The best ways to keep a hydrangea blue all summer long – expert tricks to preserve its vibrant colour
It's easier than you think
A blue hydrangea is the jewel in the crown of any British garden; however, certain varieties can change colour if certain conditions are not met. This can spell disaster if the last thing you want creeping into your garden is a frothy pink or purple bloom.
If you want to keep your plant beautifully blue, rather than pink, you need to maintain high levels of soil acidity. To keep a hydrangea blue, you need a pH of less than 5.5. If the pH rises to between 5.5 and 6.5, the blooms will turn purple, and if it goes above a pH of 6.5, the flowers will turn pink.
Another key factor, however, is also knowing which hydrangea to plant. The colour-changing power of hydrangeas is limited to mophead and lacecap varieties. This means if your hydrangea isn't one of these varieties, the colour will remain the same regardless of the pH of the soil.
1. Use a hydrangea colourant
Where you plant your hydrangea is key in determining the colour of the blooms – acidic soil will result in blue flowers, alkaline soil will create pink blues. So if you want to keep a hydrangea blue, you need acid soil.
A pH soil monitor is your best friend if hoping to keep a hydrangea blue. Traditional methods of testing soil pH involved soil testing kits – like these from Amazon – where you mix a soil sample with water, dip the tester strip in the solution and compare its colour to the guide. However, there are now lots of digital measures – like Sonkir monitor from Amazon – that make monitoring the soil pH a breeze.
If you find your plot a little on the alkaline side, then you'll be relieved to discover that there are several ways to change soil pH; one of the easiest is to use a hydrangea colourant – such as this Westland hydrangea colourant on Amazon. This product – and similar ones like this Vitax colourant on Amazon – are made from iron and aluminium salts, which acidify the soil and will help keep a hydrangea blue.
Specific instructions vary, and you should always follow the manufacturer's instructions. However, applying the colourant once while planting, then using the product as a regular feed (typically weekly) will help to keep a hydrangea blue.
'Pale pink varieties tend to blue best, with the darker pink varieties tending to produce mauve/blue flowers,' says experts from Vitax.
2. Use household waste to up the acidity of the soil
If you're looking for a budget garden idea to turn a pink hydrangea blue, there are plenty of quick hacks online.
A common one is to use a teabag to change your hydrangea's colour. ‘Burying used tea bags in the soil will change the colour of hydrangeas because the tea contains tannic acid, which lowers the pH level of the soil and increases its acidity,’ says Graham Smith MCIHort, a gardening expert from LBS Horticulture.
You could also try using coffee grounds in the garden or even watering your hydrangea with a cup of coffee every day (it may be worth buying a cheap instant coffee for the project).
While these methods are a fun experiment and often do work, Graham does note that using aluminium sulphate (£5.35 on Amazon) is a more reliable method.
Also, be careful in which hacks you try. If your goal is to keep a hydrangea blue, then avoid using fire pit ash in your garden, as this will increase the soil's alkalinity. For the same reason, you should never use eggshells around the base of a hydrangea.
3. Grow it in a pot
If you have no luck with maintaining the right levels of acid in the border. It is a good idea to try growing hydrangeas in pots. This gives you total control over the soil acidity.
Choosing one of the best hydrangea varieties for pots will set you up for success – just ensure they are also mophead and lacecap varieties. Crocus's Love mophead hydrangea would be a great option; you can buy a two-litre pot for £24.99.
Planting in ericaceous soil is essential if you want blue blooms, and it's readily available – B&Q stocks it at £7 for 50ltrs. Ericaceous compost is sometimes sold as azalea, rhododendron, and/or camellia compost, but it is typically the same product and will help keep a hydrangea blue.
Why are my hydrangeas turning pink?
If your hydrangeas are turning pink, then it is because your soil is alkaline. To turn a pink hydrangea blue, you need to raise the acidity in the soil. The best way to do this is with a hydrangea colourant or by adding aluminium sulphate to the soil.
How long does it take to turn a hydrangea blue?
It take a long time to turn a hydrangea blue – typically several months as a minimum. Larger plants will also take longer than smaller ones. It may be the case that you don't see the results of your efforts in the following year's blooms.
