Hydrangea flowers change colour naturally as they age, taking on hues of green and pink-red in August and September.
The showy blooms come in a rainbow of colours reflecting the acidity levels of the soils they’re grown in.
“Colour-changing flowers, late season interest and even science are some of the reasons why hydrangeas are a longstanding garden favourite,” say experts at the Royal Horticultural Society.
According to the gardening body, there are many tools gardeners can use to influence the colour of the blooms, including lime and aluminium sulfate.
Hydrangeas can turn various shades of pink and blue
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Peter Dowdall, who goes by the Irish Gardener online, explained how both ingredients work for hydrangeas.
Earlier this year, the green-thumbed enthusiast explained that summer is a time when many gardeners wonder how to tweak the colour of their flowers.
“For those who didn’t know, hydrangeas are pH sensitive, and what that means is the pH of the soil will determine the colour of the hydrangea flower," he said.
“Good old-fashioned garden lime will keep your hydrangeas pink.
“So just spread it around the base of your plant. If you have a pink hydrangea it will keep it a rich pink.
“If you have a blue one and you want to turn it pink, put some lime around it.”
Even when the flowers show hints of purple, gardeners can change this by adding lime products to change the alkalinity and get the perfect pink.
“Just to confuse you even further, none of this applies to the white ones, the white ones will stay white regardless,” added Dowdall.
In order to turn pink flowers blue, gardeners are advised to use ingredients known as aluminium sulfates.
“Aluminium sulfates will probably be called something like hydrangea colourant in a garden center,” explained the Irish gardener.
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White hydrangeas rarely change colour
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“When you’re planting your hydrangea - if it’s a blue one - put lots of this in the planting holder just as it is, and that’ll help keep it that rich blue.
“During the summer as the plant is growing, mix [the aluminium with water] - about 30 grams to five litre - [before feeding].
“If you have a pink hydrangea do this four or five times during the summer and it will turn blue.”
With prolonged use, several natural ingredients can also change hydrangea colours, including coffee grounds, eggshells and vinegar.