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Growing up in Eastern North Carolina, blooming hydrangeas became synonymous with the spring and summer months. I have vivid memories of driving through my neighborhood and seeing the vibrant pink, white and lilac blooms on almost every front lawn.

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

This is a common mistake made by both novice and experienced gardeners. You probably assume that flowers will certainly look the same planted in your yard as in the nursery, right? Well, not necessarily when it comes to hydrangeas. There is a particularly scientific explanation as to why your hydrangeas may not achieve the color you want.

To get the lowdown on hydrangea colors, I spoke with expert Mal Condon, curator of hydrangeas at the Heritage Museums and Gardens — or more aptly known as the “Hydrangea Guy” — to find out what makes hydrangeas tick. color and get some tips. how to actually get the color bloom 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 different shapes, colors and sizes. While the most common colors are pink, blue and purple, hydrangeas can also be red, white and green.

During his 50 years of working with hydrangeas, Condon is constantly asked why hydrangeas don’t bloom in their intended colors. Here’s what he has to say.

What changes colors?

While you may want a particular hydrangea color—a crimson red or a brilliant 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 sources will say that hydrangea colors depend on the pH of the soil, which is not the case enough true.

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

Hydrangeas act as a sort of mood ring to tell you the soil conditions of your garden. In general, more aluminum will give you blue blooms, while soil with little or no aluminum will give you more pink or red blooms. Condon explains that to achieve blue blooms, you need to have soil that is definitely more acidic with a pH lower than 5.5.

Alkaline soil — with a pH of 7.0 or higher — generates pink and red blooms, while white hydrangeas will thrive in soils with a neutral pH between 6.0 and 6.2.

Can you change the color of your hydrangeas?

Hydrangeas are unique in that, unlike most other varieties of plants or flowers, the color of their blooms can be changed with a little chemistry.

The easiest way to acidify your soil and turn those 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 a drink, using a watering can with one tablespoon per gallon of water.

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

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

Condon said the best practice when changing hydrangea color is to be patient — don’t be overzealous. He recommends adding materials to the soil only twice a year. “It’s not something you want to get crazy about,” he said.

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

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