How to make hydrangeas blue
Blue hydrangeas need acid soil. Sulphur is how you get it.
Mophead and lacecap hydrangeas flower blue when the soil is acid enough, roughly pH 5.5 or below, for the roots to take up aluminium. From about pH 6.5 they flower pink, and in between they turn mauve. Lowering the pH with sulphur is the lasting fix; expect the colour to shift over one or two flowering seasons.
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- Only mophead and lacecap types (Hydrangea macrophylla and H. serrata) change colour. White varieties stay white.
- Test first. Blue needs pH 5.5 or below; most UK garden soil sits higher.
- In pots, skip sulphur: use ericaceous compost, rainwater and a hydrangea colourant.
How much sulphur
| 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, loam | 110 g/m² per pH unit, in two doses | Spring to early autumn, six months apart |
| New bed, sandy soil | 55 g/m² per pH unit, one dose | Spring to early autumn |
| New bed, clay | 175 g/m² per pH unit, in three doses | Six months apart |
| Pots | Do not use | Ericaceous compost instead |
Rates as on our product page: University of Georgia and Cornell (established plants); Ohio State, Purdue and University of California (beds); RHS (pots). Never more than 73 g/m² in one dose on beds, or 112 g/m² a year on planted ground. Around established plants, work the pastilles lightly into the surface and water in. In a new bed, water them, let them break up for a few days, then fork them into the top 15 cm.
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 sulphur will not turn them blue
- The soil fizzes with vinegar. That is free chalk or lime, and it cannot realistically be acidified. Grow blue hydrangeas in a large pot of ericaceous compost instead.
- You water with hard tap water. Every can pushes the pH back up. Use rainwater.
- It is the wrong hydrangea. H. paniculata and H. arborescens, including 'Annabelle', do not change colour.
- The soil has little aluminium. Lower pH frees the aluminium already in the soil. Where there is little, a hydrangea colourant (aluminium sulphate) supplies it directly.
Common questions
How long does it take to turn hydrangeas blue?
Sulphur is turned into acid by soil bacteria, so it is slow in cold soil. Apply from spring to early autumn, retest after three to six months, and expect the change over one or two flowering seasons. Mauve on the way is normal.
Do coffee grounds or vinegar turn hydrangeas blue?
Not reliably. Vinegar is washed through in days, and used coffee grounds are close to neutral. Sulphur is what holds the pH down for years.
Is aluminium sulphate better than sulphur?
It is faster, but you need about seven times as much for the same pH drop, and repeated doses can build aluminium up in the soil to harmful levels (RHS). Use it for a quick result in a pot; use sulphur for borders. See aluminium sulphate for hydrangeas.
Why have my blue hydrangeas turned pink?
The pH has crept up, usually from hard tap water, lime from nearby mortar or concrete, or mushroom compost. Retest, switch to rainwater and dress with sulphur at up to 25 g/m².
How do I keep hydrangeas pink?
Keep the soil at about pH 6.5 or a little above with Granulated Lime, water with tap water, and avoid ericaceous compost. See how to make hydrangeas pink.
More: Sulphur Soil Acidifier · aluminium sulphate for hydrangeas · soil pH calculator · how to make hydrangeas pink · how to make soil more acidic · what is ericaceous compost? · how to test soil pH
Next summer's blue starts in the soil
Sulphur-bentonite pastilles that spread without dust and break apart when wet. Up to 25 g/m² around established hydrangeas, spring and late autumn.
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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.