Good morning, Bonnie, and thank you for your question to Ask a Master Gardener/Ask Extension. I will be happy to help you. You asked: “When and how should spirea, butterfly and hydrangea be trimmed back?”

Pruning spirea (Spiraea species):  Some spirea varieties bloom in the spring, while others bloom in the summer. Spring blooming spirea flowers on “old wood”, that is last years’ growth. It should be pruned after flowering in late spring or early summer. Summer blooming spirea flowers on new growth, so should be pruned before new growth emerges in the spring. In this “light pruning” deadwood is cut to the ground and the shrub can be shaped and shortened. If your spirea needs more severe pruning, sometimes called “hard pruning”, it should be done in late winter to early spring. In this case, spirea can be cut to six inches from the ground to rejuvenate; this should only be done every several years. Spirea can also be “deadheaded”, removing spent flowers and shaping the plant, which may encourage further blossoming, especially in summer-blooming spirea.

Pruning Butterfly Bush (Buddleja davidii):  There are two schools of thought as to pruning Butterfly Bush. The first espouses that they should not be pruned. The second that they should be pruned within one foot of the ground in early spring before new growth begins. Clicking on the embedded links will give more detail.

Pruning Hydrangea: (Hydrangea species):  There are several types of hydrangea; those that bloom on old wood (oakleaf, bigleaf, mountain and climbing) should be pruned after flowering. Those that bloom on new wood (smooth and panicle) should be pruned in late winter. Remontant or re-blooming varieties of mountain and bigleaf can be pruned in late winter, but it may reduce and/or delay blooming. This article has great information about pruning hydrangea.

How to prune: Pruning is an art. It is one that I am constantly trying to improve upon. This article has some helpful tips in the “Technique” section. I have also found “The Pruning Book” by Lee Reich to be an invaluable resource. Be sure to clean your pruning tools before you begin and as you move from plant to plant to avoid spreading disease. Isopropyl alcohol works well, as does a ten-minute soak in dish soap and water.

In preparing to answer your questions, I verified my responses with research-based sites. The best of those sites are embedded in the text above and/or listed in the Resources section below.

I wish you all the best in pruning your shrubs and hope that you have a bounty of blooms!

If you have further questions about this, or if you need clarification please contact me by responding to this email; it will come directly to me. I would love to hear from you!

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Resources:

Pruning Flowering Shrubs – Penn State Extension https://extension.psu.edu/pruning-flowering-shrubs

Spirea - Clemson Cooperative Extension https://hgic.clemson.edu/factsheet/spirea/

Butterfly Bush – Clemson Cooperative Extension https://hgic.clemson.edu/factsheet/butterfly-bush/

Butterfly Bush – Mississippi State Extension https://extension.msstate.edu/southern-gardening/audio/2019/butterfly-bush

History and Fun Facts: Butterfly Weed – The Ohio State University Extension https://plantpath.osu.edu/sites/plantpath/files/imce/images/NativePlants/ButterflyWeed.pdf

Pruning Hydrangea - University of Maryland Extension https://extension.umd.edu/resource/pruning-hydrangeas/

Pruning Hydrangeas for Best Bloom - University of Minnesota Extension https://extension.umn.edu/planting-and-growing-guides/pruning-hydrangeas-best-bloom

A Few Tips to Remember in Pruning Those Hydrangeas – The Ohio State University Extension https://u.osu.edu/thenews/2024/05/08/a-few-tips-to-remember-on-pruning-those-hydrangeas-2/

Pruning Deciduous Shrubs - University of New Hampshire Cooperative Extension https://ceinfo.unh.edu/resources/files/Resource000593_Rep615.pdf