Hydrangea pruning in the City of Toronto

(Question)

I planted a new hydrangea two years ago. It did not blossom the first year. This year is finally blossomed during late summer and was beautiful until the temperature dropped recently. Will you kindly advise me when and how the prune the plant which is now about 3 feet high.

(Answer)

This a common conundrum for gardeners, as there are different varieties of hydrangea that require different pruning tactics.

Perhaps the most common variety of hydrangea, ‘Annabelle’ hydrangea (Hydrangea arborescens ‘Annabelle’) is not too particular: it blooms on “new wood”, meaning the new growth produced by the current season. Some people like a clean look and cut it down in late fall/winter, and some leave the brown flowers on for a more natural look. Then they cut it down to healthy looking buds in the Spring.  ‘Annabelle’ hydrangea is easily identified by its round balls of greenish-white flowers and densely mounded shape. Flower sizes can vary both by variety and by cultural methods. For example, larger flowers are more likely to be produced on shrubs that are pruned down very hard, whereas larger and more naturally growing shrubs tend to have smaller flowers .

Another popular variety is Panicle hydrangea (Hydrangea paniculata), common varieties include: ‘Phantom’ and ‘Limelight’. These similarly bloom on new wood although gardeners often opt to keep the brown flowers over winter. Panicle hydrangeas generally get pruned much less drastically than ‘Annabelle’ as they can create interesting height and structure in the garden. Panicle hydrangeas can be large or small but tend to grow more upright, and have more open and lacy cone-shaped blossoms which can sometimes appear bi-coloured like ‘Pinky Winky’ (pink and cream or off-white) as well as one solid colour, typically cream or off-white such as ‘PeeGee’.

Mophead (H. macrophylla), lacecap, and oakleaf (H. quercifolia) hydrangeas bloom on old wood, and some of the newer cultivars such as ‘Endless Summer’ bloom both on old and on new wood. These are more frequently the category of hydrangeas that fail to bloom. That is because these shrubs are sometimes pruned too late after blooming (thereby cutting off next year’s buds), or may experience a very cold winter (which kills the flower buds for the next year). Typically, the pruning these should be restricted to maintenance pruning: i.e. pruning out only dead, crowded or unhealthy stems. It is best not to prune any of these until the Spring, when active growth begins again. It is sometimes possible to cover this category of hydrangeas to help them overwinter better.

Mopheads and lacecaps are typically the hydrangeas you will find at grocery stores and florists shops in Toronto. They bloom in stunning pink and blue balls or lacy caps, and have large, luscious leaves. Where winters are mild or in a sheltered spot, they can grow into spectacular garden specimens.

Oakleaf hydrangeas, similarly, can thrive or struggle as they are just on the edge of hardiness in Canadian Zone 6 (USDA Zone 5). They are easily distinguished from other hydrangeas by their unique, large, oak shaped leaves that turn beautiful shades of maroon and bronze in the fall. Their flowers are similar in appearance to panicle hydrangeas.

Finally there are new hybrids to confuse easy identification. For example, on the market now is a pink Hydrangea arborescens called ‘Invincibelle Spirit’ which somewhat resembles a mophead type in flower colour but nonetheless retain the overall original appearance of ‘Annabelle’. There are also dwarf varieties of panicle hydrangea like ‘Little Lamb’, which slightly resembles an ‘Annabelle’ but with more crushed and flattened cone shaped blooms and a low, mounded form.

A good rule of thumb for hydrangeas, if you truly cannot tell what you have, is to simply not prune it at all until the Spring, then cut it down to the healthiest buds that formed from the previous year. If it fails to bloom the next year anyways, try wrapping the shrub loosely in a couple of layers of burlap in late fall. I’ve made frames similar to the ones in the link below, and filled with dry, loose leaves as well as a couple of layers of burlap. If it then manages to bloom the next summer, you will have your answer as to whether it blooms on new or old wood.

This link to a previous Ask question, discusses winter kill as a possible reason for lack of bloom:

https://www.torontomastergardeners.ca/askagardener/hydrangea-blooming-growing-pruning/

These links provide more information on winterizing Hydrangeas:

https://www.hydrangeashydrangeas.com/winter_protect.html

https://www.finegardening.com/shelter-plants-winters-worst