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What Determines Hydrangea Bloom Color? An Expert Breaks Down the Science

Hint: The secret is in the soil.

High Angle View Of Blue Hydrangea Flowers Blooming At Park
You can find hydrangeas in shades of pink, red, blue, and purple -- but the colors aren't exactly what they seem.
Luca Goemans / EyeEm / Getty Images

This story is part of Home Tips, CNET's collection of practical advice for getting the most out of your home, inside and out.

Growing up in Eastern North Carolina, the cloud-like blooms of hydrangea shrubs 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 blooms in almost every front lawn. 

My family even had a few bushes in the back lawn where the hydrangeas could enjoy a lot of direct sunlight with pockets of shades scattered throughout the day. And while I loved the soft pink blooms on our hydrangeas, my mom would remark how they never bloomed bright blue like she intended them to.

This is a common mistake made by novice and seasoned gardeners alike. You probably assume that the blooms will surely look the same planted in your yard as they did at the nursery, right? Well, not necessarily when it comes to hydrangeas. There's a particularly scientific explanation as to why your hydrangeas might not achieve the color you want. 

To get the lowdown on hydrangea colors, I spoke to expert Mal Condon, curator of hydrangeas at Heritage Museums and Gardens -- or more aptly known as "the Hydrangea Guy" -- to find out what make hydrangeas change color and get a few tips on 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 blooms come in a variety of shapes, colors and sizes. While the most common colors are pink, blue and purple, hydrangea blooms can also be red, white and green. 

Over his 50 years of working with hydrangeas, Condon gets asked all the time about why hydrangeas aren't blooming in the colors intended. Here's what he has to say. 

What changes the colors?

While you might desire a specific color hydrangea -- a raspberry red or a brilliant blue -- it actually isn't up to you. Condon said it depends on the makeup of the soil. Specifically, it depends on the aluminum available in the soil. 

Many resources will say the hydrangea colors depend on soil pH, which isn't quite true. 

"Many talk about pH, and that is important, but the first requirement in the soil is you have to have aluminum," Condon said. "It's a strange thing because aluminum is toxic to most plants, but hydrangeas, particularly the macrophyllas and serratas, 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. Generally speaking, more aluminum will give you blue blooms, while soil with little to no aluminum will bloom more pink or red. Condon explains that to achieve blue blooms, you must have soil that is decidedly more acidic with a pH lower than 5.5. 

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

Can you change the color of your hydrangea?

Hydrangeas are unique in that, unlike most other plant or flower varieties, the color of their blooms can change with a little chemistry. 

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

"The reason to do this is because you can subject the plant to over-acidification," Condon said. "If we gave it dry aluminum sulfate or sulfur -- another good acidifier -- you can deter the plant's growth process, even kill the plant." 

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

Condon did say the best practice when altering 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 go nuts about," he said. 

For more hydrangea information, you can check out Condon's hydrangea care tips here

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