Aluminium sulphate for hydrangeas
Aluminium sulphate blues hydrangeas fast. In borders, sulphur is safer.
Hydrangeas flower blue when their roots can take up aluminium, which happens in acid soil, roughly pH 5.5 or below. Aluminium sulphate supplies aluminium and acidifies at once, so it works quickly. The RHS warns that repeated doses can build aluminium up in the soil to harmful levels. In a border, lower the pH with sulphur and let the aluminium already in the soil do the work.
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- In pots: a hydrangea colourant, which is aluminium sulphate, at the pack rate.
- In borders: sulphur, up to 25 g/m² around established plants, early spring and late autumn.
- White, paniculata and 'Annabelle' hydrangeas do not turn blue on anything.
Side by side
| Sulphur | Aluminium sulphate | |
|---|---|---|
| Speed | A season: soil bacteria do the work | Weeks |
| Adds aluminium | No: frees what is already in the soil | Yes |
| Product for the same pH drop | 1 | Several times as much |
| Build-up risk | None from aluminium | Aluminium can build up to harmful levels (RHS) |
| Best for | Borders and beds | Pots, or a quick colour change |
Aluminium build-up: RHS, Acidifying soil. Aluminium sulphate takes several times as much product as sulphur because each gram releases far less acid.
How much sulphur for blue hydrangeas
| Where | Rate | When |
|---|---|---|
| Established hydrangea in the ground | Up to 25 g/m² over the root area | Early spring and late autumn, at most twice a year |
| New bed, per pH unit | 55 g/m² sand, 110 loam, 175 clay | In doses of up to 73 g/m², six months apart |
| Pots | Do not use sulphur | Ericaceous compost and a colourant |
Work out your exact amount. Enter your area, your test reading and your soil type, and the calculator gives the dose, the timing and the bags to buy.
When neither will turn them blue
- The soil fizzes with vinegar. That is chalk, and it cannot realistically be acidified. Grow blue hydrangeas in a large pot of ericaceous compost.
- You water with hard tap water. Every can pushes the pH back up. Use rainwater.
- It is the wrong hydrangea. Only mophead and lacecap types (H. macrophylla and H. serrata) change colour.
Common questions
How much aluminium sulphate per hydrangea?
Follow the pack. Colourants vary in strength, and we do not sell aluminium sulphate, so we will not guess a rate. Do not repeat it beyond the pack directions.
Is aluminium sulphate safe for the soil?
Used now and then in a pot, yes. Used year after year in the ground, aluminium can build up to levels that harm plants (RHS). Sulphur avoids that.
Can I use sulphur and aluminium sulphate together?
In the ground, you rarely need both. Sulphur lowers the pH and frees the soil's own aluminium. If your soil is very low in aluminium, a light dose of colourant can help once the pH is down.
How long does it take to turn hydrangeas blue?
With aluminium sulphate, a season. With sulphur, expect the change over one or two flowering seasons, with a mauve stage on the way.
Why are my hydrangeas still pink after aluminium sulphate?
Usually chalky soil or hard tap water pulling the pH back up. Test the soil, switch to rainwater, and for chalk use a pot of ericaceous compost.
More: how to make hydrangeas blue · how to make hydrangeas pink · Sulphur Soil Acidifier · soil pH calculator · iron sulphate vs sulphur
Blue that lasts, without the build-up
90% elemental sulphur in pastilles that spread without dust. Up to 25 g/m² around established hydrangeas, spring and late autumn.
Buy the size for your border
Free UK delivery over £40. Buy now goes straight to checkout.
Free UK delivery over £40. Dispatched in 1 to 2 working days. Packed and posted from Stockport.