Growing up in eastern North Carolina, the cloud-like blooms of hydrangeas were synonymous with the spring and summer months for me. I vividly remember driving around my neighborhood and seeing the bright pink, white, and purple flowers in nearly every front yard. My family even had a few bushes in our own yard, where the hydrangeas enjoyed plenty of direct sunlight and occasional spots of shade throughout the day. I loved the soft pink blooms of our hydrangeas, but my mother often commented that they never bloomed as bright blue as she would have liked.
This is a common mistake made by both novice and experienced gardeners. You probably assume that the flowers in your garden will look the same as they did in the nursery, right? Well, with hydrangeas, that’s not necessarily the case. There’s a particularly scientific explanation for why your hydrangeas may not reach the color you want.
To learn more about hydrangea colors, I spoke with expert Mal Condon, curator of hydrangeas at Heritage Museums and Gardens—also known as “the hydrangea guy”—to find out why hydrangeas change color and to get a few tips on how to actually get the bloom color you want. For more plant tips, check out “The Best Locations for Plants in Your Home” and “The Best Houseplants for Low Light, Clean Air, and Easy Care.”
Which flower colors are possible?
Hydrangea flowers come in a variety of shapes, colors, and sizes. The most common colors are pink, blue, and purple, but hydrangea flowers can also be red, white, and green.
During his more than 50 years of working with hydrangeas, Condon is often asked why hydrangeas don’t bloom in the colors they want. Here’s his answer.
What changes the flower colors?
Even if you want your hydrangea to be a certain color – raspberry red or bright blue – it’s not up to you. Condon said it depends on the soil conditions. In particular, it depends on the aluminum content of the soil.
Many sources say that the colors of hydrangeas depend on the pH of the soil, which is not the case. quite TRUE.
“A lot of people talk about pH, and that’s important, but the most important requirement for soil is that it contains aluminum,” Condon said. “It’s strange because aluminum is toxic to most plants, but hydrangeas, especially the macrophyllas and serratas, can tolerate a small amount of it and that’s what gives us the blue color.”
Hydrangeas act as a sort of mood ring, telling you the soil conditions in your garden. Generally speaking, the more aluminum, the bluer the flowers, while soil with little or no aluminum tends to bloom pink or red. Condon explains that for blue flowers, you need soil that is significantly more acidic, with a pH below 5.5.
Alkaline soil – with a pH of 7.0 or higher – produces pink and red flowers, while white hydrangeas bloom in soil with a neutral pH between 6.0 and 6.2.
Can you change the color of your hydrangea?
The special thing about hydrangeas is that, unlike most other plants and flowers, the color of their flowers can be changed with a little chemicals.
The easiest way to acidify your soil and turn the flowers blue is with aluminum sulfate, which is available at almost any garden center. Condon explained that the best way to add aluminum sulfate to the soil is to use it as a drench from a watering can, using one tablespoon per gallon of water.
“The reason for this is that it can expose the plant to acidification,” Condon said. “If we give it dry aluminum sulfate or sulfur – another good acidifier – it can hinder the plant’s growth process or even kill it.”
To get pink flowers, you can use a fertilizer high in phosphorus to prevent aluminum absorption, or garden lime, a completely natural plant supplement that increases the pH of the soil to make the hydrangeas pinker.
Condon said the best course of action when changing the color of hydrangeas is to be patient — don’t be overzealous. He recommends adding material to the soil only twice a year. “It’s nothing to go crazy over,” he said.
For more information on hydrangeas, check out Condon’s Hydrangea Care Tips here. You can also learn how to get rid of honeysuckle or read our more general gardening and lawn tips.
