How to Keep Hydrangeas Blue

Keeping a hydrangea blue isn’t luck. It comes down to two things in the soil: it needs to be acid, and it needs aluminium the roots can reach. Get both right on the right type of hydrangea and you’ll get those gorgeous clear blues. Get either wrong and you’ll get pink, however much colourant you buy.

The short version: only some hydrangeas change colour, blue needs acid soil plus aluminium, and it takes a season to show.

Which Hydrangeas Actually Change Colour

Most hydrangeas can’t change colour at all. Only mopheads and lacecaps (Hydrangea macrophylla) switch between blue and pink. Mountain hydrangeas (H. serrata) can too.

White hydrangeas have no colour to change. That includes ‘Annabelle’ and the cone-flowered paniculata types like ‘Limelight’ and ‘Vanille Fraise’. You could drench an ‘Annabelle’ in colourant for years and it would stay white, then fade to lime-green and dusky pink all on its own.

So check your plant before spending a penny. A round ball of small flowers is a mophead. A flat plate with a ring of bigger flowers round the edge is a lacecap. A cone-shaped head is a paniculata and will never go blue. Our guide to types of hydrangeas shows how to tell them apart.

A mophead hydrangea in July. This is the type that turns blue in acid soil.

Why Acid Soil And Aluminium Turn Flowers Blue

Here’s the simple version. In acid soil, aluminium dissolves and the roots take it up, which turns the flowers blue. In chalky or limy soil, the aluminium gets locked away where the roots can’t reach it, so the same plant flowers pink.

Most acid soils have enough aluminium, but some sandy ones don’t, so you may need to add some as well. A simple soil pH test tells you where you stand.

If your garden sits on chalk or limestone, blue in the ground is an uphill battle. Growing in pots of ericaceous compost is the honest answer. If you’d rather make a border more acid, our guide on how to make soil more acidic has the details.

How To Apply Aluminium Sulphate, And When

Timing matters. Apply it every few weeks from March to May, while the plant is growing, then stop by midsummer. Three or four doses through spring is plenty for an established bush.

A simple spring routine:

1. Test your soil in late February
2. Mix the colourant with rainwater, following the pack
3. If the soil is dry, water it well first so you don’t harm the roots
4. Water the mix evenly around the plant, out to the edge of the leaves, in March
5. Repeat every few weeks until late May
6. Mulch with ericaceous compost or composted bark
7. Judge the colour next summer

Wear gloves, keep it off the leaves and store the powder somewhere dry. It’s for the soil, not a leaf spray.

Watering in hydrangea colourant in late March. Damp the soil first, then water it evenly round the plant.

Why Tap Water Turns Hydrangeas Pink

Hard tap water is a sneaky reason hydrangeas drift back to pink. In hard-water areas, much of the south and east of England included, tap water is full of lime. Every can of it nudges the soil back towards pink and undoes your good work.

Rainwater is the easy fix, and it’s free once you have a water butt. If you have to use tap water in a dry spell, lean on the ericaceous mulch and keep up the spring colourant. Our guide to rainwater harvesting shows how to set up a butt.

Keeping Hydrangeas Blue In Pots

Pots give you full control of the soil, so they’re the surest way to blue on chalky ground. Plant in ericaceous (lime-free) compost, never multipurpose, which isn’t acid enough.

Pick a pot at least 40cm wide so the roots don’t dry out or cook. Top up with fresh ericaceous compost each spring and keep up the colourant, as watering washes things through a pot quickly. Feed with an ericaceous feed. Garden centres sell hydrangea feeds that suit this.

Watch the watering in summer. A pot in full sun can need a soak every day in July, which is a lot of rainwater. A spot in light shade with some sun cuts the watering and stops the flowers bleaching.

A blue mophead in a pot of ericaceous compost. On chalky soil, a pot is the reliable route to blue.

Feeds And Mistakes That Wreck Blue Colour

Phosphate is the hidden enemy of blue. It locks up the aluminium so the roots can’t use it. Some general fertilisers and rose feeds are high in phosphate, so check the label and stick to an ericaceous or hydrangea feed.

Bonemeal, mushroom compost, garden lime and wood ash all make soil less acid. Keep them well away from a blue hydrangea.

Common mistakes that turn blue back to pink:

• Using a high-phosphate feed near the plant
• Watering with hard tap water
• Adding lime, bonemeal, mushroom compost or wood ash
• Trying to change a white or paniculata hydrangea
• Expecting a change in weeks rather than a season

One last quirk. A single bush can be blue on one side and pink on the other. The roots on the acid side make blue flowers, while those reaching more neutral soil make pink. Treat the whole root area evenly and it settles down in a season or two.

One mophead, two colours. The soil underneath varies, and even treatment fixes it.

The RHS hydrangea growing guide covers pruning and planting too. And our guide to changing hydrangea colour explains how to go from pink to blue, or back the other way.

Frequently asked questions

Why is my hydrangea pink when I want it blue?

Your soil isn’t acid enough, or lacks aluminium. Blue needs acid soil with aluminium the roots can take up. On neutral or chalky ground, hydrangeas flower pink. Test your soil first, then add hydrangea colourant in spring.

What do I add to make hydrangeas blue?

Aluminium sulphate, sold as hydrangea colourant or blueing compound. Dissolve it in rainwater as the pack says and water it around the roots every few weeks from March to May. Repeat each spring. Keep lime and bonemeal well away.

How long does it take to turn a hydrangea blue?

At least a season, often two. Start in March and judge the colour in high summer. If it’s still mauve, keep going the next spring. There’s no quick fix.

Can you make a white hydrangea blue?

No, white hydrangeas have no colour to change. ‘Annabelle’ and most paniculata types stay white whatever your soil. Only mophead and lacecap hydrangeas, and some mountain hydrangeas, turn blue.

Why is one hydrangea bush blue and pink at the same time?

The soil varies under the plant. One side may sit over acid soil with plenty of aluminium, while the other reaches more neutral ground. The roots in the acid patch make blue flowers and the rest stay pink. Treating the whole root area evenly sorts it out in a season or two.

‘Annabelle’ stays white whatever you do. Only mopheads and lacecaps turn blue.

Now plan the wider hydrangea year

Blue is just one part of the story. Pruning at the right time in late March and protecting buds from late frost keep the flowers coming, and the RHS guide and our colour-change guide above will help you plan the rest of the year.