Pruning Bigleaf Hydrangeas

Hydrangeas are the most googled shrub on the internet. And it seems that blue or pink mophead Hydrangeas (H. macrophylla) cause the most swooning. Except, that is, in colder zones where frustrated gardeners stare at large leafy shrubs with few or no flowers.

Pruning at the wrong time could be the culprit (if pruning to control size is needed, do so in August), but typically winter/early spring weather is to blame. This is because all bigleaf Hydrangeas form their flower buds on old wood (like Forsythia, Rhododendron, Magnolia). If a winter is especially cold (including wind chill factors), coupled with little snow protection, many Hydrangeas experience severe stem dieback, taking flower buds with them. Another cause of poor flowering is wild temperatures swings. There will be a period of unseasonably warm temperatures followed by a cold snap. Dormant flower buds are fooled into breaking dormancy and then destroyed by a ‘big chill’. If a Hydrangea flowers on both old AND new wood (like Endless Summer), then a second round of flower buds should form on new wood for flowers later that season.

I have adopted a new approach to growing bigleaf Hydrangeas in my zone 5 Maine garden. First of all, I only grow those that bloom on new and old wood. After years of staring at tall brown sticks emerging from green leaves, and hoping that these sticks would eventually leaf out, bursting forth in bloom, I now whack these ‘sticks’ back to the ground sometime in May. I simply wait for the second round of flowers (those formed on new wood), sparing myself months of looking at ‘stick shrubs’.

I have also made the decision to grow mostly Mountain Hydrangea (Hydrangea serrata). This species also has mophead or lacecap flowers that can be blue or pink, BUT the species tends to have better winter bud hardiness.

NOTE:I took both photos within a week of each other. The top Hydrangea was at the U.S. Botanic Garden in Washington D.C and the bottom was near my home in Kennebunk, ME