How to Grow Blue Hydrangeas [And Keep Them Blue] (2024)

Written by Daniel Woodley. Reviewed by horticulturist Elizabeth Smith. Published to Hydrangeas. Updated 1st March2023.

We independently research, combine and grow plants in our gardens. If you buy something via links, we may earn a commission. Explore our editorial process.

Page Contents:

You’ve probably heard that you can change the colour of hydrangea flowers by making some changes to the soil.

It’s true, and it’s a fairly simple task, I know as I did it a few years back at my previous property.

That’s why some blue hydrangeas purchased in garden centres change colour after a year or two; they were initially grown in soil with a different pH.

On this page, I’ll show you exactly what you need to do to change the colour of hydrangea blooms.

Don’t worry if you dislike the new colours, changing them back is just as easy!

Whether you’ve purchased a blue hydrangea or want to change a pink or red to blue/purple, explore my quick guide below.

I experimented a few years back, and here’s what happened after I made changes to the soil:

• Pink hydrangeasturned light blue.
• Redturned purple.
• Cream gained a very slight pink tinge on the tips.
• Blue turned pink or light reddish.
• White stayed white.

To change the colour of hydrangea flowers, you’ll need to alter the pH of the soil or compost.

This is easy in pots and containers, but I’ve found it a little more complicated in borders and beds where other plants grow, but it’s still achievable.

Most plants in the garden thrive in soil with a pH between 6 and 7, but if you want to turn a pink hydrangea flower into a blue one (or keep a blue one the same colour), you’ll need to make sure the soil is more acidic.

When I tried this, I found that the ideal pH for blue hydrangeas is around or just below 5.5. At this level, blue hydrangea flowers stayed blue; pink turned blue, and red flowers turned purple.

When I changed the soil pH to between 6.5 and 7, those same hydrangeas started to turn pink or red, and at 7 or above, they were a deeper red and more vibrant pink.

Here’s an overview:

• Below 6 is acidic.
• Between 6 and 7 is slightly acidic and perfect for most plants.
• Above 7 is alkaline.

The easiest way to change the pH, so it’s more acidic is to add aluminium sulphate in powder or liquid form.

I used and preferred the powder as it was cheaper, lasted longer, and I’ve been told that the liquid can easily burn the leaves if spilt.

I just added a scoop of powder (about 50g) to the existing soil, and then every 7-14 days, I added another scoop to 5 litres of water and poured it around the base of the hydrangea.

If you try this, you’ll need to continue adding the aluminium sulphate to the soil for as long as you want to keep the blue or purple colour.

Acidic soil and compost will gradually turn more neutral over time, so the hydrangea flowers will slowly return to pink or red unless you keep topping them up with aluminium sulphate.

For best results, I also feel that the hydrangea should also be grown in ericaceous compost.

Another option is to use ericaceous compost (acidic compost).

This type of compost is usually marketed toward gardeners growing blueberries, rhododendrons and azaleas, but its high acidic content makes it perfect for growing blue or purple hydrangeas.

If you’ve purchased a blue or purple hydrangea in the mophead or lacecap family and you want to retain the original colour, you should either grow it in ericaceous compost or add aluminium sulphate or do both.

Ericaceous compost will, over time, turn more neutral, so you’ll need to top it up throughout the year.

Tap water in hard water areas has higher levels of alkaline, which will counteract the acidic compost or additives that make or keep hydrangeas blue.

If you live in a hard water area, consider watering your hydrangeas with rainwater or filter the tap water first.

If you’re worried about your fertiliser changing the pH of the soil, choose a feed that was specifically formulated for hydrangeas.

I’ve been using Vitax’s hydrangea feed for three years, and all of my shrubs are healthy and produce colourful blooms:

Vitax Hydrangea Feed is:

• A mixture of fine powder which is quickly absorbed and granules which provide feed for up to 8 weeks.
• Specially formulated for hydrangeas giving them everything they need.
• Tested by us here at DIY Gardening – see our review of this hydrangea feed here.
• Neutral, so it won’t change the pH of the soil or alter the colour of the flowers.
• Comes in a resealable bag for freshness.
• Only 2-3 applications are required each year.

If you have a blue hydrangea, it means it’s growing in acidic soil.

To change it from blue to pink (or from purple to red) you’ll need to reduce the acidic content in the soil by adding alkaline.

The first step is to stop adding aluminium sulphate and mulch with regular compost rather than ericaceous compost.

The next step is to add garden lime which is a chalky material with a very high alkaline content.

The RHS website has more information about garden liming.

Based on my experience, it can take months and sometimes years to change the soil pH from very acidic (5) to alkaline (7 or above), but you can monitor the progress by regularly testing the soil with cheap pH testing strips.

Growing a blue hydrangea in a pot is easy as the acidic content stays within the pot, but in a typical border, it leeches out and is gradually replaced with more neutral organic matter.

Through my testing, experimenting and generally having fun in the garden, I’ve found that the easiest way to grow a blue hydrangea in a border or bed with neutral or alkaline soil is to dig a hole 8 inches wider and deeper than the root ball and fill in the void with ericaceous compost.

I would then add aluminium sulphate as per the instructions, which usually means every 14 days.

I always make my hydrangeas twice a year and if I wanted to keep the new colour, I would make sure I use ericaceous compost.

Hydrangeas will only produce blue flowers in acidic soil, and if your hydrangea is blue and you haven’t made any changes to the soil, it’s most likely because the soil is naturally acidic.

Pine needles and cones and the leaves from some other plants and shrubs are acidic and will naturally acidify the soil, but without a fresh supply of this organic matter, the soil will gradually turn neutral and blue leaves will turn pink.

You may also like:

The ultimate Hydrangea Growing Guide

5 reasons your hydrangeas aren’t flowering

How to grow hydrangea macrophylla “ab green shadow”

Seeing yellow hydrangea leaves? This is the solution but act quickly

Causes and solutions of hydrangea leaf spot

Why are my hydrangea leaves curling and wilting? (solutions)

This quick guide to growing blue hydrangeas was created by Daniel Woodley here at DIY Gardening and was first published on the 6th of June, 2022.

Discover morehydrangea growing tips here.

Daniel is a gardener who has a long history working in landscaping and building trades. He completely redesigned his garden and is continually experimenting with new ornamental plants and products.

He also enjoys growing vegetables and fruits in addition to his herbaceous border and container garden.

More About Daniel Woodley

Thanks for reading our guide to growing blue hydrangeas. Find out more about the team behind DIY Gardening

About Us