Friday, July 8, 2011

Hard Lessons

'Nikko Blue'  mingling with agastache.
Pruning hydrangeas can be tricky business.  Those blooming on new wood, like the native hydrangea arborescens, are pretty foolproof.  Cut almost to the ground each winter, they grow back and bloom even bigger than with no pruning.

However, my 'Nikko Blue' mopheads (Hydrangea macrophylla) have caused me much angst in the past.  So much conflicting advice out there:  "prune them right after blooming",  "prune them in winter",  "don't prune them at all".  I've tried each of these with poor results.  Some years, like last year,  I had no blooms at all.   I had such envy driving by gardens where the Nikko Blues were loaded.

Planted next to rear windows, mine needed some pruning each year.  So I read more on pruning, talked to more successful gardeners, and thought about my own experiences.  Then a light went on.

One year, I just didn't get around to pruning them until early March when the late snow melted.  Discovering the top buds enlarging,  I rushed out and pruned, hoping I hadn't waited too late.  That year I had the most spectacular display of blooms.

This year, I patiently waited for March to arrive before grabbing my Felco #11 pruners.  After the trim, I added some Plant-tone fertilizer for added insurance.  Spread a little ammonium sulfate to intensify the blue color (if they bloomed).  And after all that, I still wasn't sure what the outcome would be.

So when late May rolled around and I saw tons of flower buds forming on my Nikko Blues, I did a little happy dance.  I still can't believe my outcome.  And the blooms keep coming.


I can't say this lesson applies to every garden in every region.  But I did find some rational explanations to my success this year.  Michael Dirr, in his beautiful book, Hydrangeas for American Gardens, mentioned that hydrangea macrophylla flower buds develop on old wood of the previous year.  However, flower buds are often present along the entire length of the stem.  When the tops are cut, lower buds are called into action.  

Another source explained why August and September pruning failed.  Before winter dormancy, pruning  simulates new growth that uses up energy reserves needed to set next year's flower buds.

I may not have formal training, but when it comes to hydrangea macrophylla,  the school of hard knocks has given me a pretty good education.

Small additions of ammonium sulfate produce a pleasing blue.

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