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How to turn hydrangeas blue on acid soil or in a pot

Updated: October 2026

The mophead you bought as blue opened pink, and a tub on amazon.co.uk says it will turn hydrangeas blue. Blue is a soil and species outcome, not a promise in the packet. This page says which plants can move, which stay as they are, and why a chalk border often will not shift however much salt you add. It does not rank the listings and it does not give a gram recipe. The [hydrangea fertiliser comparison](/hub/fertiliser-soil-and-bark-mulch/hydrangea-fertiliser) is the product table. The plant facts are from the Royal Horticultural Society and BBC Gardeners' World, not from a trial in our garden. A qualifying purchase can earn a commission at no extra cost to you.

The mophead you bought as blue opened pink, and a tub on amazon.co.uk says it will turn hydrangeas blue. Blue is a soil and species outcome, not a promise in the packet.

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Generic Generic aluminium sulphate hydrangea acidifier, 100 g

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Vitax Ltd Vitax hydrangea colourant, 500 g

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Vitax Ltd Vitax hydrangea colourant, 500 g
9.8/10about £8.48
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Westland Westland hydrangea colourant, 500 g

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Westland Westland hydrangea colourant, 500 g
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On this page
  1. 1.Which hydrangeas can change
  2. 2.Plants that stay the colour they are
  3. 3.Acid soil, aluminium, and rainwater
  4. 4.A pot when the border will not move

Which hydrangeas can change

The RHS group that changes with soil is mopheads and lacecaps of Hydrangea macrophylla, plus H. serrata and H. involucrata. In acid soil the flowers can be blue. Nearer neutral they tend to mauve. On alkaline soil they tend to pink. Some cultivars are bred to stay pink. Being a hydrangea is not enough. Oxford’s Plants 400 profile says blue below about pH 5.5 and pink above about pH 7. The RHS band for staying blue is about pH 4.5–5. Do not average those into one threshold. The Vitax granule label says the feed does not change colour. The hydrangea fertiliser buying guide comes before any tub.

Plants that stay the colour they are

White, red, and green cultivars do not follow pH the way a blue-or-pink mophead does. BBC Gardeners' World notes that Lanarth White and Fireworks can tint in the inner florets. That is not a white head turning blue. H. paniculata, H. quercifolia, and H. arborescens, including Annabelle, sit outside the shifting group. Flower heads often fade green and then pink. Blue in July and green in September is ageing, not a failed tub. Once the plant can shift, the feed-versus-tub choice is on hydrangea feed vs colourant.
Generic Generic aluminium sulphate hydrangea acidifier, 100 g

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Generic Generic aluminium sulphate hydrangea acidifier, 100 g
7.6/10about £5.30
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Acid soil, aluminium, and rainwater

The RHS puts a stable blue around pH 4.5–5, where aluminium is available. Sulphur is the usual soil acidifier, and it is slow. Aluminium sulphate is fast. You need about seven times as much aluminium sulphate as sulphur to move the pH, and large amounts interfere with phosphorus. A weekly drench is a label routine with a cost, not a tonic. Hard mains water pushes a pot the other way. BBC Gardeners' World and the RHS prefer rainwater for blue. If the butt is empty, use tap water rather than let the plant dry out. Limestone or chalk at 75–100 g per square metre in winter pushes a shifting variety toward pink.

A pot when the border will not move

On alkaline ground the RHS route is a pot of ericaceous compost, not a campaign to acidify the bed. Peat-free ericaceous compost is the mix BBC Gardeners' World points to. Rainwater matters more in that small volume than another handful of salt. The colourant, if you still use one, goes on at the label rate. Rates are on how to use hydrangea fertiliser. Doubling because the garden beyond the pot is chalk, then doubling again because the first week looks pink, is how roots get damaged. If a crumb of soil fizzes, free lime or chalk is present and the RHS says that soil cannot realistically be acidified. A 100 g packet will not overturn it. A soil lab is the next step before you dose a whole border, or after a shrub has collapsed.

Common questions

Which hydrangeas can turn blue?
Mopheads and lacecaps of Hydrangea macrophylla, H. serrata, and H. involucrata can, if aluminium is available in acid soil. The RHS puts a stable blue around pH 4.5–5. Some cultivars are bred to stay pink. Check the plant label. A feed does not make this happen.
Will my white one turn blue?
Usually, no. Most white varieties do not follow pH. BBC Gardeners' World notes that Lanarth White and Fireworks can tint in the inner florets. That is not a white mophead turning blue all over. Do not buy a colourant for a white shrub and expect a new colour.
Will a panicle or an Annabelle turn blue?
No. Panicle hydrangeas and smooth hydrangeas such as Annabelle are outside the group that shifts with aluminium. Ageing heads may blush pink. They do not become blue. A tub sold for hydrangeas does not change the species.
What pH do I need?
The RHS says blue cultivars stay blue around pH 4.5–5. Oxford’s Plants 400 profile says blue below about pH 5.5 and pink above about pH 7. Keep both as they are written. Mauve sits between the acid and alkaline ends on plants that can shift. A meter reading on its own does not create aluminium.
Can I do this in chalk soil?
Not in the open bed, if the soil fizzes with vinegar. The RHS says that ground cannot realistically be acidified. Use a pot of ericaceous compost and rainwater. Doubling a colourant on the chalk itself is not a substitute for that pot.
Should I use rainwater?
Yes, when you are trying to keep blue. Hard tap water works against an acid compost over the summer. If rainwater runs out, the RHS would rather you use tap water than let the plant dry out. Drought is the worse failure.
Why did the flowers go green in September?
Ageing. Mophead heads often fade green and then pink late in the season. That is not proof the colourant failed, and it is not proof a feed changed the chemistry. Judge the colour in the main flowering window, not on the faded head.
Will a fertiliser change the colour?
Not the feeds in this set. The Vitax label says the feed does not change colour. Colourants are the salts aimed at a shift, and only on the species above. A high-phosphate lawn or tomato feed can lock aluminium. That is not a reason to invent an analysis for a bottle that prints none.

Related pages

Hydrangea soil comparison

Hydrangea fertiliser

Best hydrangea fertiliser UK by the job, not the stars

Hydrangea fertiliser buying guide: feed, colour or none

Hydrangea feed versus colourant: nutrients, blue, or both

How to use hydrangea fertiliser: rates and when to stop

Sources

Public UK and maker sources from the research digest, used for this page in October 2026. Prices, stars and review counts are from the ten amazon.co.uk listings captured on 19 October 2026. Scores follow the 21 October 2026 shelf note. The NPK 8-4-12 line is the Vitax label, and it is not printed in the amazon bullets. Aluminium percentages and the eye-damage class are from the maker sheets named below, not from an amazon bullet. The March 2026 GOV.UK item was a consultation, not a shopping rule.

  1. RHS, shrubby hydrangea growing guide (retrieved 19 October 2026)
  2. RHS, acidifying soil (retrieved 19 October 2026)
  3. RHS, soil pH (retrieved 19 October 2026)
  4. Agriculture Act 1970, section 68 (retrieved 19 October 2026)
  5. Fertilisers Regulations 1991 (retrieved 19 October 2026)
  6. GOV.UK, new fertiliser regulations, 19 March 2026 (retrieved 19 October 2026)
  7. Vitax Hydrangea Feed label (retrieved 19 October 2026)
  8. Westland Hydrangea Colourant (retrieved 19 October 2026)
  9. Westland Hydrangea Colourant safety data sheet (retrieved 19 October 2026)