Hydrangeas will bloom "brighter" and be "more beautiful" with one unusual ingredient, according to a gardening expert. The unexpected ingredient will change the pH of the soil, which in turn makes the colour "bolder" in blue flowers - although there is another hack for pink flowers too. 

When it comes to blue hydrangeas, soil with a higher acid content makes the flower "bluer", so if your soil test has a pH of more than five which leans towards neutral or alkaline, gardening expert Melinda Myers suggested adding an acid to lower the pH. She recommended using aluminum sulfate or another acid to "create a brighter, bolder blue" on Martha Stewart's website, adding that the "acidity of aluminum is what influences the blue colour of flowers".

She added: "In alkaline soil, where there isn't as much aluminum available to the plant, try adding aluminum sulfate — or an equally acidic element like ammonium sulfate — to the soil to brighten the blues of your blooms."

Aluminum sulfate can be used to help treat the soil of many acid-loving plants, such as azaleas, rhododendrons or camellias, and it can be bought through many online marketplaces. 

However, when it comes to pink or red hydrangeas, these flowers flourish in a soil that is slightly less acidic, with a pH of about six. Myers recommended spreading wood ashes or limestone to lower the acid content instead.

Hydrangeas should also be pruned annually to encourage growth, and the best time is late March after the last frost, to help protect any tender new buds from an unexpected cold snap.

According to the Royal Horticultural Society (RHS), hydrangeas require little attention, but annual pruning encourages new growth. To do so, winter-damaged wood should be cut back to the first strong, healthy bud.