What Determines Hydrangea Bloom Color? An expert breaks down the science

This is part of the story Home TipsCNET’s collection of practical advice for getting the most out of your home and inside.

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 vibrant pink, white and lilac flowers on nearly every front lawn.

My family also had a few bushes in the back lawn where the hydrangeas could enjoy lots of direct sunlight with scattered shades throughout the day. And while I loved the soft pink flowers on our hydrangeas, my mother would comment how they never bloomed into the bright blue flowers she would have liked.

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

To narrow down hydrangea colors, I spoke with Mal Condon, curator of hydrangea at the Heritage Museum and Gardens — or more aptly known as “the hydrangea guy” — to find out what makes hydrangea change color and get some tips. How to get the color you want.

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

What colors are possible?

CNET Home Tips logo

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

In his more than 50 years of working with hydrangeas, Condon is often asked why hydrangeas don’t bloom in the intended colors. Here’s what he had to say.

Does the color change?

Whether you want a particular color hydrangea – raspberry red or a brilliant blue – it’s really not up to you. Condon said it depends on the soil composition. In particular, it depends on the available aluminum in the soil.

Many sources state that hydrangea colors depend on soil pH, which is not the case absolutely the truth

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

Hydrangeas act as a sort of mood ring to tell you the soil conditions in your garden. Generally, too much aluminum will give you blue flowers, while soils with little or no aluminum will give you more pink or red blooms. Condon explains that to get blue flowers, you must have a 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 thrive in soil with a neutral pH between 6.0 and 6.2.

Can you change the color of your hydrangea?

Hydrangeas are unique in that, unlike many other plants or flower varieties, the color of their blooms can be changed with a small chemical.

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 says the best way to add aluminum sulfate to soil is to soak it, using a watering can with one tablespoon per gallon of water.

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

To get pink flowers, you can use 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 more pink.

The best practice when condoning hydrangea color change is to be patient—don’t be overzealous. It is recommended to add material to the soil only twice a year. “It’s not something you want to be crazy about,” he said.

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

More hacks for your yard and garden

Leave a Comment

Your email address will not be published.