Hydrangeas are well known for being affected by whether their soil is acidic or alkaline, nature’s litmus test. But if you would prefer your Hydrangeas to stay blue, there are a few ways you can help them out.

Put in the work

If the soil in your garden is naturally quite limey, you will find you’ll need to put in a lot of work to keep your Hydrangeas blue. However, it is worth it in the end, for their lovely, delicate colour.

It isn’t a total loss if you can’t successfully maintain acidic soil for blue Hydrangeas. If you don’t get the soil acidic enough, your flowers may become a beautiful combination of blue and pink, even turning a lilac shade in places.

Hydrangeas in blue, purple, and pink

Get potted up!

A particularly easy way to isolate your Hydrangeas is to grow them in pots! This way, you have complete control over what goes into that container. Also, it is much easier to maintain an acidic pH in a controlled environment.

By starting with ericaceous compost, and continuing to acidify your soil throughout the year, your Hydrangeas should stay blue easily!

Blue hydrangeas growing in a container

How to acidify your soil

The first thing you need to do is to test your soil before you start. This way, you will have a good idea of how far away from that pH you are to begin with.  To keep your Hydrangea truly blue, the pH of your soil will need to be 5.5 or lower.

You can do this at any time of the year, but you may need to adjust if using sulphurous products. These take longer to work when the soil is cold, so worth aiming for spring through to autumn for the best results.

There are several ways you can maintain acidity in your soil:


Organisms in the soil will convert the Sulphur into Sulphuric acid. Naturally, this will increase the acidity! Don’t worry though, this won’t burn you!

This, however, can take several weeks. Potentially even months if the soil is cool, particularly if carried out in winter months.

Using Sulphur is one of the most effective and affordable ways to acidify your soil if you can be patient.

Ferrous Sulphate (Sulphate of Iron)

Ferrous sulphate is a much quicker method than using Sulphur.

It works by binding iron ions with clay in the soil. This kicks out hydrogen ions, which combine with the remaining sulphate ions. The result is a mild solution of Sulphuric acid.

This is much quicker than using Sulphur, but you need it in much higher quantities for the same ultimate impact. Therefore, it is a more expensive option.

Aluminium Sulphate

This is often used to ‘blue’ hydrangeas and is otherwise known as a ‘blueing agent’.

Because it works so quickly, it is easy to accidentally lower the pH too much, resulting in a high build-up of aluminium in the soil. This could be toxic to the plant, so be mindful.

The cost of using Aluminium sulphate is comparable to Ferrous sulphate, so more than the cost of Sulphur.


To help your soil become acidic enough for Hydrangeas, you will need to alter the soil quite deeply. If you don’t go low enough, they will potentially be drawing water from limey soil instead.

This could mean altering your soil at least 30 cm/1 foot deep. It will be quite a lot of work, but worth it for the impact of blue flowers!

Firstly, you will want to dig in a considerable amount of ericaceous compost. This is designed to be acidic for acid-loving plants like heathers, azaleas, and rhododendrons. The name ericaceous even comes from the Latin ‘Erica’, meaning ‘heather’!

I would suggest that you use some protective equipment before moving on to the Sulphur application. Gloves, goggles, and a dust mask should provide adequate protection.

Then, apply your Sulphur. You can sprinkle it over the surface and water it into the soil, but this may take a while to penetrate. If you can, cultivate it into the ground with a rotovator.

Digging in ericaceous compost

Maintaining acidic soil

There are a few ways to try to boost your acidic soil with low effort, to maintain acidity.

You could water your plants with a very weak solution of water and lemon juice, or vinegar. All you would need is 1 tablespoon of vinegar or lemon juice in 2 litres of water. Occasionally watering your plants with this solution will add a little extra acidity. Use rainwater for this, as tap water tends to be slightly more alkaline.

You can also apply mulch to keep your soil thick and full of nutrients. This can also help to maintain acidity if you use the right materials! Aim for leaf mould, pine needle, ericaceous compost, and bark chippings for your Hydrangea mulch.

A mulched blue hydrangea

By following my advice, you can be proud of your blue Hydrangeas in your garden. Use science to your advantage and keep your Hydrangeas blooming blue every year.

To find out more about growing Hydrangeas:

Or check out my Pinterest board for more ideas: