When it comes to adding a pop of colour to your garden there's a huge array of flowers to choose from, but hydrangeas are among the most sought-after.

Not only are the plants popular because of their striking ball and cone-shaped flowerheads, but they also have the unique ability to change colour. Bigleaf hydrangeas (Hydrangea macrophylla) and mountain hydrangeas (Hydrangea serrata) are both capable of colour changes, specifically shifting from pink blooms to vibrant blue and vice versa. The colour change is partly due to the plant's ability to absorb aluminium, which is influenced by how acidic or alkaline the soil is. It's possible for gardeners to influence the colour that hydrangeas bloom by changing the pH of the soil, so if you want to achieve blue flowers there are things you can do to encourage this.

True blue flowers are rare in nature with less than 10% of flowering plants having this colour, so the fact hydrangeas can turn blue makes them pretty special, but achieving a bright blue bloom does require some patience.

To get blue hydrangeas your soil will need to have a pH level of 6.5 or lower, so if it's not at this level you'll need to make it more acidic - and the quickest way to do this, according to gardening experts, is by using aluminium sulfate.

Experts at Proven Winners explain: "In acidic soil, aluminum is more readily available for hydrangeas to absorb. The aluminum interacts with the pigments in the petals, leading to a stunning blue colour.

"Only bigleaf and mountain hydrangeas can change their colour in a predictable, controllable way. Smooth and panicle hydrangeas undergo some colour change as they age, but their flowers cannot be turned blue."

The experts add: "So what if your soil pH isn't low enough for blue flowers? To turn hydrangeas blue, you'll need to make your soil more acidic. You may have heard of tricks like adding coffee grounds, vinegar or even pennies or a bundle of rusty nails to the soil. However, it's best to use aluminum sulfate, which is a proven method for changing flower colour.

"Achieving the desired shade of blue in your hydrangeas is a process that requires patience. It may take up to a year for hydrangea flowers to transition to the stunning blue that gardeners covet."

The experts say turning hydrangeas blue won't happen instantly and will be a gradual process that takes time, and may require multiple applications of aluminium sulfate.

If the pH of your soil is lowered too quickly or too much then plants can struggle to absorb the nutrients it needs so it's important to monitor the levels as you go.

If you want to use a more natural method to achieve blue flowers, you can try applying coffee grounds, the peel of citrus fruits like oranges, lemons, limes and grapefruits, or crushed egg shells, although these methods will be slower.

Experts at Dammanns Garden company add: "Organic materials, such as coffee grounds, egg shells or citrus fruit peels are another, more natural method of making soils more acidic. Simply grind them up and work them into the ground. These changes will happen gradually, so it might take an entire year of doing this consistently for it to reach the right acidity."