What determines the color of the hydrangea bloom? An expert breaks down the science

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Growing up in eastern North Carolina, the cloud-like flowers of hydrangea bushes became synonymous with the spring and summer months. I have vivid memories of driving around my neighborhood and seeing the vibrant pink, white, and lilac flowers on almost every front lawn.

My family even had a few bushes in the back lawn where the hydrangeas could enjoy plenty of direct sunlight with scattered pockets of shade throughout the day. And while I loved the soft pink flowers on our hydrangeas, my mother commented that they never bloomed bright blue like she wanted them to.

This is a common mistake made by both novice and experienced gardeners. You’re probably assuming that the flowers will surely look the same planted in your garden as they did in the nursery, right? Well, not necessarily when it comes to hydrangeas. There is a particularly scientific explanation for why your hydrangeas may not be getting the color you want.

To learn about hydrangea colors, I spoke to expert Mal Condon, curator of hydrangea at Heritage Museums and Gardens, or more aptly known as “the hydrangea guy,” to find out what makes hydrangeas change color. and get some tips. on how to actually get the color you want.

Read also: This sweet flower is a secret garden killer. Here’s how to get rid of it

What colors are possible?

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Hydrangea flowers come in a variety of shapes, colors and sizes. Although the most common colors are pink, blue and purple, hydrangea flowers can also be red, white and green.

In his 50 years of working with hydrangeas, Condon is constantly asked why hydrangeas don’t bloom in the desired colors. This is what he has to say.

What changes colors?

While you may want a specific color hydrangea, raspberry red or bright blue, it’s really not up to you. Condon said it depends on the composition of the soil. Specifically, it depends on the available aluminum in the soil.

Many resources will say that the colors of hydrangeas are dependent on the pH of the soil, which it is not enough truth

“A lot of people talk about pH, and that’s important, but the first requirement in soil is that you have to have aluminum,” Condon said. “It’s a strange thing because aluminum is toxic to most plants, but hydrangeas, especially macrophyllas and serrates, tolerate a small amount and that’s what gives us the blue.”

Hydrangeas act as a sort of mood ring to let you know the soil conditions in your garden. Generally speaking, more aluminum will give you blue flowers, while soil with little or no aluminum will give you more pink or red blooms. Condon explains that to get blue blooms you need to have decidedly more acidic soil 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?

Hydrangeas are unique because, unlike most other varieties of plants or flowers, the color of their flowers can change with a little chemistry.

The easiest way to acidify the soil and turn these flowers blue is with aluminum sulfate, which can be found at almost any garden center. Condon explained that the best way to add aluminum sulfate to the soil is to apply it as water, using a watering can with one tablespoon per gallon of water.

“The reason for doing this is because you can subject the plant to excessive acidification,” Condon said. “If we give it dry aluminum sulfate or sulfur, another good acidifier, you can deter the growth process of the plant, even kill it.”

For pink flowers, you can apply a high-phosphorus fertilizer to discourage aluminum uptake, or Garden Lime, an all-natural plant supplement formulated to raise soil pH to make hydrangeas pinker.

Condon said the best practice when altering the color of hydrangeas is to be patient — don’t be too enthusiastic. He recommends adding materials to the soil only twice a year. “It’s not something you want to go crazy over,” he said.

For more information on hydrangeas, you can check out Condon’s hydrangea care tips here.

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