Q. My butterfly bush (Buddleia spp. ) pushed new growth early due to the warm weather and that has been killed by frost. Should I cut it back now, or should I wait to see if it starts to grow again?
A. While people were enjoying the much-above-average temperatures in mid-March, many plants joined in the fun and started growing far earlier than normal. It was not a big surprise that normal March weather returned, including a few heavy frosts that damaged such tender new growth. While most plants will bounce back with no problem, some may not be so lucky, especially plants such as butterfly bush 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 annually. They are more susceptible to freeze damage over the winter and to frost damage as new growth begins in spring.
Subshrubs do not go completely dormant in winter as hardy woody or herbaceous plants do; instead they are quiescent or resting. This allows them to respond to warm temperatures more quickly than other plants. Sometimes they do not have the stored energy to push new growth a second time if the first flush is killed. It is not unusual to lose these plants when we have a hard frost after they push new growth in spring.
Avoid the temptation to prune your butterfly bush back until we are past danger of severe frost, usually mid-May. In more "normal" spring weather, we would still recommend not pruning subshrubs back until late spring because pruning pushes new growth. That is why it is not a good idea to prune them in fall, either. The new growth might not harden off in time for winter.
Be careful when you prune it to not go lower than five or six leaf buds from the ground. Beyond that, you get into old wood that will not break new growth.
In addition to butterfly bush, other common subshrubs include:
• Root beer hyssop (Agastache rupestris)
• Artemesia (Artemesia spp.)
• Blue-mist shrub (Caryopteris spp.)
• St. Johnswort (Hypericum spp.)
• Lavender (Lavandula spp.)
• Ornamental oregano (Origanum rotundifolium, O. laevigatum)
• Common oregano (Origanum vulgare and cultivars)
• Russian sage (Perovskia atriplicifolia)
• Rue (Ruta graveolens)
• Culinary sage (Salvia officinalis and cultivars)
• Lavender cotton (Santolina chamaecyparissus)
• Thyme (Thymus spp.)
Send questions to Sandy Feather by e-mail at email@example.com or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.