Hydrangea Colour and Soil pH
Why bigleaf flowers turn blue or pink, how far you can move them, and how long it takes.
Hydrangea colour and soil pH are linked through one metal: aluminum. When a bigleaf hydrangea can take up aluminum, its flowers turn blue; when it cannot, they stay pink. Soil pH decides how available the aluminum is.
Only bigleaf hydrangeas (mophead and lacecap) change colour this way. Penn State Extension notes that smooth hydrangeas such as Invincibelle Spirit keep their colour whatever the soil, and white bigleaf flowers stay white.
Hydrangea colour at a glance: Blue needs aluminum, which plants take up in acidic soil. The University of Massachusetts gives pH 5.5 and below for blue and 6.5 and above for pink. To go bluer, lower the pH (Penn State suggests elemental sulfur); to go pinker, add lime. Expect more than a year for a change. Test your soil first.
Growing bigleaf hydrangeas? See How to Prune Hydrangeas in Canada for the pruning that keeps them flowering.
pH and Colour
| Soil pH | Flower Colour | Source |
|---|---|---|
| 5.5 and below | Blue | University of Massachusetts |
| 5.2–5.5 | Target for blue | Penn State Extension |
| 6.0–6.2 | Target for pink | Penn State Extension |
| 6.5 and above | Pink, rose or red | University of Massachusetts |
The two extension guides use slightly different numbers; between them, the in-between soils give mauve or purple. How deep the colour gets also depends on the cultivar: the University of Massachusetts notes some are always pastel and others rich.
Test Before You Change Anything
Soils differ across Canada, and from one side of a house to the other. Penn State Extension warns that lime can leach from house foundations and sidewalks, which can make blue flowers difficult or impossible right beside them. A soil test tells you where you are starting.
Going Bluer
Penn State Extension suggests lowering the pH with elemental sulfur or a soil acidifier, and avoiding high-phosphorus fertilizer, which keeps aluminum out of the plant. It notes that commercial growers use aluminum sulfate in potting mixes, but in soils that already hold aluminum, adding it is unnecessary and may damage the roots.
Going Pinker
Penn State Extension’s route to pink is the reverse: add lime to raise the pH and use a fertilizer with more phosphorus to keep aluminum out. It says a change can take more than a year to show.
Why a New Plant Changes Colour
A hydrangea bought blue in a pot was grown in a mix set up for blue. Planted in your garden, it starts responding to your soil, so the colour can drift in the first year or two.
Frequently Asked Questions
What makes a hydrangea blue or pink?
Aluminum. Bigleaf hydrangeas turn blue when they can take up aluminum and stay pink when they cannot. Soil pH controls how available it is.
What soil pH gives blue hydrangeas?
The University of Massachusetts gives pH 5.5 and below for blue; Penn State Extension aims for 5.2 to 5.5.
What soil pH gives pink hydrangeas?
The University of Massachusetts gives pH 6.5 and above for pink, rose or red; Penn State Extension aims for 6.0 to 6.2.
How do I make my hydrangea more blue?
Lower the soil pH, for example with elemental sulfur as Penn State Extension suggests, and avoid high-phosphorus fertilizer. Test the soil first.
How long does it take to change hydrangea colour?
Penn State Extension says it may take more than a year to see a change.
Do all hydrangeas change colour?
No. Only bigleaf hydrangeas respond to soil this way. Smooth and panicle hydrangeas keep their colour, and white bigleaf flowers stay white.
Sources
- University of Massachusetts Amherst Extension — Growing Hydrangeas
- Penn State Extension — Why Doesn’t My Hydrangea Bloom?