Gardeners can help ensure their blue hydrangeas retain their stunning hue with one simple tip, experts say. You may have previously bought a blue variety of the flowering woody shrubs only to find that the next year the bloom is pink, with the cause being your soil PH, according to growing aficionados.

Thankfully, there are ways of counteracting the colour change, including a small tweak to how you feed them. While many plant lovers may just rely on a garden hose or watering can, the type of water you use and even where you are in the world can have a profound effect on how they develop. This true of blue hydrangeas, which can produce different coloured blooms depending on how they're looked after.

Tap water, especially the kind you'll find in hard water areas, can be alkaline and cause blue-flowered hydrangeas to take on a pink hue.

However, rainwater is naturally more acidic, which helps hydrangeas produce blue flowers.

Gütegemeinschaft Substrate für Pflanzen e.V. (GGS), a German quality assurance organisation for substrates used on plants, recommends using only rainwater.

To give hydrangeas "special care" it recommends having them in a "shaded or partially shaded spot to maintain the blue flower colour" bcause bright sunlight can bring about a brighter blue hue.

"Water regularly and use exclusively rainwater with a low calcium content," it adds, explaining that using "water with a high level of calcium content will increase the soil pH level in the medium term".

"As a result, aluminium ions will no longer be available to the plants and consequently not reach the flowers."

It also recommends feeding them exclusively with special hydrangea fertiliser, as it helps "maintain the soil’s acidity and also contain aluminium sulphate". 

You can collect rainwater "from the roofs of homes, garages, greenhouses and other garden structures as long as they have gutters and a down pipe that enters the drain at ground level", the Royal Horticultural Society (RHA) explains on their website.

The water can then be collected in water butt to be used on your plants.

Meanwhile, if you're looking for another simple way to get your hydrangeas looking as colourful as possible, Susan Conforte McNeill, a gardener and founder of Susan Said What, has shared a hack that involves something you may never thought of using.