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Soil content determines hydrangea bloom colors

Posted: June 21, 2014 - 11:11pm

Hydrangeas are a staple in most southern gardens. Bigleaf hydrangeas, also known as Japanese, French and snowball hydrangeas, bloom in colors of blue, pink or different shades inbetween. Did you know that the bloom color of hydrangea can be changed? Hydrangeas are unique in that their bloom color is based on the pH of the surrounding soil. If you are a regular reader of my articles, you know that the pH of soil is important for nutrient availability.

Most major plant nutrients are accessible to plants in the pH range of 6.0-6.5. A soil with a pH above this can inhibit a plant from absorbing the nutrients. We consider some of the nutrients unavailable to a plant because of the soil’s chemistry. Bloom color in hydrangeas is due to nutrient availability issues based on soil pH. The color variation is based on the presence or absence of aluminum compounds in the flowers. If aluminum is present in the flowers the bloom color is blue. If aluminum is absent, the flowers appear pink. If aluminum is present in small quantities, the bloom color varies between blue and pink. Soil pH indirectly changes bloom color in hydrangeas by affecting the availability of aluminum in the soil. If the soil is acidic, or has a pH of 5.5 or lower, then aluminum is more available to the roots, resulting in blue flowers.

Overall, Georgia soils tend to be more on the acidic side, so most of our hydrangeas are blue. As you move to soils which are more alkaline, those with a soil pH over 7.0, the availability of aluminum decreases and hydrangea blooms are pink. What if you have blue hydrangeas and would like them to have pink flowers? You can change the pH of your soil and gradually shift your hydrangea’s flower color. You will need to raise your pH, making your soil less acidic, by adding lime. Do this by adding one cup of dolomitic lime per 10 square feet and watering it into the soil. However, dolomitic lime is slow to act and may take up to a year to see any desired effects. Liquid soil drenches are a faster way to achieve the color change. Dissolve one tablespoon of hydrated lime into a gallon of water. Drench the soil around the plant three times during the spring with the lime solution. Take care not to let the hydrated lime touch the leaves as they may be damaged. Several brands of hydrated lime can be found at garden centers and hardware stores. All should work sufficiently.

Even though most of our soils are acidic, some alkaline soils do exist in Georgia home gardens. If you have pink hydrangeas and would prefer blue blooms, lowering the soil pH is your solution. Make up a soil drench using one tablespoon of aluminum sulfate dissolved in one gallon of water. Drench the soil around the plant three times in the spring. Also be sure to avoid the solution contacting the hydrangea leaves.

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