Why are my hydrangea flowers pink instead of blue?

How soil chemistry controls hydrangea colour, which varieties can change and which cannot, and the realistic way to keep a blue hydrangea blue.

Difficulty: beginner
Time: 30 min
Read: 3 min

Short answer

Blue hydrangeas need aluminium, and aluminium is only available to the plant in acid soil below about pH 5.5. In neutral or alkaline soil the plant cannot take it up and the flowers turn pink. White varieties never change, and no treatment turns a pink-only variety blue.

The colouring is not a pigment difference — it is the same pigment binding aluminium or not. That is why the change is gradual, why it is often patchy across a plant, and why watering with hard tap water slowly turns a potted blue hydrangea pink.

Step by step

Check the variety can change.Only mophead and lacecap macrophylla types shift between pink and blue. White varieties stay white. Paniculata varieties go from white to pink as they age, and nothing you do affects that.
Test the soil pH.Blue needs about pH 5.5 or lower. Between 5.5 and 6.5 gives mauves and muddy purples. Above 6.5 gives pink regardless of what you add.
Use rainwater, not tap water.Hard tap water is alkaline and raises the pH of the compost with every watering. A potted blue hydrangea watered from the tap turns pink within two seasons.
Apply a hydrangea colourant if you want blue.The products sold for this are aluminium sulphate. Apply from early spring, repeatedly through the season, following the label. It is a maintenance treatment, not a one-off.
Grow blue varieties in pots on alkaline soil.In chalky ground you will never hold blue in the open border. A large container of ericaceous compost, watered with rainwater, is the reliable route.
Encourage pink deliberately if that is what you want.Adding garden lime raises the pH and pushes the plant towards pink. This direction is much easier than the other.
Avoid superphosphate feeds if you want blue.High phosphorus locks aluminium up in the soil. Use a low-phosphorus or specifically ericaceous feed on blue hydrangeas.
Expect a season or two of transition.The change happens as new buds form, so it is gradual and often uneven across the plant while it shifts. Judge it the following summer, not the same one.

Tips

Hard tap water is the usual reason a bought blue hydrangea goes pink in its pot.
It is far easier to turn a hydrangea pink than blue.
White hydrangeas cannot be coloured. Nothing sold will change them.

Common mistakes

Burying rusty nails or coffee grounds at the roots — Neither delivers available aluminium and neither meaningfully changes soil pH. Aluminium sulphate applied properly is the only thing that works.
Applying colourant once and expecting it to hold — Rain and irrigation leach it out and the soil buffers back towards its natural pH. It needs applying repeatedly through the growing season, every year.

If it doesn't work

Bought blue, gradually turning pink

Cause: Alkaline tap water or alkaline soil — Fix: Switch to rainwater, use ericaceous compost, and apply aluminium sulphate through the season.

Muddy purple flowers

Cause: pH sitting between the two, around 5.5 to 6.5 — Fix: Commit in one direction — aluminium sulphate for blue, lime for pink.

Different colours on the same plant

Cause: Uneven soil chemistry around the roots — Fix: Normal during a transition. Treat evenly and expect it to settle over a season.

No change after a year of treatment

Cause: Alkaline soil buffering, or a variety that cannot go blue — Fix: Move it to a container of ericaceous compost, or accept the colour it wants to be.

Questions people ask

Can I turn a pink hydrangea blue?

Only if it is a mophead or lacecap macrophylla variety, and only by getting the soil pH below about 5.5 and supplying aluminium sulphate repeatedly. In chalky ground, use a container instead.

Do coffee grounds turn hydrangeas blue?

No. They are close to neutral once composted and they supply no aluminium. It is a persistent garden myth.

Why did my hydrangea change colour on its own?

Usually water. Hard tap water raises the pH of the compost over time, and a plant that was blue at the nursery under rainwater turns pink in a pot on a domestic supply.