When to cut back buddleia – get the timing right with this expert advice

Q: I have recently planted a butterfly bush in my backyard and it's growing rapidly. When is the right time to prune it? It currently has beautiful flowers and is great for attracting butterflies, but I don't want it to get out of control.

A: Buddleias are extremely fast-growing shrubs, which is why pruning them is important. This is usually done by cutting the flowered shoots back to within 4in of the main framework, gardening expert John Negus explains. 

It's a job that – in most cases – is best reserved for spring, and it's essential to time it right. Otherwise, you can stunt their flowering potential, or make them more vulnerable to frost damage.

John Negus

John has been a garden journalist for over 50 years and regularly answers readers' questions in Amateur Gardening magazine. He has also written four books and has delivered many talks over the years on horticulture.

The right time to cut back your buddleia

Some perennials can be cut back in the fall to neaten up a garden ready for winter. However, don't be tempted to do this to your butterfly bush. This is because the loss of insulating branches and stored sugars may reduce their winter hardiness, explains Tony Avent, a plant expert and the CEO of Plant Delights Nursery in North Carolina.

'Generally, the best time to prune buddleia begins in early spring, but this will depend on the weather in your area and zone for that year,' explains Nikki Bruner of Perfect Plants nursery in Florida. 'A good rule of thumb is to use the average last frost date for the zone which is determined by the USDA,' she advises.

Keep an eye on the weather forecasts, too, particularly if you live in a colder zone. If frost is predicted in your area, then hold off on pruning until the weather warms up. Some gardeners will be able to commence pruning in March or even late February, whereas others may have to wait until early May.

'Feed shrubs with general-purpose fertilizer after pruning to encourage strong growth,' recommends John. Burpee's Organic Granular Food, from Amazon, is well-rated.

Nikki Bruner

With a passion for plants and expertise in marketing, Nikki combines her knowledge of the nursery industry with her creative thinking to promote and highlight the exceptional offerings of Perfect Plants Nursery. She is dedicated to ensuring customer satisfaction and providing valuable insights for plant lovers.

It's also important to note that the correct time to prune can also depend on the variety of buddleia that you're growing.

Most of the Buddleia davidii cultivars flower on new wood, so by pruning in spring you will encourage a bounty of new blossoms. 

However, some buddleia cultivars, including B. alternifolia and B. globosa, flower on the previous season's growth, so cutting back in spring would remove the flower buds. Therefore, for these exceptions, it's best to prune them in mid to late summer, after they've finished flowering. This is similar to pruning lilac and other flowering shrubs that grow on the previous year's growth.

When to prune buddleia in containers

As John Negus explains, pruning container-grown buddleia is the same as pruning larger varieties planted in the ground. Simply cut them back to nearly ground level or to a low framework of main stems in early March, or later in cooler areas to avoid late hard frosts, he says. 

Start feeding them with a balanced fertilizer when temperatures are a little warmer, and continue doing so throughout spring and summer, he advises. Ensure the size of their container is sufficient for their root run, keep it somewhere sunny, and remember to water them often, too, he says.

As mentioned above, the few exceptional varieties that only flower on old wood should be pruned in summer, instead.

When to deadhead buddleia

Although buddleia plants should be given a proper prune in spring (or summer, depending on the variety), deadheading is different. 

This is when you remove any faded blooms, and you can do this all through the flowering season to encourage more to grow. Other plants that benefit from deadheading include zinnias, dahlias, and coneflowers.

If you forget to prune your butterfly bush, it will still produce new growth. However, this will generally be by extending the existing growth, making for a tall and spindly plant, as John Negus points out. So, an annual trim is worth the effort if you want to keep these shrubs looking their best. Just remember to stay clear of the common pruning mistakes while you carry out the task.