Last updated on September 2nd, 2021
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If you have hydrangeas in your garden, you know that they produce beautiful flowers. Regardless of the variety you are sure to get large blooms that take on many different shades. Of course you can change the shade that you’re blooms 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 the flowers because of 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 regularly and they should be used intermittently. Changing the soil level is best done before you actually plant the hydrangeas that way the hydrangeas don’t succumb to shock. But to do this you want to slowly change the soil levels. Rapidly trying to change it could cause damage to pre-existing plants.
After you have tested the 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 those 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 added to maintain the colours you prefer which we discuss a little further down.
Which hydrangeas can’t you change the colour of? Any other variety. If you have hydrangeas that produce white flowers, for example, they won’t change. If you plant the smooth hydrangea, the Oakleaf 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 you want the Ph to be around 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 sulfate which can either be added granular or water in, this will increase the acidity level of the soil over time and hydrangeas will start 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 superphosphates and bone meal.
If you want to give it your best try we would mix ericaceous compost into the soil and add aluminium sulfate to the soil and then with an ericaceous feed and continue the process every season carefully checking the Ph on a regular basis.
Making hydrangeas pink
In order to make hydrangeas pink, you need to increase the alkalinity 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 fertilizers and this will discourage the absorption of aluminium so this is a good place to start.
You can also apply lime a few times a year which makes the soil more alkaline and is available at most garden centres, ideally, you want to try and aim at getting a pH of about 6.0 to 6.2.
It’s also worth noting that it’s much easier to turn pink hydrangeas blue than it is to turn blue hydrangeas pink. If you have blue hydrangeas and you have been unsuccessful changing the flower colour to pink it may be better to grow pink hydrangeas in large pots and your soil may be too acidic to alter. The same goes for if the soil is very alkaline it can be much easier to show them in pots.
What’s in a name?
Do not be fooled by the name of the mop head or lacecap variety you choose. Just because it has the term blue or pink in the name does not mean that that is the 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 a pink no matter the type 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 accordingly.
Grow hydrangeas in pots is much easier to control the soil Ph level
It’s worth noting that its much is easier to control the colour of hydrangeas in pots as you can pot into ericaceous compost if you have a blue hydrangea or normal compost which should produce pink flowers and then you can add as mentioned above to change the compost to get the correct Ph level which can often be just achieved by using the correct feed and potted into the correct compost to start with.