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How to Change the Colour of Your Hydrangeas

July 21, 2020
Written by Aaron

Hydrangeas are some of the most beloved flowering shrubs around the world—you only need one look at their big, voluminous flowerheads to see why they’re so easy to love. There are hundreds of hydrangea varieties, each belonging to one of six main types. One type, in particular, known as mophead hydrangeas, is famous for a very peculiar and whimsical trait: the flowers can change colours!

The Colour-Changing Shrub

The flowers of the mophead hydrangea change as a reaction to their soil chemistry, resulting in flowers from bright pink to neon blue and every shade in between. However, the shift in shade doesn’t happen overnight—it requires some planning, and a fair bit of patience, to change your hydrangea’s colour.

The good news is that anyone can encourage their mopheads to change colours with the right supplies and a few tips! Here’s how to do it in your yard.

From Pink to Blue

When planted in alkaline soil, mophead flowers bloom in brilliant shades of pink. By increasing the acidity of the soil over time, the flowers will gradually shift to a lovely shade of lilac, and eventually, turn vivid blue with a low enough (more acidic) pH. It’s a true delight to behold this colour change taking place throughout the summer. Some crafty gardeners have even managed to create an “ombré” effect by keeping a row of soil more alkaline on one end and more acidic on the other!

pink purple and blue hydrangea flowers

There’s one caveat to the fun; however: while pink hydrangeas will readily fade to blue, it’s nearly impossible to reverse back to pink. Those who have attempted this tend to end up with a rosy purple, which is pretty enough, but never quite the same. With that said, blue mopheads are so gorgeous; there’s a good chance you’ll be happy to keep them around as-is!

How to Turn Your Hydrangeas Blue

Changing your soil pH requires the use of soil amendments, which can gradually alter the soil chemistry. If your hydrangeas are blooming pale pink to lilac, you can intensify the pink colour by adding garden lime to the soil. Follow the package directions to ensure you’re using the right amount at once, and over time, those hydrangeas will start blushing!

However, most mophead growers are in it for the blue blooms. Blue is a relatively uncommon colour in the gardening world, so those gorgeous blue blooms are highly coveted! Mopheads start blooming blue flowers once the soil pH reaches 5.5 or lower. The best way to increase the acidity of the soil is with the use of aluminum sulfate, a common soil amendment that we carry here at Eising. You’ll need to mix a quarter-ounce of aluminum sulfate into a gallon of water and soak the soil around your mopheads. You’ll get the best results by starting in spring, but you can start at any point during the season. However, once you start treating the soil, you’ll need to reapply the amendment every month until you reach the desired result, and then maintain it with an application every other month during the growing season.

To maintain the acidity of your soil, you can also fertilize with acidic compost like orange peels, coffee grounds, eggshells, or peat. While you won’t see results right away, these will help increase soil acidity in the long term, so that you eventually won’t have any need for the aluminum sulfate. 

how to change the colour of your hydrangeas blue flowers need pH 5.5 or lower

Considerations

Before you start modifying your soil acidity, keep your other plants in mind, as well. Each plant has a different tolerance to acidity—some love it, some can’t tolerate it. As your soil acidity changes, so will your soil’s suitability for your other favourite plants, so make sure to plan around that. Keep an eye on the nursery tag when you shop for plants to grow around your mopheads to ensure you’re leaving with a good fit. We recommend using soil tests regularly to monitor soil acidity throughout your garden. These tests are quick and easy to use, and lots of fun to do with kids!

 

A garden full of pink, purple, and blue hydrangeas is a truly magical place—and the pride of many Southern Ontario gardeners. Chat with us in-store for plant and product recommendations as you embark on this ultra-popular gardening project. Then, prepare to be spellbound by these enchanting shrubs!

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