Why Do Hydrangeas Change Colour?

Plant a hydrangea by the front door, watch it flower sky-blue, move house, and see the very same shrub bloom candy-pink in its new garden. No other common garden plant performs this trick, and it has fascinated gardeners for two centuries. The explanation is pure chemistry — a litmus test you can grow — and once you understand it, you can nudge the colour yourself. Here is what is actually happening beneath the mopheads.

The short version: it's aluminium, not pH directly

The colour-shifting hydrangeas are the bigleaf hydrangea (Hydrangea macrophylla — the mopheads and lacecaps) and its close relative the mountain hydrangea (H. serrata). Their sepals carry a pigment called delphinidin-3-glucoside. On its own, that pigment reads pink. But when aluminium ions bind to it, the complex it forms reflects blue instead. So the real question is not 'how acidic is my soil?' but 'can my hydrangea get hold of aluminium?' — and that is where pH comes in. Aluminium is abundant in most soils, but it only dissolves into a form roots can drink in acidic conditions. In neutral or alkaline soil it stays chemically locked away, and the flowers default to pink.

• Acidic soil, roughly pH 5.5 and below: aluminium available → blue flowers.
• Around pH 5.5–6.5: partial aluminium uptake → purples, mauves, or a patchwork of blue and pink on one shrub.
• pH 6.5 and above: aluminium locked up → pink flowers.

This also explains the exceptions. White cultivars — along with the cone-flowered Hydrangea paniculata and 'Annabelle'-type H. arborescens — lack the pigment altogether, so no soil on Earth will turn them blue. And a hydrangea in a pot of ordinary peat-free compost may sulk pink even at low pH, simply because the compost contains little aluminium to absorb.

Why the 'petals' behave like a litmus test

A quirk worth knowing: the showy parts of a hydrangea head are not petals at all but sepals — modified leaves that surround the true, tiny flowers at their centre. It is these sepals that hold the delphinidin pigment, which is a member of the anthocyanin family, the same pigments that colour blueberries, red cabbage and autumn leaves. Anthocyanins are natural pH and metal indicators — red cabbage water famously turns pink in acid and blue-green in alkali — and the hydrangea has, in effect, planted that chemistry lesson in your border. Curiously, it runs opposite to litmus paper: acid soil gives blue hydrangeas, not red.

How to turn a hydrangea blue

• Check your pH first with an inexpensive soil test kit. If your soil is naturally alkaline or chalky, blue will be a running battle — a large pot of ericaceous (acidic) compost is far easier.
• Apply aluminium sulfate around the root zone in spring, diluted and watered on according to the packet, several times through the growing season. This both acidifies the soil and supplies the aluminium itself.
• Choose feeds low in phosphorus, because phosphorus binds aluminium in the soil before roots can take it up. A feed formulated for acid-loving plants is ideal.
• Water with rainwater where you can: tap water in hard-water areas slowly nudges soil back towards alkaline.
• Mulch with pine needles or composted bracken, and be patient — colour shifts take months, and the full effect usually shows the following summer.

How to turn a hydrangea pink

Going the other way means denying the plant aluminium. Raise the pH by working garden lime into the soil around the shrub in autumn or early spring, and repeat annually — soil chemistry drifts back. A high-phosphorus fertiliser helps by locking up whatever aluminium remains. In heavy, naturally acidic soils, true pink can be harder to achieve than blue, and the surest route is a container of ordinary (non-ericaceous) multi-purpose compost, where you control the chemistry completely. Either direction, work gradually: swinging pH violently in one season stresses the shrub more than it changes the flowers.

Tip: a hydrangea that opens one colour and drifts towards green, dusky pink or burgundy in late summer is not reacting to soil at all. That slow 'antiquing' of the heads as they age is normal, driven by fading pigments and the season winding down — and it is why dried hydrangea heads hold those vintage shades.

What colour changes can tell you about your garden

Because bigleaf hydrangeas broadcast their soil chemistry, they make a rough-and-ready diagnostic for everything nearby. A reliably blue hydrangea tells you the bed suits acid-lovers like camellias, rhododendrons and blueberries; a stubbornly pink one suggests alkaline conditions that lavender, clematis and most Mediterranean herbs will enjoy. Neighbours' hydrangeas are a preview of your own soil, too — a street of blue mopheads is practically a geological survey. It is worth adding that the shrub itself does not care: pink, purple or blue, the plant is equally healthy. The colour is information, not a symptom.

If you have inherited a mystery hydrangea and want to know whether it is a macrophylla that will respond to soil treatment — or a paniculata that never will — photograph it in PlantMe. The app identifies your exact plant from 35,000+ species and cultivars, tells you which type of hydrangea you have, and gives pruning and care advice matched to it. It works in 28 languages, and it is rated 4.4★ by more than 26,000 plant lovers.