Every gardener wants their hydrangeas to look their best for summer, and there is a simple way to make the flowers more colourful that will not cost you a penny. Susan Conforte McNeill, a gardener and founder of Susan Said What, has shared that you can make hydrangea blooms a beautiful blue by just adding coffee grounds to their soil. 

She said: “I’ve learned that turning a hydrangea blue naturally, with no harsh chemicals, can actually be easy. Just add… coffee!” Hydrangeas change colour based on the acidity of the soil, as more acidic conditions allow aluminium to be absorbed by the plantwhich reacts with pigments in the petals to produce a breathtaking blue hue. 

Coffee grounds are acidic, which greatly helps change the colour of hydrangeas. However, coffee also has some other positive effects for gardens, such as making flowers bloom even brighter. 

Adding more organic material to soil improves its structure, allowing more water to be absorbed. This helps the plant roots stay hydrated for longer in summer.  

Earthworms also love coffee grounds, and having them wriggling around a garden will greatly improve soil health, which will also help boost hydrangeas. 

However, slugs are known to be greatly irritated by the scent of coffee and are more likely to stay away from hydrangeas as they will smell less appetising to them. 

Susan said: “In addition to gorgeous blue blooms, adding coffee grounds to the soil has some other benefits, too. Coffee grounds will improve drainage, water retention and aeration of the soil. They also help microorganisms that are beneficial to plant growth thrive, and attract earthworms.” 

How to make hydrangeas blue by feeding them coffee grounds 

Before adding coffee grounds, you need to test how acidic the soil is using a pH kit to see if it needs to be amended. Soil testing kits can be found at most gardening centres or on Amazon. 

A pH of 7 is neutral, while anything higher will mean the soil is alkaline, and therefore, it is fine to add coffee grounds. Anything lower means you should not add anything to the soil. 

Once you have done that, save any used-up coffee grounds from your morning brew in a dry container. Only use leftover grounds that have already been brewed and diluted, as fresh coffee is too potent for hydrangeas. 

Before using coffee grounds in the garden, make sure they have dried out and are not extremely wet; otherwise, you can encourage mould growth. 

Then, all you need to do is sprinkle a thin layer of coffee around a hydrangea, but avoid touching the stem, as this can cause too much moisture to build up at the base of the plant. 

Use a spade to lightly mix the coffee grounds into the soil to help them break down quickly, and then water the ground thoroughly.

This is a slow process and will not change the hydrangea's colour overnight, but if you apply the coffee grounds every two to four weeks throughout summer, they should turn blue.  

However, it should be noted that natural feeds are a lot more potent than the fertilisers you buy in stores, so monitor your hydrangeas carefully and make sure to water them each time you use coffee grounds.