Last updated on February 27th, 2022
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If you have Hydrangeas in your garden, you know that they produce beautiful flowers. Regardless of the variety you have, you are guaranteed to get large blooms that take on many different shades. If the flower colour isn’t to your preference, you can change the shade that your Hydrangeas produce.
How to change the soil pH for Hydrangeas
Changing the bloom colours from pink to blue or blue to pink is based on the soil pH. The pH level of the soil will only change the colour of the mop head or lacecap Hydrangeas.
If you have Hydrangeas with green flowers, you may not be able to change the colour of these flowers based on the type of Hydrangea you have. Read more about Hydrangeas with green flowers here.
What you need to test the soil pH
You need to understand that when doing this, the first step is to test the soil with a soil pH kit at home. These can be purchased online and should be used intermittently. Changing the soil level is best done before you actually plant your Hydrangea, this way, your Hydrangeas won’t succumb to shock. To achieve this, you want to slowly change the soil levels because rapidly trying to change it could cause damage to pre-existing plants.
After you have tested the pH levels, you need to add material to the soil in order to change it in whichever direction you prefer based on the blooms that you want.
This is not a one-time requirement. Your soil will naturally return to its original pH, which is why you will need to purchase these kits regularly (unless it’s a reusable digital model) so that you can test year after year and season after season. As the pH levels return to normal you will need to continue adding whatever element you originally added to maintain the colours you prefer, and this is something we discuss in more detail further down.
Which Hydrangeas can’t you change the colour of?
If you have Hydrangeas that produce white flowers, for example, they won’t change. If you plant the smooth Hydrangea, the Oak Leaf Hydrangea or the Panicle Hydrangea, you will not be able to alter the colour of the flowers.
Making Hydrangeas Blue
In order to make your Hydrangeas blue, you need to increase the overall acidity. To increase the acidity you can naturally add organic material like coffee grounds. Ground up citrus peels also work well when mixed into the soil but you usually need to do a little more than just this.
As these organic materials decompose they add aluminium to the soil and it is the increased aluminium that makes the soil more acidic. Ideally, the pH of the soil should be low, around 5.2-5.5.
If you don’t want to save your old coffee grounds and citrus peels you can always buy aluminium sulphate which can either be added granular or watered in. This will increase the acidity level of the soil over time and your Hydrangeas will begin to turn blue. You can also mix ericaceous compost into the soil around the base of the plants and feed with an ericaceous feed, you want to avoid super phosphates and bonemeal.
If you want to give it your best try we would recommend mixing ericaceous compost into the soil, adding aluminium sulphate and then using an ericaceous feed and continuing the process every season, carefully checking the pH on a regular basis.
Making Hydrangeas Pink
In order to make your Hydrangeas pink, you will need to increase the alkalinity of the soil, which can be done using lime. In order to do this, you want to limit the amount of aluminium that is absorbed by the soil. You can buy high phosphorus fertilisers and this will discourage the absorption of aluminium, so this is often a good place to start.
You can also apply lime a few times a year and this makes the soil more alkaline. It is available at most garden centres and ideally, you want to try and aim at getting a pH level of around 6.0 to 6.2.
It’s also worth noting that it’s much easier to turn pink Hydrangeas blue rather than turning blue Hydrangeas pink. If you have blue Hydrangeas and you have been unsuccessful at changing the flower colour to pink, then it may be better to grow a pink Hydrangea in a large pot because your soil may be too acidic to alter. The same goes for if the soil is very alkaline.
What’s in a name?
Do not be fooled by the name of the mop head or lacecap varieties you choose. Just because it has the term blue or pink in the name does not mean this is the true shade you will get for the flowers. If it is a mop head or lacecap Hydrangea you can change the flowers to a blue or pink colour, no matter the variety or the name.
More importantly, whatever blue or pink or purple varieties you see already on display when you purchase your Hydrangea from the nursery is not indicative of the flowers you will get season after season once you plant it at home. Rather, it is a reflection on the soil pH inside that container, so once you transplant to your garden you will need to measure and change the soil levels accordingly.
Grow Hydrangeas in pots because it is much easier to control the soil pH level
It is worth noting that it’s much is easier to control the colour of Hydrangeas in pots because you can put them straight into ericaceous compost if you have a blue Hydrangea or normal compost should you want to produce pink flowers, and then you can add as much (as those mentioned above) to alter the compost to get the correct pH level. This can often be achieved by simply using the correct feed and potting them into the correct compost, to begin with.
