Key Takeaways
Bloom color is generally controlled by soil pH and aluminum availability.Only certain hydrangeas can change color, and some cultivars will stay white no matter what you do to the soil.Color shifts happen slowly, often taking a full season or more.
Hydrangeas are one of those hardy perennials that can feel almost magical: One year, your neighbor has a surge of sky-blue blossoms, the next they’re a blushing pink. But hydrangeas changing colors isn’t random—it’s some basic chemistry at work in the soil (think back to that pH experiment in science class).
By making a few easy soil adjustments, you may be able to influence whether your hydrangeas lean blue, pink, or somewhere in between. To explain the method behind it, we turned to horticulture experts who specialize in soil health and bloom behavior. Here’s how to give your blooms the best chance at a color change.
How Soil Affects Hydrangea Color
Some hydrangeas can morph colors from season to season based on the soil composition and acidity. “The main factor in changing the color of hydrangeas is the pH of the soil,” says Laura Janney, garden expert and the founder of The Inspired Garden Masterclass. “The lower the number, the higher the acidity (pH at or lower than 6), the bluer your blooms will be.” The opposite is also true: A higher pH and more alkaline soil will shift the color more toward pink.
First, you need to identify the hydrangea variety in question. Janney says you won’t be able to change the color of smooth, panicle, or oakleaf hydrangeas. Instead, you’ll need bigleaf or mountain hydrangeas, which are the only type capable of this color change, says Lorraine Ballato, a resident hydrangea expert at the New York Botanical Garden and the author of Success With Hydrangeas. But there’s a caveat: “Even within that parameter, some of these varieties are white and will stay white no matter what a gardener does. So it’s important to know if your hydrangea is pH sensitive.”
If that wasn’t complicated enough, Ballato says some of the newer bigleaf cultivars have been bred to lean either blue or pink, so altering the soil won’t do much to change their color. “Again, it’s critical to know if your plant’s pH will affect the flower color.”
Start by using varieties that are more known to change color easily, including Nantucket, Endless Summer, and Blue Enchantress, says Janney.
Turning a Hydrangea Blue
Hydrangeas can turn blue with acidic soil, but Janney says you also need to make sure aluminum is present in the soil. (Lowering pH alone won’t shift the hue unless aluminum is available to the roots.)
“Your first step should be to test the pH and the aluminum level of your soil, especially if you’ve never planted hydrangeas in your garden before,” says Janney. “I recommend using a fertilizer like a soil acidifier, which can help turn hydrangeas from pink to blue. You can also try leftover coffee grounds.” With this method, Janney says to add coffee grounds to your soil a few months before the blooming season begins, like in the late fall.
Unfortunately, coffee grounds won’t add the required aluminum—they just make the soil slightly more acidic and favorable. For more reliable results, Ballato says to amend your soil with aluminum sulfate.
Turning a Hydrangea Pink
It can feel like a balancing act to get perfectly pink hydrangeas, and unfortunately, one wrong ingredient commonly found in fertilizers can undo your efforts. “To turn hydrangeas pink, you need to increase the alkaline levels of the soil by adding dolomitic lime around the base of the plant,” says Janney.
Both experts say you shouldn’t add phosphorus to the soil. “When you start to mess around with high phosphorus fertilizers, you run the risk of putting other nutrients out of balance,” says Ballato.
Target pH Levels to Alter Color
Despite your best efforts, this pH and color change is usually not permanent. “Soil will always revert to its natural state, so testing pH and amending on a regular basis will be required,” says Ballato. “How much to add depends on the results of your soil test and the difference between the desired reading and the current reading. One size does not fit all.”
For a quick reference, Ballato shared the recommended pH readings for a range of hydrangea flower colors:
Soil pH
Flower Color
4.5
Deep, vivid blue
5.0
Medium blue
5.5
Lavender purple
6.0
Purplish pink
6.5
Mauve pink
6.8
Medium pink
7.0
Deep, vivid pink
Other Tips for Changing Hydrangea Color
Changing hydrangea color can be tricky, especially if you expect results in just a few weeks. To give your plants the best chance, here are a few other expert-approved tips.
Be Patient
“None of the above practices happen overnight. Have patience,” says Janney. “You should start amending the soil in the fall and then again in early spring. It can take months and up to a full growing season, and sometimes longer.”
You may also need to adjust your expectations. Janney says the color change can start off gradual, but, in theory, it should be more dramatic the following season. “A lot depends on the age and health of the plant. Healthier plants will be quicker to adapt. Also, going from blue to pink is generally quicker and easier.”
Try Rose Food
The best fertilizer for any hydrangea is rose food or a granular shrub product, says Ballato. “I know it sounds strange, but rose food is ideal (actually for most shrubs except those that prefer acid soils, like azaleas, camellias, etc.). And I don’t recommend using Holly-Tone [an organic fertilizer for acid-loving plants] unless the soil test indicates a need for more acidic soil.”
Switch to White Hydrangeas
If you tend to be a perfectionist in the garden, pink or blue hydrangeas may not be the best choice for you. “I’m personally not a believer in trying to change the color of your hydrangeas because it’s actually more difficult to control than people think,” says Janney. “If you are going to be disappointed in not having perfectly Nantucket Blue, I encourage people to stick with white hydrangeas.”
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