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Close your eyes and think of a country garden - a traditional, chocolate-box affair without any hint of timber decking nor so much as a single tree fern. Are there great mopheads by the kitchen door? Naturally. For no country garden would be complete without its hydrangeas.

Blue hydrangeas, at that. And not just blue, but several blues, all powder pastel shades that merge dreamily one into the other like so many patches of summer sky. In reality, of course, your hydrangeas are as likely to turn out pink or white rather than any of the colours that paint manufacturers call "hydrangea blue". As generations of gardeners have known, it's all down to iron filings in the soil. Or aluminium salts. Or rusty nails. Or chalk. OrI "Blue hydrangeas are much admired," says Beeton's Dictionary of Everyday Gardening, published as a companion to the better-known cookery book."It is some peculiarity in soil and situation which produces this variety." One way of "procuring" blue flowers, the authors suggest, is "planting in strong loam and watering freely with soapsuds".

By the 1930s, technology had caught up with the hydrangea."Where it is desired to change the colour from pink to blue," says The New Illustrated Universal Reference Book,"special blueing powders are used. These are sold by all nurseries."

Whether or not wartime shortages swept blueing powders off the nursery shelves isn't clear, but a few years later the Daily Express was advising readers simply to water hydrangeas with iron sulphate.

In recent years, advice has tended to focus on soil acidity, the rule of thumb being that alkaline soils produce pinker flowers while acid soils produce bluer ones. So what happened to those iron filings?

In fact, the colour of a hydrangea depends on a combination of factors, including the variety in question and the age of the plant. Blue flowers require both iron and aluminium to be available, but the plant can only take up these elements in an acid soil.

That's the science bit. Now close your eyes again, and think of that perfect country garden.

David Newnham

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