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French hydrangeas bloom on old wood

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By Jeneen Wiche

It seems that we have been spoiled: A decade of mostly mild winters has led us to believe that all those borderline hardy plants would never get knocked back by a cold winter.
Well, I have seen quite a few crape myrtles, figs and French hydrangeas that are struggling to come back on old wood.
Fear not, however, because these plants are root hardy and will sprout new growth from the roots.
It used to be that all of these plants were expected to die to the ground each year, but improvements in plant hardiness, placement in the garden and proper pruning have resulted in improved performance.
However, after the winter of 2013-2014, we have to reasonably admit the limitations of some plants.
Crape myrtles and figs will sprout and bloom on new growth but older varieties of French hydrangea need old wood to bloom on. Typically these get killed to the ground if temperatures consistently drop below 10 F like they did this winter.
Newer varieties like the popular “Endless Summer” and “Penny Mac,” among others, will bloom on new growth.
Hydrangea macrophylla got its common moniker because they were predominately cultivated in France starting in the early 20th century.  Hydrangea macrophylla prefer cool, moist and shady conditions.
Properly pruning and fertilizing hydrangeas will certainly factor into health and if you get summer blooms ... but you must first answer one question. What kind of hydrangea do you have?  Different species require different pruning techniques and timing is everything.
Newly planted H. macrophylla can go two years without pruning, but every year thereafter it is advisable.
Without pruning, the older growth will become leggy and the plant can easily out grow its boundaries if winters continue to be mild.
Once the blooms have faded, the safest bet is to prune the shoots that bear this year’s blooms.
These can be pruned all the way to the ground or leave two nodes above ground to encourage bushier growth.
If you take this approach, you will know for certain that you are removing the older wood and leaving the current season’s growth (those shoots that just produced foliage) which will bear next year’s blooms.
White cultivars of H. macrophylla are always white; however, the intermediate hues between pink and blue are determined by soil pH.
For blue, the soil needs to be more acidic with a pH between 5.5 and 6.5.
Add lime if you desire pink, maintaining a soil pH between 6.8 and 7, but be careful, too much lime causes an iron deficiency (chlorosis) which causes a yellowing, mottled effect on the leaves called mosaic.
To prevent this you can also fertilize Hydrangeas a couple times a year with a fertilizer formulated for acid-loving plants; there are some sea-based fertilizers that work well. Give them a shot of fertilizer now, another in late summer and again in early spring.
Not all hydrangeas bloom on old wood.
Hydrangea paniculata, or the peegee hydrangea is a very vigorous grower that blooms on the current season’s new growth.  Hydrangea arborescens, native to the eastern United States, also blooms on new growth.  “Annabelle,” probably the most popular and most dramatic hybrid of the native species, is what many refer to as a ‘snowball bush’ because of the mass of white flower ‘balls’ that cover the shrub from early summer until frost.
They are very easy to grow if you can provide a well-drained, shady location.
H. arborescens can be heavily pruned to the ground or lightly pruned, about one-third of the shoots, in late winter.  For maximum blooms I recommend pruning ‘Annabelle’ more dramatically.