The blue hydrangea has something of a fascination, as urban legend would have it that it’s hard to obtain. It’s actually more or less true in that the soil composition will determine whether your hydrangea’s petals turn blue… or not. You just finished pruning a hydrangea and you’re wondering how to make it blue at the next bloom? You’re in the right place!
Why does a blue hydrangea exist?
There isn’t, strictly speaking, a white hydrangea, a pink hydrangea, or a blue hydrangea. It’s mainly about hydrangeas that grow in different soils, even if some varieties stay white or pink no matter what. There’s a nuance: those that become the easiest to blue are Japanese hydrangeas (Hydrangea serrata) and Hydrangea macrophylla.
Hydrangeas differ from one another far more by their growth habit, such as with the climbing hydrangea.
Thus, whether you grow the hydrangea in a pot or in the ground, it’s the soil composition and, specifically, its acidity that will define the color of its petals. The more acidic the soil, the more likely your hydrangea will be to turn a vivid blue.
Your first goal to obtain a blue hydrangea will therefore be to determine the soil pH!
How to make a hydrangea blue
You’ve probably gathered that if you don’t live in a region with naturally acidic soil below a pH of about 6, you’ll need to take action with amendments. Don’t rely solely on peat soil; while it can help guide things, it can also dry out your substrate too much.
White vinegar
It’s not the most ideal solution, as it requires a bit of finesse and can pose a risk if overused over time or at the moment of watering. Plan to dilute about 1/4 cup (60 ml) of white vinegar per liter of water, and you’ll be set.
Crushed slate
The result is a slightly more unpredictable outcome, but if you dose it, you’ll also have an aesthetically pleasing mulch that will complement your hydrangeas. Apply it twice a year to ensure such mulch influences the soil beneath.
Market products
There are several products specifically designed to blue hydrangeas, all primarily composed of aluminum sulfate, sometimes called the “Brittany blue.” The method is fairly effective but requires watering at the right moment, namely when the first flower buds appear. Always follow the directions on the packaging, as they can vary by product.
Pay attention to your watering water
If your soil is at the right pH but you water with limestone (hard) water, the result won’t be… blue. The best solution for your watering, and this holds true for turning a hydrangea blue or for any other reason, is to use rainwater. This way you won’t risk altering your soil’s composition.
by Yves-Marie Allain
The hydrangea is indeed an exotic plant… hailing from Japan. A houseplant and greenhouse plant, it becomes a permanent garden staple, starting pink, until we discover why and how to make it blue!
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