Hydrangeas are bold and beautiful plants that can add a much needed splash of colour to gardens, but they are also unique in the fact some varieties can actually change colour.
The Hydrangea macrophylla variety – also known as a bigleaf hydrangea – has the ability to change the colour of its flowers based on the pH levels of the soil. According to the Royal Horticultural Society (RHS), if the soil in your garden is acidic (with a pH of 5.5 or lower) then this should produce striking blue or purple flowers thanks to the availability of aluminium, whereas alkaline soils (with a pH of 6.5 or higher) encourage more pink or red blooms. White hydrangea varieties generally stay white regardless of the soil pH levels.
So if you’re particularly fond of the pink variety and want to bring a bit of vibrancy to your garden, there are ways you can make your soil more alkaline using natural ingredients. One method recommended by gardening experts is to add crushed egg shells to the soil around the base of your hydrangeas.
Egg shells are primarily composed of calcium carbonate, so when they break down into the soil this mineral is released which then makes the soil more alkaline. The increase in alkalinity then reduces the availability of aluminium to the hydrangea plant, which encourages pink blooms.
Garden Soul explains: “One of the key arguments in favour of using eggshells in the garden is their potential to balance soil pH. When crushed eggshells are mixed into the soil, they slowly release calcium carbonate, which can help raise the pH level, making the soil more alkaline. This is welcome news for gardeners seeking those gorgeous pink or white hydrangea blossoms.
“Egg shells are not just about pH; they also offer a nutrient boost to your hydrangeas. Alongside calcium, they contain small amounts of essential minerals like magnesium and potassium. These nutrients can improve overall soil health and contribute to more strong, healthier plants.”
But gardeners should be aware that the process of turning hydrangeas a vibrant shade of pink won’t happen instantly overnight and will take a long time for the colour change to appear using just egg shells alone. As such, it’s recommended that you add some garden lime to the soil as well to give the plant an extra helping hand.
Epic Gardening says: “The greatest gardening benefit the eggshell has to offer is its amount of calcium. Unfortunately, this calcium is not immediately available to your gardens. You will need about a year of decomposing before the calcium breaks down enough to permeate into your soil.
“The calcium found in eggshells is calcium carbonate. This is one of many micronutrients essential for cellular formation in plants. Calcium carbonate is a main component of limestone, which is widely known for increasing the pH of your soil to a more alkaline level. Soil pH below 6.0 will turn your hydrangea blossoms blue, and the presence of acidic materials such as sulfur or aluminum sulfate can lower the soil pH. Above 7.0, alkalinity causes hydrangeas to skew towards the pink range, and calcium or limestone will turn the flowers pink.”
It adds: “In theory, eggshells will eventually change the color of your bigleaf hydrangea flowers. It will take some time for the eggshells to break down properly and actually affect the pH of your soil. All in all, I think there are better ways to change your flowers from blue to pink.
“Garden lime, which is made of calcium carbonate just like egg shells, will work much quicker at raising the pH of your soil. Once you have garden lime on hand, it is important to follow package instructions to ensure that you are using the correct application rate.”