I have a blue hydrangea that I planted last year that looks like it mostly died back. Did I do something wrong, and is there something I should do now to help it?

— Jennifer Duff, Waukegan

It is not unusual for bigleaf hydrangea (Hydrangea macrophylla) cultivars like your blue hydrangea to die back each year due to the cold. Simply cut back the dead stems to the point of new growth. Do not routinely prune them back hard in late winter or early spring as you might with ‘Annabelle' hydrangeas. Give the plants extra time in spring to leaf out before determining whether the stems are dead. You should be able to determine what is dead or alive by now. Try to cut the plant back evenly so that you do not leave any stems that are dramatically taller than others.

You do not say exactly which blue hydrangea you planted; type will affect your results. An older variety of hydrangea, ‘Nikko Blue,' is at the edge of its hardiness zone in the Chicago area. This hydrangea blooms on large buds formed on growth from the previous season. After a harsh Chicago winter, a large number of flower buds are usually lost with the flowering. Late spring frosts and deer browsing can also decrease flowers. If this blue hydrangea cultivar dies back to the ground, there will be no flowers that year. The hydrangea will re-sprout from the base and develop healthy green foliage but no flowers. From my experience, in most Chicago gardens, because of winter damage, it is unusual to get any flowers from the ‘Nikko Blue' hydrangea or any other cultivar that only flowers on old wood. The proper way to prune the ‘Nikko Blue' is to cut just below the flower clusters as the flowers are fading in late summer. Then in spring, prune just above the first set of buds if the plant has not died back to the ground.

There are new varieties of bigleaf hydrangeas (not all have blue flowers) that flower on old and new wood. These are more reliable in flowering. Some of the new cultivars are ‘Endless Summer,' ‘Twist-n-Shout' and ‘Blushing Bride.' The soil pH affects the flower color, with more blue in acidic soils and pink in slightly acidic to alkaline soils. Add elemental sulfur in spring and fall to lower the pH of your garden soil to promote the blue color. Your plant will also benefit from a light fertilization and mulching this spring. Then provide supplemental water during dry periods this summer as the plant is still establishing. In general, on the spectrum from easy to difficult, I find these plants more difficult to grow.

If you have plant or gardening questions, contact the Chicago Botanic Garden's Plant Information Service at 847-835-0972.

Tim Johnson is director of horticulture for the Chicago Botanic Garden in Glencoe.

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