My first question as Columbia County’s agriculture and natural resource agent came from a gardener from my hometown of Lincolnton, asking about her hydrangeas. She had planted pink hydrangeas in the spring, but now the blooms were blue. This knowledgeable gardener understood the importance of adding lime to the soil to influence pH level and thus the hydrangea’s color, but needed specific information about the kind of lime and method of application.
As Charles Phillips wrote in previous columns, the best gardening practice is to take a soil sample. A soil sample indicates the nutrients and pH level of the soil, eliminating the guesswork on how much fertilizer or lime is needed.
Let’s look at why pH is so important.
Most major plant nutrients are more available at a pH of 6 to 6.5. A pH that is too high or too low can cause a lack of nutrient uptake from the soil and manifests itself in a variety of ways. Hydrangeas exhibiting color changes in their blooms, for example.
Color variation in hydrangeas are due to the presence or absence of aluminum compounds in the flowers. If aluminum is present, the color is blue. If it is present in small quantities, the color is variable between pink and blue. If aluminum is absent, the flowers are pink.
Soil pH indirectly affects flower color by affecting the availability of aluminum in the soil. When the soil is acidic (pH 5.5 or lower), aluminum is more available to the roots. The flowers are blue. The majority of Georgia soils are acidic and therefore, the flower color likely will be blue. When the soil is alkaline (pH 7.0 or higher), the availability of aluminum is decreased. The flowers are more pink.
To return my gardener’s hydrangeas to their original pink color, I recommend broadcasting one cup of dolomitic lime per 10 square feet and watering it into the soil. It might take a year to see a noticeable change in flower color. A faster way to achieve a change is through liquid soil drenches. Dissolve one tablespoon of hydrated lime in one gallon of water. Drench the soil around the plant in March, April and May with the solution. Avoid getting the solution on the leaves, because damage will occur.
Dolomitic limes have several names, including Pennington, Greenacres, Lilly Miller and Green Earth. There are also several brands of hydrated lime on the market, including Hi-Yield, Bonide, Hoffman, and Lee’s. All are effective choices.
While Georgia homeowners will often encounter acidic soil, alkaline soils do exist. To gradually change flower color from pink to blue, broadcast ½ cup of wettable sulfur per 10 square feet and water it in. Again, a faster way is through liquid soil drenches. Dissolve one tablespoon of aluminum sulfate in one gallon of water and drench the soil around the plant in March, April and May. Avoid getting the solution on the leaves.
A general recommendation for gardeners who are not concerned with bloom color but need specifics for proper hydrangea planting this fall is to amend the soil in the projected planting area. Apply 50 pounds of a composted organic matter per 10 square feet and incorporate it thoroughly into the top 8 to 12 inches of soil with a tiller or shovel.
When planting, dig the hole two times larger than the root ball. Don’t plant too deep. Make certain the top of the root ball is level with or slightly above the soil surface. Water thoroughly after planting and apply 3 to 5 inches of mulch.
Hydrangeas respond to several light applications of fertilizer during the growing season. .
Hydrangeas are water-demanding plants and are best suited for the moderate water-use landscape zones.
Fellow gardeners of Columbia County, please remember that the Columbia County Extension office is now located at 6420 Pollards Pond Road in Appling. I can be contacted at (706) 541-4011 or firstname.lastname@example.org. Please keep the questions coming.