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Hydrangeas respond well to coffee grounds in the soil. Most people use coffee grounds to help change the colour of their hydrangea blooms from pink to blue as it makes the soil more acidic but it’s also useful to retain moisture in the soil between watering. To be clear you can only change the colour of the mop head and lacecap hydrangeas. As mentioned, coffee grounds make the soil more acidic which in turn helps turn the flowers blue.
Here’s how you use coffee grounds with hydrangeas and how the process works.
Can Coffee grounds change the pH to make the soil more acidic?
The pH of the soil measures how acidic or alkaline the soil is. Some plants prefer acid soil (pH 1-6), others like neutral soil (pH 7) and yet others are happy in an alkaline environment (pH 8-11). You can measure the pH of your soil with a special pH meter, this one on amazon is the model I recommend.
Once you know what the starting pH is, you can add materials to make it more acidic, neutral or alkaline to change it to what your plants prefer.
To turn hydrangea flowers blue (remember only certain varieties can do this) you need a more acidic soil so a pH reading of between 1-6 and this is where coffee grounds come in as they make your soil more acidic.
The effect of increasing the acidity of your soil is to make your hydrangea blooms bluer or to turn them blue from white or pink. It is worth noting that if your soil is already slightly acidic this process will be much more effective.
However, if you have more alkaline soil with a high pH reading, simply using coffee grounds may not be enough to turn the soil acidic. Even if the process works, you will most likely need to do it regularly to retain the acidic pH as it will naturally become more alkaline again.
Do coffee grounds help retain moisture?
Adding coffee grounds, which are an organic matter, to your soil adds nitrogen and in turn, this helps the stem, leaf and root development of your plants. The soil retains moisture, keeping the roots wetter for longer. If your soil is a bit sticky, add the coffee grounds to your compost to help the soil drain much more easily.
How to add the coffee grounds to the soil
You can sprinkle the coffee grounds on top of the soil and leave them there to decompose into it. To keep your garden tidy (and less smelly) though, dig the grounds right into the soil.
Alternatively, if you regularly compost your plant beds, add the grounds into your compost bin. Remember though, if you do this you will be making your compost heap more acidic which may not be ideal for some of the other plants in your garden.
How much coffee grounds should you add to your soil?
You need to keep testing the pH of the soil to see that it is in fact becoming more acidic. If your starting pH is neutral (7) you don’t need to add a huge amount of coffee grounds.
If you have a start point on the alkaline side of the scale, however, you need to take your soil through the neutral point (ph 7) to the acid side. This takes more work and more coffee grounds and is not always very effective.
You’re looking for pH 6 to pH 6.5 to get the best blue flowers.
Will the soil revert back to its orginal pH?
As already mentioned, this is not a one-off happening. Over time, your soil reverts to its original state and pH. Your hydrangea blooms will go back to the colour that suits the soil’s pH. Make a point to check the pH of the soil at the beginning of the growing season and then at the end and add coffee grounds if needed.
This won’t work on all varieties of hydrangeas. Check with your greenhouse experts to see if your hydrangea is suitable for colour changing or just try it out. In general, you can change the colour of most mop head and lacecap hydrangeas.
For those who aren’t great coffee drinkers, eggshells will also increase the acidity of your soil and turn the blooms blue. Ericaceous compost also makes your soil more acidic which you can learn more about in this post.
The colour of the hydrangea blooms of your new shrub may not be their colour after a time in your garden as the pH of your soil may be different to that in the pot they were planted in.
For more information about how to change the colour of your hydrangea shrub – including how to turn it pink – Read our How to change the colour of hydrangeas article.