If you’ve ever admired those stunning blue hydrangeas, you’re not alone! I’ve always been drawn to the elegance of their deep blue blooms, but I was curious—how can you achieve that perfect shade of blue in your own garden? It turns out, with the right conditions, it’s actually possible to turn your hydrangeas blue without much fuss.
In this guide, I’ll walk you through everything you need to know—from choosing the right variety to preparing your soil and using alum to get those gorgeous blue flowers. With a little bit of effort, you can have the blue hydrangeas you’ve always dreamed of. Let’s get started!
Blue hydrangea blooms, particularly on Hydrangea macrophylla (mophead hydrangeas), look incredibly elegant. However, these blue flowers will only appear if the right conditions are met. Here’s how you can turn your hydrangeas blue.
The key mineral responsible for the blue blooms is alum. Alum is an aluminum salt (aluminum sulfate) that typically contains aluminum ions, sulfate, potassium, and ammonium, a nitrogen compound. While all of these components are essential plant nutrients, the blue color of the blooms is caused exclusively by the aluminum ions.
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Not All Hydrangeas Can Turn Blue
However, alum can’t work miracles: For your Hydrangea macrophylla blooms to turn blue, you need to start with a variety that is capable of changing color. Most pale pink varieties of mophead hydrangeas, such as Endless Summer, can easily be turned blue. However, varieties with intensely pink flowers, like Masja hydrangea, cannot achieve a blue color.
Important: Acidic Soil
Another crucial requirement for blue hydrangeas is the soil’s pH. Only in acidic soils can the aluminum ions accumulate in the soil solution and be absorbed by the plants. Hydrangeas will show their brightest blue hues at pH levels below 5.0. At pH levels above 5.5, the blooms will start turning pink, and at pH levels above 6.0, the flowers will become a lilac-pink color.
To achieve a lower pH, you can add plenty of leaf compost, needle litter, or rhododendron soil to the ground. Sandy soils will lower the pH relatively quickly, while clay soils have a high buffering capacity and may remain around 6.0 even after adding acidic compost. In this case, a complete soil replacement around the roots may be more effective, or growing hydrangeas in pots, where you can more easily control the soil’s pH. You can easily measure the pH of your soil with test strips available from garden centers.
Applying Alum with Water
Once you have met the necessary conditions, it’s time to use the alum. It’s available at pharmacies, but you can also find it in garden stores, often combined with hydrangea fertilizer. If using pure alum, add three grams per liter of watering can and stir until it dissolves. Water your plants with soft tap water or collected rainwater, as hard water (which contains a lot of calcium) will raise the soil’s pH and weaken the effect of the alum.
From early May to early June, water your hydrangeas once a week with the alum solution, four to five times. If you use a “blueing” fertilizer, follow the package instructions. However, this method is usually somewhat weaker than using pure alum.
And that’s it! With the right variety, acidic soil, and a little alum, you’ll soon be enjoying the stunning blue hydrangea blooms you’ve always wanted. It’s a simple process that can make a big difference in the look of your garden.
I hope these tips help you transform your hydrangeas into beautiful blue flowers. If you give it a try, I’d love to hear how it goes—feel free to reach out with any questions or share your results. Happy gardening!
By following these steps, you’ll be able to enjoy beautiful blue blooms on your hydrangeas, adding a touch of elegance to your garden!
