the previous season, so if these were cut back in spring, you would remove the flower buds,' says Graham from LBS Horticulture.
'Neither of these varieties should be hard pruned, and instead, you should cut out a selection of the oldest branches around mid-summer.'
Pruning in summer should be tackled with a feather-light touch. 'Trim back about one-third of the plant's growth to promote new growth and flowers,' Christopher advises.
FAQs
What happens if you don't prune buddleja?
Pruning is an important task in the garden, but why should you cut buddleja back? And what happens if you don't?
'If you don't prune buddleja, it will continue to grow and produce new growth, but it will do so by extending existing growth, making the plant tall and spindly,' explains Graham from LBS Horticulture. 'A lack of regular pruning may also cause the plant to become sparse and it may flower less.'
Does buddleja grow on old wood?
'Some varieties of buddleja do grow on old wood, such as Buddleja alternifolia or Buddleja globosa,' says Graham.
Now you know when to cut buddleja back, you can prune your way to vibrant displays year after year.
