Lee Valley Tools    Gardening Newsletter
   Vol. 6, Issue 4
    August 2011
   The World of Hydrangeas

This versatile plant is available in varieties to fit many gardening needs. Some of the most popular species are outlined below.

The Mophead Hydrangea (Hydrangea macrophylla)
With its lovely dark-green, glossy leaves and huge blue or pink flowers that tend toward a rounded shape, this hydrangea is the one most people recognize. While common, it's actually one of the trickier species to grow, particularly in cold climates. Winter protection is necessary to ensure its survival and to prevent winter dieback that hinders flowering. The mophead flowers almost exclusively on old wood, so cutting back should be avoided.
Blue mophead hydrangea
Blue mophead hydrangea blooms
Pink mophead hydrangea
Pink mophead hydrangea blooms

Perhaps the most interesting thing about the mophead is that its flower color varies depending on the plant's uptake of aluminum, which is largely dependent on the soil's acidity level. Hydrangeas draws aluminum more readily from soil that has a lower pH level (i.e. more acidic soil). When planted in such soil, blue flowers result. The soil where I live tends to be very alkaline. This means the shrub's aluminum intake is less and the resulting flowers are pink. It's possible to change the soil's acidity level in localized areas, and I have even seen these plants flowering blue on one side and pink on the other, with every shade of purple in between!

To make your soil more acidic, and your flowers blue, there are a few things you can do. Keep in mind that neighboring plants may not take kindly to the change in acidity level. For a quick fix, mix 1/2 oz (1 tbsp.) of aluminum sulphate with 1 gallon (3.8l) of water and apply it to your plant's base. This will need to be done regularly throughout the growing season to maintain the soil's pH. (Be cautious, as applying too much can burn the roots.) You can increase the acidity more slowly by adding used coffee grounds and tea leaves to the soil at the plant's base every day, or by working acidic compost, such as pine needles, into the soil regularly. While effective, this method can take a long time to work. To make your soil less acidic (and your flowers pink), add dolomitic lime to it several times per season. Both aluminum sulphate and dolomitic lime should be available at your local garden center.

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