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Notebook · Guide 8 October 2026 · seasonal

Turning a hydrangea blue: what really works

Blue needs acid soil so the roots can take up aluminium. On acid soil a blueing compound helps; on chalk it won't. White hydrangeas never turn blue. Pots are the honest route.

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You bought a hydrangea covered in sky-blue flowers, and two summers later it's pink or a muddy mauve. Nothing is wrong with the plant. Blue is the result of acid soil, and your garden is probably less acidic than the nursery's compost was. You can push it back towards blue, but only if your soil is already on the acid side, or if you grow it in a pot. On chalky soil, no product will hold it there for long.

01At a glance

  • Blue comes from aluminium: flowers turn blue when the plant takes up aluminium, and roots can only do that in acid soil.
  • Only some hydrangeas change: mophead and lacecap (Hydrangea macrophylla), plus H. serrata and H. involucrata. White, red and green varieties keep their colour.
  • The numbers: the RHS says blue varieties stay blue at pH 4.5–5. Pink comes in alkaline soil, and mauve in between.
  • On chalk, use a pot: ericaceous compost and rainwater. Blueing compounds won't work on very alkaline soil.
  • Start in spring: treatments go on as growth begins, from March to May.

02Why it's pink, purple or blue

Hydrangea flower colour depends on soil pH, but indirectly. In acid soil there's plenty of aluminium available to the roots, and when aluminium gets into the plant the flowers are blue. As pH rises towards neutral, less aluminium is available and flowers turn mauve or purple. In alkaline soil they turn pink.

So a purple or mauve hydrangea is telling you the soil is close to neutral. That's the case where a blueing treatment has the best chance of helping.

One more thing to know: in late summer, blue and pink flowers naturally fade to mixes of green and pink-red. That's ageing, not your soil changing.

03Step one: test your soil

Before you buy anything, find out your pH. A simple soil test kit is cheap and tells you which of the two routes below is yours. If you can see lumps of chalk in the soil, you already know the answer.

04If your soil is acid or neutral

  1. Use a blueing compound containing aluminium sulphate if your blues are drifting to purple or pink. The RHS suggests this where blue colour is unreliable.
  2. Follow the pack's rates and timing exactly. More is not better.
  3. Apply in spring. University of Georgia Extension advice is to drench the soil in March, April and May, and to keep the solution off the leaves, which it can damage.
  4. Water with rainwater where you can. The RHS recommends rainwater for blue varieties to hold the colour. In a drought, use tap water rather than let the plant suffer.
  5. Be patient. UGA calls the spring drench its quicker method; its slower soil-acidifying treatment can take a year to show a noticeable change.

05If you're on chalk or very alkaline soil

The RHS is clear that blueing treatments won't work on very alkaline soil or where chalk is visible. You'd be fighting the whole garden, and losing.

The honest route is a large pot of ericaceous compost, watered with rainwater. Pick a plant that's already blue in flower when you buy it, and use a blueing compound in spring if the colour starts to drift.

Or grow it in the ground and enjoy pink. The RHS even suggests a winter dressing of ground limestone or chalk at 75–100g per square metre to make pinks and reds stronger.

06Mistakes that waste money

  • Treating a white hydrangea. White varieties stay white. They often age to pink, but they won't go blue.
  • Pouring blueing compound on chalky borders year after year. It's a running cost, and the soil wins.
  • Expecting instant results. Colour shifts gradually; slower soil-acidifying treatments can take a year to show.
  • Folk remedies. Rusty nails, coffee grounds and tea leaves are widely suggested. We found no RHS or university support for them, so we don't recommend relying on them.
Save your money
Test before you treat. A cheap pH test tells you whether blueing compound can work at all.
Don't buy it for chalk soil. Put the money towards a big pot and a bag of ericaceous compost instead.
Collect rainwater. A water butt costs once and keeps a potted blue hydrangea blue for years.

Before handling or planting where children or pets play, check the safety flags on our hydrangea profile.

How we checked this

Every care and safety claim on this page is verified against horticultural and veterinary sources.

Sources: RHS — How to grow shrubby hydrangeas · RHS — Hydrangea by colour · University of Georgia Extension — Growing Bigleaf Hydrangea (Circular 973) · Mississippi State University Extension — How to Change the Color of Hydrangeas

Checked 8 October 2026. If our advice ever conflicts with the RHS, believe them and tell us.