Q: Is it true that you can change the flower color of hydrangeas? I have a pink hydrangea in a large flowerpot on the patio, but I would like to dye it blue – how do I do that?
A: The coloring of pink flowers into blue and vice versa is actually possible with some hydrangea varieties: Hydrangea macrophylla And Hydrangea serrataThis seemingly magical transformation depends entirely on the type of soil the plants are growing in and can be controlled by amateur gardeners with a green thumb, a little know-how and patience.
Below, gardening experts explain the science behind this fascinating phenomenon and offer tips on how you can try it yourself when growing hydrangeas at home.
Expert tips for changing hydrangea color
Whether you’re growing hydrangeas in pots or in the ground, these tips will help you maintain or change their color.
Why hydrangea flowers can change color
Hydrangea flowers are pink or blue depending on the aluminum content in the soil, explains Kristen Pullen of Star® Roses and Plants. The pH of the soil plays a big role, as aluminum ions are only mobile (and available) under acidic conditions.
“For hydrangeas to turn blue, there must be both mobile aluminum and acidic soil,” she says. If that’s the goal, aim for a pH of 6.0 or less.
On the other hand, a pH above 7.0 means the soil is alkaline and will produce pink or reddish flowers. If the pH is somewhere in between, the flowers will likely be purple or a mix of blue and pink.
How to change the color of your hydrangea flowers
“A soil test is a good place to start if you want to change the color of your hydrangea flowers,” says Anna Ohler, owner of Bright Lane Gardens. Soil pH testers are available on Amazon – this one also measures moisture and light levels.
Once you know where your floor is on the scale, you can take steps to adjust it to the color you want.
Remember that if you want blue flowers, you need to make sure the soil is acidic. Kirsten says, “The best soil acidifier for this is a product that contains aluminum sulfate. It lowers the soil pH and adds aluminum.” You can also lower the soil pH by adding garden sulfur, Anna notes.
Niwar Nasim, president of Nasim Landscape, also recommends using an organic, acidic mulch, such as pine needles—”a great way to keep the pH low for extended periods of time.” If you’re growing your flowering shrub in a pot, using a potting mix for acid-loving plants, like this one from Coast of Maine on Amazon, is also a good way to keep the blooms blue.
For pink flowers, Anna recommends working garden lime into the soil to raise the pH. This garden lime from Jobe’s Organics on Amazon is a well-reviewed choice.
This product can be used on all types of soil to lower the pH and help hydrangeas turn blue. It can also be used when growing acid-loving plants such as azaleas.
How long does it take for hydrangea flowers to change color?
The color change of your hydrangeas does not happen overnight – it can take several months.
Anna Ohler says, “In some areas, the soil can be highly acidic or highly alkaline. In such areas, it can be difficult to change the color of your hydrangeas and it may take several seasons to successfully adjust the pH of your soil.”
“It’s important to test your soil pH regularly to ensure it stays in the right range for the desired hydrangea color,” adds Niwar.
Does the color of white hydrangeas change depending on the pH of the soil?
White hydrangeas such as ‘Annabelle’ are becoming increasingly popular and look fantastic in both modern and cottage gardens. Their colour does not change depending on the pH of the soil. However, the flowers of some varieties, such as the Bobo® panicle hydrangea from Nature Hills, take on a pink hue as they age.
Need more advice on these flowering beauties? Our guides on pruning and watering hydrangeas are full of practical tips.
