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Blue hydrangea in a pot UK

Updated: October 2026

You repotted a blue hydrangea into a patio pot, watered it from the kitchen tap, and the next summer the flowers came up pink. A blue hydrangea in a pot in the UK loses that colour when the compost is ordinary multipurpose and the water is hard mains, because aluminium becomes harder for the roots to take up as conditions turn alkaline. The colourant is the top-up once the pot is right. The wrong compost still has to come out. Prices are on the [blue fertiliser comparison](/hub/fertiliser-soil-and-bark-mulch/blue-fertiliser). We did not measure tap water in your street.

You repotted a blue hydrangea into a patio pot, watered it from the kitchen tap, and the next summer the flowers came up pink.

Our top picks

Topline Westland hydrangea feed and colourant bundle

Best overall

Topline Westland hydrangea feed and colourant bundle
6.8/10about £16.99
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Vitax Vitax Hydrangea Feed, 1 kg, pack of 2

Preis-Tipp

Vitax Vitax Hydrangea Feed, 1 kg, pack of 2
8.2/10about £11.98
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Westland Westland Hydrangea Colourant, 500 g

Rank 3

Westland Westland Hydrangea Colourant, 500 g
9.1/10about £6.99
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On this page
  1. 1.Why a pot hydrangea loses its blue
  2. 2.Ericaceous compost in the container
  3. 3.Rainwater, and tap water when the butt is empty
  4. 4.Colourant as a top-up, and what the pot still cannot do

Why a pot hydrangea loses its blue

Mophead, lacecap and Hydrangea serrata can move between pink and blue. In a container, multipurpose compost plus hard tap water is the usual reason a blue plant drifts pink, because aluminium becomes harder for the roots to take up. We have no calcium figure for a named town. In the Amazon reviews, people who then watered a colourant in at the base from spring reported a move back towards blue. If the pot dried out, the Royal Horticultural Society would rather you use tap water than let the shrub wilt. Compost versus a feed versus a colourant is on hydrangea colourant or ericaceous compost.

Ericaceous compost in the container

On alkaline soil, the Royal Horticultural Society says to grow blue-flowered hydrangeas in containers of ericaceous compost. That is the main buy. A colourant tub is the follow-up. Gardeners' World pairs peat-free ericaceous compost with rainwater. Where pink is the aim, they mention John Innes No. 3 blended with multipurpose. Recovering blue wants the ericaceous bag. At potting, shake loose the old multipurpose if the roots allow. According to the manufacturer, the potting unit is 50 g per 5 litres of compost. The RHS guidance we used gives no pot diameter, so this page leaves diameter out.
Topline Westland hydrangea feed and colourant bundle

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Topline Westland hydrangea feed and colourant bundle
6.8/10about £16.99
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Rainwater, and tap water when the butt is empty

Water blue cultivars with rainwater when you have it. That is the Royal Horticultural Society line, and Gardeners' World repeats it. In a dry July the butt runs out, and you use tap water so the plant is watered at all. Drought beats a perfect pH. The flowers may drift pinker on the mains. Hardness numbers change by postcode, so this page leaves them to your water company. A heavily scaled kettle is a household clue to collect rainwater. Who buys which tub is on the best hydrangea colourant page.

Colourant as a top-up, and what the pot still cannot do

With ericaceous compost in the pot, according to the manufacturer, the container rate is 50 g of colourant per 5 litres of compost at potting, then 50 g dissolved in 5 litres of water every seven days on the compost, from April to September. The solution stays off the leaves. The grams, the eye hazard and the point where you stop are on how to use hydrangea colourant. A small rootball scorches more easily than a border shrub. One reviewer said the suggested amount nearly killed a plant. Doubling the rate because the tap is hard is the alkaline-soil alternative printed for the border. In a pot the ericaceous compost is already that alternative. Keep the weekly figure at the label rate. A generic 30 ml tub is a different label.

Common questions

Why did my blue hydrangea turn pink in a pot?
The usual patio pair is multipurpose compost plus hard tap water. Responsive mopheads, lacecaps and Hydrangea serrata move towards pink when aluminium is less available, which is what alkaline conditions do. The Royal Horticultural Society points blue flowers on alkaline ground to a container of ericaceous compost, watered with rainwater when you can. A colourant on top of the old compost is the slower fix. Repot first.
Which compost should I use?
Ericaceous compost, peat-free if that is what you can buy, for a blue-flowered hydrangea in a pot. That is the RHS advice where the garden soil is alkaline, and it is the right bag even on acid ground if the pot was previously filled with multipurpose. John Innes No. 3 blended with multipurpose is the mix they mention when pink is the aim. Use the ericaceous bag if you are trying to hold or recover blue.
Can I keep using tap water?
Yes, when the water butt is empty, because a dry hydrangea is the worse outcome. The RHS says to use tap water rather than let blue cultivars drought. Rainwater is what you return to for colour. Heavy scale in the kettle is a clue that your mains is hard, and it is a number for your postcode. Collect what you can from a shed roof, and water from the tap in a dry spell.
What if I have no rainwater in a dry spell?
Use the tap, and water properly, so the rootball does not bake. Colour may drift while you are on hard mains. When rain returns, fill the butt and switch back. A colourant programme from April can still run on the weeks you have rainwater, at the label rate, on the compost. Skipping water to save the blue is how the shrub suffers. The drought exception is explicit in the RHS container advice.
Can compost turn a white hydrangea blue?
White, red and green cultivars stay that colour. Ericaceous compost and rainwater set the conditions for a responsive pink or blue mophead. They do not add a blue pigment to a white flower. If the label says white, panicle or Annabelle, the pot can still be a good home, and the flowers will stay white. Leave the colourant for a plant that can actually shift.

Related pages

Blue fertiliser comparison

How to use hydrangea colourant

Colourant or ericaceous compost

Best hydrangea colourant

Blue fertiliser for hydrangeas: which pack to buy first

Sources

Public UK and maker sources from the research digest, used for this page in October 2026. Prices, stars and review counts are the amazon.co.uk listings in that digest. The Westland dose and the 93% aluminium sulphate figure are from the manufacturer pages, not from the amazon bullets. A 5.0 from three ratings has no review text.

  1. RHS shrubby hydrangea growing guide (retrieved 22 October 2026)
  2. RHS acidifying soil (retrieved 22 October 2026)
  3. Westland Hydrangea Colourant (retrieved 22 October 2026)
  4. Westland safety data sheet, 8 January 2021 (retrieved 22 October 2026)
  5. BBC Gardeners' World, how to grow hydrangeas (retrieved 22 October 2026)
  6. Grow Scotland, hydrangea flower colour (retrieved 22 October 2026)
  7. Agriculture Act 1970, section 68 (retrieved 22 October 2026)
  8. Fertilisers Regulations 1991, Schedule 2 (retrieved 22 October 2026)