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How to keep hydrangeas blue with soil, pH and water

Updated: October 2026

You may buy a blue hydrangea, put it on a patio or in a border, and find the next flowers mauve or pink. How to keep hydrangeas blue starts with the cultivar, not with a tub of colourant or the most acid-looking bag. Colour-changing mophead and lacecap hydrangeas need acid conditions, available aluminium and steady care. White and panicle hydrangeas will not become blue because you change the compost. Our [hydrangea soil comparison](/hub/fertiliser-soil-and-bark-mulch/hydrangea-soil) covers ericaceous bags on amazon.co.uk. This page explains what that growing medium can do, why rainwater matters, and when a container is more dependable than trying to alter alkaline ground.

You may buy a blue hydrangea, put it on a patio or in a border, and find the next flowers mauve or pink. How to keep hydrangeas blue starts with the cultivar, not with a tub of colourant or the most acid-looking bag.

Our top picks

SIC SIC Ericaceous 50 L

Best overall

SIC SIC Ericaceous 50 L
9.2/10about £12.49
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Green Haven Green Haven Ericaceous 50 L

Rank 2

Green Haven Green Haven Ericaceous 50 L
9.4/10about £12.49
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ecoblaze TerraGrade Ericaceous 50 L

Best value

ecoblaze TerraGrade Ericaceous 50 L
8.4/10about £9.99
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On this page
  1. 1.Which hydrangeas can turn blue
  2. 2.Soil pH, aluminium and rainwater
  3. 3.Ericaceous compost is not a colourant
  4. 4.What to do on alkaline or chalky ground

Which hydrangeas can turn blue

Mophead and lacecap forms of Hydrangea macrophylla, and some Hydrangea serrata cultivars, can change colour. The RHS describes blue flowers in acid soil where aluminium is available, mauve near neutral, and pink in alkaline soil. The right conditions support a blue potential. They cannot create it. White, red and green cultivars generally keep their colour. Hydrangea paniculata and arborescens types such as 'Annabelle' are not made blue by ericaceous compost. A white flower may age through pink or green on its own. Check the label, or ask the nursery if the cultivar is unknown. When an acid bag is unnecessary is in the hydrangea soil buying guide.

Soil pH, aluminium and rainwater

For a responsive hydrangea, the RHS gives roughly pH 4.5 to 5 for blue flowers, because aluminium is more available then. A low-pH compost supports that process. It is not blue dye. Of the seven products here, only SIC prints a number, pH 4.0. Other listings may say ericaceous or low pH without a figure. Hard mains water pushes a lime-free potting mix away from acid conditions. Rainwater is preferred, particularly in a container. A covered water butt makes that practical. Follow any hosepipe restriction in a dry spell. If the butt is empty, use tap water rather than leave the plant dry. A wilted pot will not protect flower colour.
SIC SIC Ericaceous 50 L

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SIC SIC Ericaceous 50 L
9.2/10about £12.49
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Ericaceous compost is not a colourant

Ericaceous compost is a lime-free growing medium for rhododendrons, azaleas, camellias, blueberries and colour-changing hydrangeas. A colourant is a separate treatment, commonly based on aluminium salts. Aluminium sulphate may act faster than elemental sulphur and adds the aluminium involved in blue colour. The RHS notes that repeated heavy use can interfere with phosphorus and let aluminium build towards harmful levels. The dose on that label belongs to that product. There is no universal dose to copy from another tub. On strongly alkaline or chalky ground, repeated treatment may be costly and temporary. Peat is not a general soil acidifier, and one bag in a planting hole does not convert the surrounding soil. If foliage is scorched or you do not know the pH, stop and get advice.

What to do on alkaline or chalky ground

A container is the clearer route on alkaline soil when you want blue flowers: a compact colour-changing cultivar, a pot that drains, and an ericaceous medium. A hole of acid compost is a temporary island. Roots meet the native soil, and alkaline conditions return. If the plant must go in the ground, test first and accept that pink or mauve may be the result you can keep. A small hydrangea often needs a pot around 45 cm across, though cultivar and pot shape matter. Mix, bag size and drainage are in the hydrangea compost for pots guide. Ground versus container, once you have the plant, is the hydrangea planting guide.

Common questions

Will ericaceous compost turn any hydrangea blue?
No. It can support blue colour in responsive mophead, lacecap and some serrata cultivars by maintaining acid conditions, but genetics still determine whether the flowers can change. White, panicle and arborescens hydrangeas do not become blue simply because you repot them into an acid mix.
What pH keeps hydrangeas blue?
The RHS associates blue flowers with acid soil at about pH 4.5 to 5, where aluminium is available to the plant. That is a growing-condition range, not a guarantee of colour. The cultivar, aluminium availability, water and changes in the pot over time all affect the result.
Can I use tap water on a blue hydrangea?
Yes, especially if the alternative is allowing the pot to dry out. Rainwater is preferable because hard tap water gradually raises the pH of a lime-free mix, but drought is a more immediate threat. Use rainwater from a water butt when you have it, and water thoroughly with tap water when you do not.
Is hydrangea colourant the same as compost?
No. Ericaceous compost is the root-growing medium. A colourant is a separate aluminium-based treatment intended to influence flower colour in responsive plants. Follow the exact colourant label and do not invent or combine doses, because excessive aluminium sulphate can damage soil chemistry and plant health.
Can one bag change a chalk border?
One bag in a planting hole will not reliably overcome chalk or strongly alkaline surrounding soil. Water and expanding roots reconnect the plant with the native ground. A container filled with ericaceous compost offers a more controllable root zone if blue flowers are your priority.
How can I make a hydrangea pink instead?
Colour-changing cultivars tend towards pink in alkaline conditions. The RHS gives winter applications of limestone or chalk at 75 to 100 g per square metre as guidance, but only after you understand the existing soil and plant. Do not lime acid-loving neighbours, and remember that non-responsive cultivars will retain their natural colour.

Related pages

Best ericaceous compost by job

Hydrangea soil

Ericaceous compost for hydrangeas: what to check first

How to plant a hydrangea in compost: depth and water

Hydrangea compost for pots: mix, size and watering

Sources

Public UK sources checked for this page in October 2026. Prices, stars and review counts are from the seven amazon.co.uk listings on file. A peat-free line, a pH and a feed period are listing claims. Title litres are not treated as the fill when the volume field disagrees. This is not a laboratory test. A 2022 Defra announcement is not a ban already in force.

  1. RHS, how to grow shrubby hydrangeas (retrieved 23 October 2026)
  2. RHS, acidifying soil (retrieved 23 October 2026)
  3. RHS, peat-free compost (retrieved 23 October 2026)
  4. Gardeners' World, ericaceous compost (retrieved 23 October 2026)
  5. Gardeners' World, how to grow hydrangeas (retrieved 23 October 2026)
  6. Defra, sale of horticultural peat to be banned (retrieved 23 October 2026)
  7. Peat Free Partnership, legislation timeline (retrieved 23 October 2026)
  8. Consumer Rights Act 2015, section 11 (retrieved 23 October 2026)