Hydrangea: The Types, and Why the Colour Changes

Hydrangea is the only common flower whose colour is set by soil chemistry rather than by variety, which is why the same plant produces blue flowers in one garden and pink in another. Understanding that explains most of what people find confusing about them.

The Colour Is Aluminium, Not Genetics

In Hydrangea macrophylla — the mophead and lacecap types — flower colour depends on how much aluminium the plant can take up from the soil, and that availability depends on pH.

Acidic soil, below about pH 5.5, makes aluminium available and the flowers come blue. Alkaline soil, above roughly pH 6.5, locks the aluminium away and the same plant flowers pink. In between you get purples and muddled intermediate shades.

This region's soils tend acidic, which is why local hydrangeas skew blue.

Two things follow. White varieties do not change, because they have no pigment to shift — a white hydrangea will not turn blue whatever you do to the soil. And changing an established plant's colour takes a season or more of soil amendment, not a quick treatment.

Four Hydrangea Types

Macrophylla — mophead and lacecap. The florist's hydrangea, the colour-changing one, and the most tender.

Paniculata — conical white heads that age to pink and then to rust. Not pH-sensitive. Firmer sepals and better cut-flower behaviour than macrophylla.

Arborescens — very large white domes, Annabelle being the familiar one. The most prone to wilting as a cut flower and the hardest to revive.

Quercifolia — oakleaf, with lobed foliage that colours dramatically in autumn. Grown as much for the leaves as the flowers.

They Are Sepals, and That Is the Key to Everything

The showy parts of a hydrangea head are not petals but sepals, and they are unusually long-lived because their job is to keep advertising while the tiny true flowers at the centre set seed.

That is why hydrangea dries so well while most flowers do not, and why a head can look identical for months once it has firmed up.

It also explains the wilting. Sepals present an enormous surface area losing water continuously, on a woody stem that draws slowly — so a hydrangea can go from perfect to collapsed in a couple of hours.

Reviving a Wilted Head

Because the sepals are broad and thin, they absorb water directly through their surface. That makes the rescue possible and it looks wrong.

Fill a sink with cool water. Re-cut the stem sharply with secateurs, ideally under water. Then lay the whole flower in, head fully submerged, and leave it one to two hours. Take it out, shake gently, and put it in fresh water. A fully collapsed stem is frequently standing again within the hour.

For a stem that keeps failing, dip the freshly cut end in powdered alum before it goes back in the water. It is astringent, it keeps the cut surface open, and it works more often than it has any obvious right to.

The Autumn Harvest

Left on the plant into late summer and autumn, macrophylla heads begin to lignify — laying down structural material and turning papery — while the pigment breaks down unevenly into mottled green, burgundy, bronze and dusty rose.

That mottling cannot be produced by dyeing and cannot be rushed. It is the antique hydrangea sold at a premium in autumn, and it is harvested roughly late August into October in this region.

Heads cut then hold their form as they dry and are usable for the following year, which makes autumn the one time it is worth buying more than you need. Our Preserved Flower Dome is built from this material.

Drying, Which Almost Everyone Does Wrong

The standard advice to hang flowers upside down in the dark is wrong for hydrangea.

They dry best standing upright in a vase with two or three inches of water, left to take up what they want and then simply allowed to run dry over a week or two. The gradual loss lets the sepals firm up without collapsing. Hung dry and fast, the heads shrivel and the colour goes flat.

Keep them out of direct sun afterward, which bleaches the burgundies toward beige within a couple of months.

Buying Fresh Hydrangea

Press a sepal between finger and thumb. Soft and fleshy means an early-season head that will wilt readily and revive readily. Papery and slightly stiff means a late-season head that will dry rather than die.

Give them deep water, unlike most of what shares their vase — the woody stem needs a substantial column to draw from. Strip the lower foliage hard, because hydrangea leaves hold a great deal of water and compete directly with the head.

And keep them cool and out of sun. Our Mixed Hydrangea is built around the deep-water requirement, which is the opposite of what tulips or ranunculus want in the same arrangement.