It is something I have heard people ask a lot: “Why do hydrangea flowers change color according to soil pH?”
I remember seeing my friend’s hydrangeas years ago, and they were such a vibrant blue.
I immediately want to turn my own pink hydrangeas blue.
So I did exactly what any man does when he wants the answer to a question he doesn’t know.
I went straight to Google!
It is easy to get bogged down in the science of how the pH of soil affects the color of a hydrangea, but I will try and explain it to you here.
So let’s get started.
Why Do Hydrangea Flowers Change Color According To Soil Ph?
It isn’t just the pH level of soil that affects the color of the flowers of hydrangea, it is the amount of aluminum in the soil. It is the aluminum a pink hydrangea absorbs that changes its blooms to blue. In more acidic soil, aluminum is available in a more easy-to-absorb state. When soil becomes more alkaline, any aluminum in the soil solidifies and can’t be as freely taken in by the hydrangea.
#1: The More Acidic the Soil the Bluer the Flowers
There are very few flowers that are affected by soil in the same way hydrangeas are.
The rule is that the more acidic the soil, the more blue the flower, so:
- Soil pH of 5.5 or lower: blue flowers
- Soil pH of 5.5 to 6.5: purple flowers
- Soil pH of 6.5 or higher: pink flowers
But it isn’t that simple.
If you want blue hydrangeas, having more acidic soil is just one part of the puzzle…
#2: It is All About the Aluminum
The real factor in whether your hydrangea turn from pink to blue is the amount of aluminum in the soil.
Pink hydrangeas absorb aluminum from the soil, and it is that that makes them appear bluer.
There is a link between pH levels and aluminum though.
RELATED ===> Are Rusty Nails Good For Hydrangeas?
The pH levels of soil change the availability of aluminum to hydrangeas.
In acidic soil, aluminum is found in a more soluble state, which means it is easier for hydrangeas to access and to therefore change the colors of their blooms.
In alkaline soil, aluminum is more solid and those more difficult for a hydrangea to absorb.
So it can be summarised like this:
- Acidic soil with aluminum: blue flowers
- Acidic soil without aluminum: purple or pink flowers
- Alkaline soil with aluminum: purple or pink flowers
- Alkaline soil without aluminum: purple or pink flowers
So as you can see it takes a particular set of circumstances for your hydrangea to produce blue flowers.
#3: But Not All Varieties of Hydrangea Change Color
One further slight complication, not all hydrangea will change color according to soil pH and aluminum availability.
Lacecap and mophead varieties are the ones that can change color. White hydrangeas won’t change color at all.
RELATED ===> How Often To Water Hydrangeas With White Vinegar?
Can Soil Be Too Acidic For Hydrangeas?
Whilst hydrangeas love acidic soil if the soil is too acidic it can be harmful to them.
If the pH is below around 3 it will mean many of the nutrients a hydrangea needs to survive, such as nitrogen, phosphorus and potassium, are not as easily obtainable.
Highly acidic soil can also make it a less favorable environment for beneficial bacteria and soil organisms.
It is unlikely your garden soil will be this acidic, but if you are concerned it is you can get basic pH tests from any garden center to test your soil.
How Long Does It Take To Turn Hydrangeas Blue?
This is a bit like the old adage of how long is a piece of string.
It depends on your soil originally, the type of hydrangea you want to grow and some other factors.
For instance, watering your hydrangea with soft water will speed up the process. That is because soft water contains fewer minerals and is acidic.
If you live in a hard water area try watering with rainwater.
It is easier to turn a pink hydrangea blue than vice-versa.
That said, it will usually take more than one season to change the pH of your soil
What Chemicals Make Hydrangeas Blue?
We have already touched on what chemicals and minerals make hydrangeas blue.
Essentially it is all down to aluminum.
One of the most common ways substances use to grow blue hydrangeas is aluminum sulfate, also known as papermaker’s alum.
This can be purchased from any garden center.
The sulfate acidifies the soil and the aluminum is absorbed by hydrangeas to give them their blue hue.
If you google how to turn hydrangeas blue you will come across all kind of ideas.
Some are weirder and wackier than others.
Let’s take a look at some of the most popular theories and see what works and what doesn’t.
Will Pennies Turn Hydrangeas Blue?
RELATED ===> Can Preen Be Used Around Hydrangeas?
One of the most common suggestions is that planting pennies will help hydrangeas grow blue flowers.
Unfortunately, this isn’t true.
Hydrangeas need aluminum in the soil to encourage them to bloom blue, and pennies are made of zinc with a copper coating.
In the 1970s the US Mint did briefly experiment with aluminum pennies, but they are long gone.
There is a small bit of science here that may explain this theory.
For many years pennies were made completely of copper.
Copper could potentially work in tandem with hydroxyl ions in soil to lower pH levels and free up aluminum.
But as copper is very stable and corrodes very slowly, this would take A LONG time.
And, as we mentioned above, pennies are now made of zinc with a copper coating, so it is even more of a moot point!
Will Baking Soda Turn Hydrangeas Blue?
This is another old wives’ tale that has gained popularity over the years.
Baking soda will not turn your hydrangeas blue.
If you google, you will also find people saying it can make hydrangeas pink.
This is more likely as baking soda is alkaline, and as we know alkaline soil means pink flowers when it comes to hydrangeas.
Baking soda is not a good addition to your soil full stop.
It is basic, it is salty and it won’t do anything for your plants.
Will Epsom Salt Turn Hydrangeas Blue?
Epsom salt is neutral in pH, so it is unlikely to have any effect on your hydrangeas and will certainly not turn them blue.
It does supply magnesium so could potentially benefit old soil by replenishing the supply of this chemical.
However, you need to be careful when adding Epsom salt to your plants, as it can create a deficiency of calcium. Calcium is vitally important to the healthy growth of plants and must be present.
Adding too much Epsom salt, without adding any calcium at the same time, will have the same effect as adding an excessive amount of fertilizer.
That being it could cause fertilizer burn and damage your flowers.
Will Holly Tone Turn Hydrangeas Blue?
Using a soil additive such as Espoma Hollytone can help turn your hydrangeas blue.
Hollytone contains sulfur, which is an acidifier.
As well as acidifying the soil, it acts as a fertilizer to support plants and free up the necessary aluminum ions for blue blooms.
As you can see it isn’t just the pH of a soil that changes the color of the flowers of hydrangea, there also needs to be aluminum in the soil to turn a hydrangea from pink to blue.
Aluminum really is the key, whilst the pH levels activate the aluminum so it can be absorbed by hydrangeas.
Just be aware that white hydrangeas will not change color.
A popular way to ensure there is sufficient aluminum in the soil is to use aluminum sulfate or a soil additive such as Espoma Hollytone.
As for some of the old wives’ tales you might have heard to get your hydrangea to produce blue blooms?
They most likely won’t work!