Extension Educator, Horticulture
As I mentioned in last week's column hydrangeas have many personalities. Each species and its cultivars have their own quirks of winter hardiness, plant size, leaf size, flower color, flower size, and flower shape. For those of us obsessed with hydrangea's bountiful flowers an important difference is whether the cultivar blooms on old wood (last year's growth), new wood (this year's growth), or both.
The first lesson in hydrangeas -- save the plant label. Few plants reward the organized gardener as much as hydrangeas. Their identification is crucial to their happiness in the landscape and to our happiness with them. The plant name can lead you to information about the care of your hydrangea and also what to expect in a floral show.
In last week's article I highlighted the common smooth hydrangea, Hydrangea arborescens. Its white mophead cultivars include 'Annabelle' and 'Grandiflora'. The saga of hydrangeas continues this week with bigleaf hydrangea, Hydrangea macrophylla. Of all the hydrangea species and cultivars, this one dominates the landscape and florist markets. The fantastic flowers of bigleaf may be pink, white or blue and mophead or lacecap depending on the cultivar. Some of the showy sepals of the flowers are frilly and serrated.
If we were a bit warmer we wouldn't have to think too much about hydrangea's winter hardiness and all cultivars would give us some measure of flower pleasure. Unfortunately some cultivars die to the ground most winters and, unless they are marketed as blooming on new wood, will give us lush leaves but little else. In addition hydrangeas are insomniacs. Much of their cold hardiness woes stem from their habit of waking early in spring and going to sleep late in fall.
'All Summer Beauty', 'Coerulea Lace', 'Decatur Blue', Endless Summer, 'Frillibert', 'Geisha Girl', 'Nikko Blue', 'Lilacina', 'Veitchii' (one of the best lacecaps), and 'White Wave' are listed as some of the most winter hardy cultivars of bigleaf.
'Bailmer' (trademarked as Endless Summer) flowers on old and new growth. In southern regions Endless Summer blooms twice, on old growth and again on new growth. Here I have seen more cases where the new growth is killed to the ground and flowers don't appear until later on new growth (about 10 weeks into the season).
Hydrangeas need plenty of moisture, high organic matter soils, afternoon shade, and with bigleaf hydrangea a bit of soil chemistry to get the desired flower color. Bigleaf hydrangea flowers have a chameleon nature. They don't change colors to hide, but change to be noticed. The flowers turn pink or blue and all tie-died shades in-between depending on the soil pH and therefore the available aluminum. Although the big white mopheads of smooth hydrangea can take on a slight pink color as the flowers age, they do not turn bright pink or blue no matter what the soil pH.
To get blue flowers in bigleaf hydrangea aluminum sulfate is used to lower the pH and add aluminum. Generally an application of acidifying fertilizer in spring is used, and then when flower buds become visible aluminum sulfate is added to the soil. Additional applications of aluminum can be made every two weeks for three more applications. Don't overdo the aluminum, however, since it can reach toxic levels that can stunt plants. Also add little phosphorus since it can tie up the aluminum even in acid soils. Or forget all this and just enjoy the natural pink colors produced in our soils.
Several other fine hydrangeas including oakleaf, panicled, and climbing are worth a place in the landscape.
Great resource - Hydrangeas for American Gardens by Michael Dirr; Timber Press 2004.