Extension Educator, Horticulture
What started our obsession with hydrangeas is anyone's guess. Perhaps it was the foldouts of voluptuous flowers adorning tables in a decorator gardener's magazine. Perhaps it was the unforgettable appeal of a big and bold shrub as hard to ignore as a red sports car. Or perhaps it's their elusive nature to flower.
Hydrangeas have many faces. Numerous species of hydrangeas exist along with their own quirks of winter hardiness, flower color, flower size, and flower shape. In addition some of the species have hundreds of cultivars. The hydrangea confusion also stems from the stems. An important difference is whether the cultivar blooms on old wood (last year's growth), new wood (this year's growth) or both. The moral of the story - save the plant label. The name can lead you to information about the care of your hydrangea.
The hydrangea show is all about the flowers and how they hang together in clusters (inflorescence). The cluster may include small delicate fertile flowers, big and showy petal-like sepals and just about any combination of the two. Clusters of mostly big and showy sepals are called mopheads. This article highlights one of the most common white hydrangeas. Pink and blue flower lovers will have to catch next week's column.
Smooth hydrangea, Hydrangea arborescens, is a North American native of eastern forests which is a good clue to a happy home in the landscape. An aficionado of landscape hydrangeas would hardly recognize their wild side. As a native plant it has a delicate beauty. Landscape forms are about as far removed from delicate as Sylvester Stallone. The native version tends to have an open, loose habit with fewer leaves and smaller flower clusters of 3-4 inches wide. Wild hydrangea's kissing cousin cultivars have flower clusters up to 12 inches across.
Cultivated forms are full-bodied mounds of leaves and flowers. Some say too much of each for the landscape. Others marvel at its hard-to-ignore, robust, rotundness. I say that's why ice cream comes in more flavors than vanilla - everybody has different tastes.
The most common cultivars of smooth hydrangea are 'Annabelle' and 'Grandiflora'. 'Annabelle' is a true Illinoisan. Its name and subsequent rise to popularity were due to the efforts of the late, and truly great, University of Illinois plantsman, J.C. McDaniel. 'Annabelle's original home was a wooded area in Anna, Illinois.
'Annabelle' represents the quintessential picture of a mophead or snowball hydrangea. Flower buds are green maturing to white and then back to green. Her white symmetrical spheres can reach 12 inches across. Impressive for sure, but unfortunately massive flowers translates into a flattened shrub after a heavy rain. 'Annabelle' will probably need support to keep her upright.
A common hydrangea of old farmsteads is 'Grandiflora', also listed as Hills of Snow hydrangea. Once 'Annabelle' came along 'Grandiflora' was pushed aside. The individual showy flowers of 'Grandiflora' are larger than 'Annabelle' and flower two weeks earlier. However each cluster has fewer flowers held loose, lumpy clusters.
Smooth hydrangeas are reliable bloomers because they bloom on new wood. They will bloom even if a late frost or hard winter kills the stems. In late winter they can be pruned a couple different ways. One way is to remove the stems to the ground to produce large flower clusters. The more vigorous the growth of the shoot, the bigger the flower.
Another way is to remove only the old seed heads to maintain last year's stems. New growth from the old stems is not as vigorous and will yield smaller flower clusters. This is not necessarily bad. It's a good flower show, but the plants usually stay upright without the heavy larger flowers.
Great resource - Hydrangeas for American Gardens by Michael Dirr; Timber Press 2004.