Q: I bought some beautiful blue mophead hydrangeas last year that I planted in my garden, but I was really disappointed to discover that the plant’s flowers are pink this year instead of blue, and there aren’t very many of them. What’s happened? Is there any way for me to fix it? FL, Co Laois
A: You’re not the first gardener to be flummoxed by the quixotic nature of certain varieties of this handsome, summer-autumn flowering deciduous shrub, whose sensitivity to soil pH can determine the colour of its flowers. Just like litmus paper, your mophead hydrangea flowers’ change from blue to pink is an indication that the plant is now growing in a strongly alkaline or limey soil with a pH above seven, whereas when you originally bought it in its pot, it was growing in a neutral or slightly acidic compost that produced blue flowers.
I’m afraid that it’s too late to change the colour of the flowers this summer. To encourage it to produce blue, rather than pink flowers next year, you’ll need to transplant it into an ericaceous bed, or into a very large pot or tub filled with ericaceous compost (a compost with a low pH suitable for acid-loving plants).
You’ll also need to make sure to water it with rainwater rather than tap-water as the latter will also almost certainly be on the hard side. If required, you can also encourage the production of blue flowers by using an aluminium-rich liquid plant feed designed for the purpose.
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Also bear in mind that hydrangeas need a sheltered spot away from cold temperatures and icy winter winds that could damage their nascent flower buds. Carefully timed pruning in spring to remove faded flowerheads is also important to encourage good flowering as is an organic mulch of well-rotted manure or garden compost and a scattering of slow-release organic fertliser around the base of the plant in spring.
These large, thirsty, decorative shrubs really hate to dry out so if you’re growing them in pots or tubs, keep them regularly well-watered and give established plants an extra boost in the shape of occasional liquid seaweed feeds in summer-autumn.
Just to add that not all hydrangeas respond to soil pH in this way. Worthy alternatives include Hydrangea arborescens “Annabelle”, an enduringly popular variety known for its white-lime-green flowers that slowly fade to creamy pink as they age, lime-green Hydrangea paniculata “Limelight” with flowers that fade from creamy lime-green to a fruity pink, and Hydrangea paniculata “Wim’s Red”, whose flowers change from cream to pale pink and then eventually to deep red as the season progresses.
