How to Keep Hydrangeas Blue

Bigleaf hydrangea (Hydrangea macrophylla), a deciduous shrub that is hardy in U.S. Department of Agriculture plant hardiness zones 6 through 10, produces large, rounded clusters of flowers that range in color from pale green or white to pink or blue. The color of bigleaf hydrangea's flowers is influenced by the pH level of the soil in which the shrub grows; the flowers tend to be green or white when soil pH is neutral, pink when soil is alkaline and blue when soil is acidic.

Gardeners who wish to use natural or organic methods to produce blue blooms in their bigleaf hydrangea can add amendments to the soil that will gradually lower, or acidify, the soil's pH level and may, over time, result in blue flowers. Adding acidic liquid such as lemon juice or vinegar to irrigation water or pouring it directly on the soil may help, and mulching with coffee grounds, citrus peels or pine needles will also slowly lower the soil's pH level. These amendments have only a gradual effect on soil pH over a period of months or years, however, and do not immediately change the color of the flowers.

Bigleaf hydrangea's flower color is not directly determined by soil pH. Rather, its flower color is the result of the amount of aluminum available in the soil; the more aluminum the plant is able to take up, the bluer the flowers will be. Aluminum is more readily available to the plant when soil pH is acidic; so acid soil usually produces blue flowers.

Because of this chemical relationship, one reliable way to produce blue flowers is to add aluminum sulfate to the soil. Add 1 tablespoon of aluminum sulfate to 1 gallon of water, and water the plant with the solution several months before bloom time.

The addition of too much aluminum to the soil can be toxic to the plant, though. So a more benign alternative is to sprinkle 1/2 cup of wettable sulfur around the base of the plant a few months before bloom time to lower the pH of the soil.

Some types of bigleaf hydrangeas will not produce blue flowers even when grown in acidic soil. Bigleaf hydrangea cultivars such as 'Alpengluhen,' 'Pia' and 'Kardinal' produce purple-pink flowers in acidic soil and deeper pink flowers in alkaline soil. Species such as oakleaf hydrangea (Hydrangea quercifolia), the 'Alice' cultivar of which is hardy in USDA zones 5 through 9, and panicle hydrangea (Hydrangea paniculata), which is hardy in USDA zones 4 through 7, produce white flowers that may turn pink as they age but are never blue.