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"Pruning Buddleia" by Sandy Feather

How and when to prune Butterfly Bush

Q. I have a rather large Butterfly Bush (Buddleia) in my garden that needs to be pruned. Should I have done that in the fall, or should I do it now? It was so beautiful last summer that I want to be sure I care for it properly so it is just as nice this year, since it attracts so many Butterflies!

A. Your butterfly bush (Buddleia davidii) is one of a number of plants that are classified as subshrubs. These are plants that are woody near the base of their stems, but produce herbaceous, rather than woody, new growth every year. They are more susceptible to freeze damage over the winter, and to frost damage as new growth begins in spring. It is not unusual to lose these plants when we have a hard frost after they start to push new growth.

Sandy's Garden

When to Prune a Butterfly Bush

Subshrubs should never be pruned in fall; allow them to stand until we are past danger of severe frost in late spring. Pruning in late summer or fall can push new growth that does not have time to harden off before winter. When that tender growth is killed by winter cold, subshrubs may not have the stored energy to push out new growth a second time, resulting in a dead plant. They also do not go completely dormant during winter as hardy, woody plants do; instead, they are quiescent or resting. This makes them able to respond to warm temperatures more quickly than true herbaceous or true woody plants.
  

Butterfly Bush 'Peacock'
'Peacock' Buddleia


 


That also means they can break dormancy too fast in spring, only to have a late frost kill the new growth. However, you cannot treat them exactly like herbaceous plants either. For example, most herbaceous plants can be cut back to the ground in fall or early spring. They respond by growing back from their roots. Subshrubs cannot be cut back to the ground because they do not break new growth from the older, woody stems that grow close to the ground. If they are cut back that severely, subshrubs usually die.

'Honeycomb' Butterfly Bush
'Honeycomb' Buddleia

When you prune subshrubs, it is best to cut back to an outward facing bud that shows signs of new growth. Those that bloom in early spring can be trimmed after the first flush of growth to remove tips that have been killed by winter cold. They can be pruned again when they finish blooming. Subshrubs that bloom in late summer can be pruned once in early spring. Do not prune them below five or six leaf nodes from the ground. Beyond that, you get into old wood that will not break new growth. Cutting the more vigorous subshrubs, such as butterfly bushes, close to the ground in late spring is a good way to control their size and to keep them blooming well in your garden.

'Potters Purple' Buddleia
'Potters Purple' Butterfly Bush

All of the subshrubs listed below are at their best in full sun and very well drained soil. Poor soil drainage - especially through the winter months is deadly to these plants.


In addition to the butterfly bush, some other common subshrubs grown in Pennsylvania include:

  • artemesia (Artemesia spp)
  • blue-mist shrub (Caryopteris x clandonensis)
  • St. Johnswort (Hypericum spp.)
  • lavender (Lavandula spp.)
  • ornamental oregano (Origanum rotundifolia, O. laevigatum)
  • common oregano (Origanum vulgare and cultivars)
  • Russian sage (Perovskia atriplicifolia)
  • rue (Ruta graveolens)
  • culinary sage (Salvia officinalis & cultivars)
  • lavender cotton (Santolina chamaecyparissus)
  • thyme (Thymus spp.)
      

MORE

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