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Can you change the colors of hydrangea flowers? It depends on your type of hydrangea and your soil pH. We’ll explain how to change color from blue to pink or from pink to blue.
Which Hydrangeas Can Change Color?
It’s not every hydrangea that changes color. It’s the flowers of some Bigleaf hydrangeas (H. macrophylla)—especially Mophead and Lacecap types—and H. serrata cultivars whose colors change based on the pH of the soil.
Blues are best grown in acidic soil; pinks and reds do best in alkaline or neutral soil. In other words, acidic soils with a pH of less than 5.5 produce blue flowers; soils with a pH greater than 6.0 produce pink flowers.
White hydrangea color is not affected by the soil pH. The white flowers stay white (color can never be changed) and usually prefer the same conditions as the pinks and reds.
But the relationship between color and pH is more complex than just numbers on a scale. It is the availability of aluminum ions—and the degree to which a particular cultivar can absorb them—that influences color.
How Long Does the Color Change Take?
It is possible to change colors, but not instantaneously. Color correction takes weeks—even months—for the desired changes. It is easier to change blue flowers to pink than to change pink flowers to blue, and some cultivars are more prone to color variability than others.
We recommend waiting until the plant is at least 2 years old to give it time to recover from the shock of its original planting.
Have your soil tested for pH, then check with your local nursery for the recommended amount of aluminum sulfate and directions. Here are a few guidelines to follow, once you’ve given a plant time to recover from the shock of its original planting:
How to Change Hydrangea Color
Knowing all of the above, it’s fairly simple to increase acidity (for blue) or alkalinity (for pink):
From Pink to Blue
To increase acidity (for deeper-blue flowers): Apply a solution of 1⁄4 ounce aluminum sulfate per gallon of water three times per year. (Aluminum sulfate is a colorless salt obtained by the action of sulfuric acid on hydrated aluminum oxide. Buy in any garden center.) Soak the ground with the solution after the plant starts growing in the spring and repeat twice at 3- to 4-week intervals. Once a year, in the spring, apply 25-5-30 fertilizer, according to manufacturer’s instructions.
From Blue to Pink
To increase alkalinity (for changing blue flowers to pink): In the spring or fall, spread ground limestone (dolomitic lime) at a ratio of 4 pounds per 100 square feet and water it well. (Excessive alkalinity will cause chlorosis, or yellow leaves.) In the spring or fall, apply 25-10-10 fertilizer, according to the manufacturer’s instructions.
Hydrangea flowers will naturally fade and dry in the fall, often to a combination of pink and green or tan. This is simply the aging process, which cannot be reversed.
Hard water (water with a high mineral content) can also affect the flower color, turning blue flowers more pinkish, so use rainwater to water your hydrangeas, when possible.