Change the Bloom Color of Your Hydrangeas

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By Skip Richter

The vivid blues and pinks of hydrangeas announce the summer season in the South just like watermelon and lemonade. This popular plant of the old Southern gardens is a native of China and but thrives in our region if provided with the right growing conditions.

A staple of the traditional garden, hydrangeas prefer bright shade and thrive in a shaded northern exposure. They do not like soggy soil conditions but insist on consistent moisture -- otherwise they'll wilt like a popsicle on the noonday asphalt. Provide your shrub with soil amended with compost or peat moss and a surface mulch of pine needles or bark, and it will return the favors with a summer full of beautiful blooms.

The common bigleaf hydrangea, H. macrophylla, will indicate just how acid or alkaline your garden soil is by the color of its blooms. Acid soils produce blue blooms, whereas less acidic soils assure the gardener of pink flowers. Sometimes in an irregular soil with a variation of pH you will see varying shades of pink and blue on the same plant or within the same planting. City water supplies are often alkaline. With frequent watering, the soil becomes more alkaline, and the blooms brighter shades of pink.

Changing Bloom Color

The mechanism of color variation in hydrangeas is the presence or absence of aluminum in the blooms. When aluminum is present the blooms tend to be blue. When it is not, they tend toward pink. Soil pH affects bloom color by making soil aluminum more (low pH or acidic soil) or less (higher pH or basic soil) available for uptake by the plant. To change the bloom color from pink to blue, add one-half cup of sulfur per 10 square feet, mix it into the soil surface, and water it in well. To change the soil the blooms from blue to pink, instead of sulfur, mix in 1 cup of lime per 10 square feet. This procedure can take months to a year to change the soil pH and thus the bloom color.

For a faster reaction when changing from pink to blue, dissolve 1 tablespoon of alum (aluminum sulfate) in a gallon of water and drench the soil around the plants thoroughly three times, four weeks apart in spring. To rapidly change from blue to pink substitute 1 tablespoon of hydrated lime for the alum. Always apply these solutions to the soil, not to the blooms or foliage, or damage may result.

The best time to prune hydrangeas is immediately after their peak of bloom in summer. Thin out old, non-productive canes, and prune other canes to reduce height if needed, but keep some terminal shoots for next summer's blooms. If a bitter winter results in damaged canes a little clean up pruning is fine in early spring, but don't overdo it.

Skip Richter was National Gardening Association's reporter for the Lower South region. 

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