In a chilly spring whenwe yearn for floral display, pots of vividly coloured hydrangeas in corner stores and supermarkets are an easy sale.Their round heads of deeply saturated blue- and pink-hued flowers last for weeks indoors and on doorsteps, and reassure us that summer will soon arrive. Now that summer is here, what happens to those colourful hydrangeas? Not wanting to discard the plants, they’re often set out into garden beds with the expectation of more blue and pink flowers to come. That might be a gardener’s first encounter with the complexities of cold hardiness zones and the soil science that controls hydrangea coloration. Cold hardiness for hydrangea shrubs is the first consideration.They are a large plant family with a wide spread of frost restrictions. The ‘corner store’ hydrangeas sold around spring holidays (Easter, Passover, Mother’s Day) are referred to as mop heads (Hydrangea macrophylla), and they have limited cold endurance in Canadian gardens. Mop heads bloomon old wood; that is, canes that are one year old. Their root systems and above-ground cane structure must survive the winter to put out the next season’s blooms. Canadian hardiness zone 6b is probably the coldest condition that they can tolerate (with protective bundling) and still preserve their blooming ability. In zone 6a, mop heads are root hardy, but their flower-carrying canes will die. Spring-purchased mop heads can be potted up into larger containers for summer growth, and over-wintered in a heated garage to preserve both their roots and canes. A blue-flowering mop head cousin is the lace cap hydrangea (such as ‘Nikko Blue’), most often purchased at garden centres.Their flower heads are broadly flat, with central clusters of tight fertile buds surrounded by a flamboyant ring of sterile florets. Lace caps have softer blue to mauve colour tones and are cold hardy only to zone 6a. Gardeners in suitably warmhardiness zones or with heated garages have cleared the first hurdle in keeping their corner store hydrangeas for another growing season. There is the story of a gardener who purchased a blue hydrangea, protected it over the winter in a garden bed, and the following summer − it bloomed pink. (If that happened to you, rest assured that you are not alone.) Mop head and lace cap hydrangeas are colour-variable and react to the chemistry of their soil. Acid soil will turn their petals blue; alkaline soil makes them pink. Soil acidity and alkalinity is measured on a scale of zero to 14, with 7 as the neutral mid-point at which acid turns to alkaline values.This is referred to as the Gardening The mystery of blue and pink hydrangeas Enjoying a Canadian summer at home is especially valuable this year. The home garden is a safe and welcoming refuge, where we can consider the finer points (and some mysteries) of plants we love – in particular, those blue hydrangeas. 58 | www.snowbirds.org
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