Hydrangeas in Australian gardens: colour, pruning and where they fail

Two things decide whether a hydrangea works, and fertiliser is not one of them: where the plant went in the ground, and what someone did to it with the secateurs the following winter. Get both right and a hydrangea is a long lived shrub across southern Australia. Get them wrong and you own a green shrub that refuses to flower, or a crisp brown one by mid January.

Before you spend anything, work out which of the three groups you are buying. Pruning, colour behaviour and sun tolerance all change between them, and most bad hydrangea advice comes from applying one group's rule to another.

The three groups you are choosing between

Bigleaf mophead

The classic form: Hydrangea macrophylla, with thick stems, broad soft leaves and dense domed heads of showy sterile florets. It flowers on wood built the previous season, holds colour for six to eight weeks, and it is the only group where soil chemistry affects blue versus pink. Our macrophylla runs from 175mm squat pots up to 500mm, from $25.95.

Lacecap

Same species, different flower architecture. A lacecap carries a flat plate of small fertile flowers ringed by larger sterile ones, so the head reads lighter from a distance. Hydrangea macrophylla Jogasaki is a lacecap with narrow, pointed florets, in 140mm from $21.95, a sensible way to trial the group before committing to a row. Lacecaps take the same pruning and respond to pH the same way as mopheads. Hydrangea Runaway Bride sits in this looser camp, carrying flower along each stem rather than only at the tip.

Panicle

Hydrangea paniculata is a different plant with a different rulebook. The heads are conical rather than domed, they open cream or white and age through pink or red as autumn nights cool, and, critically, it flowers on the current season's growth. It also takes more sun, wind and cold than the bigleaf types. Candlelight and Hercules are panicle selections in 200mm; Diamond Rouge and Sundae Fraise are panicles too, and more compact.

Soil pH and flower colour, and where it stops applying

Soil below about pH 5.5 keeps aluminium soluble and available to the roots, and the flowers come out blue. Soil above about pH 6.5 locks that aluminium away, and the same plant flowers pink. In between you get muddy mauves, and often mixed heads on the one bush.

This is where people waste money. The mechanism applies to bigleaf types only, meaning mopheads and lacecaps. It does not apply to white bigleaf cultivars, which have no pigment to shift and stay white whatever you pour on them, Runaway Bride included. It does not apply to panicle hydrangeas at all: Candlelight, Hercules, Diamond Rouge and Sundae Fraise move from cream to pink or red with temperature and flower age, not soil chemistry, and neither aluminium sulphate nor lime will alter that.

For blue on a bigleaf, aluminium sulphate applied in autumn and again in early spring will get you there, but treat it as an ongoing job, not a one-off correction. For pink, dolomite lime lifts pH. Neither shifts quickly, so allow a full season before judging, and expect little movement in a bed against a rendered wall or concrete path. In a pot you set the medium and it stays set, which is why reliable blue usually means container-grown.

Pruning: the reason most hydrangeas never flower

Bigleaf mopheads and lacecaps set next summer's flower buds on this summer's wood, from late summer onward. Cut the whole bush down in winter to tidy it up, which looks sensible on a mess of bare sticks, and you have removed every flower bud. It regrows strongly, looks healthy and gives you nothing. Two or three years of that and people decide the plant is faulty.

Do this instead. After flowering, or in late winter if you leave the spent heads on for frost protection, cut each flowered stem back to the first pair of fat buds below the old head, usually 150 to 300mm down. Then take a quarter of the oldest stems out at ground level to keep the base renewing. Nothing else.

Panicle hydrangeas are the reverse. They flower on new growth, so late winter is the time to cut them back hard, to a framework of stubs 300 to 500mm from the ground. Harder pruning gives fewer, larger heads on stronger stems. Left alone they still flower, but the heads shrink and the stems flop under rain.

If you have inherited a hydrangea and cannot identify it, watch one full flowering first. A conical head means panicle: prune in late winter. A domed or flat head means bigleaf: cut only to the fat buds, and never take the whole bush down.

Position and the afternoon sun problem

Morning sun, afternoon shade. For bigleaf types that is the rule in almost every Australian climate, and not negotiable in the hotter ones. Four to five hours of morning light against an east-facing wall, or dappled light under a deciduous canopy, gives reliable flowering. Direct sun from 1pm in January scorches leaf margins and browns the heads within days, even with adequate soil moisture: the plant cannot move water to the leaf edge fast enough.

Panicle types take full sun through Melbourne, Adelaide, Canberra and the southern tablelands, and want morning sun with light afternoon shade in Sydney, Perth and around Brisbane. Inland, in hot dry positions, treat no hydrangea as a full-sun plant.

They also resent root competition: a hydrangea under an established gum or fig spends its life losing a water contest. In a shaded bed they sit well with camellias, gardenias and Japanese maples, and our guide to Japanese maples in Australian gardens covers the same aspect rule.

Water, and what the afternoon wilt actually means

Hydrangeas collapse theatrically on hot afternoons and stand back up overnight. That is transpiration outrunning uptake, not proof the soil is dry. Push a finger 100mm in before you reach for the hose. Damp soil under a flat plant means heat, and watering again only raises the risk of root rot.

In the ground, established plants want a long drink twice a week through summer, more often in sand, plus 50 to 75mm of coarse mulch kept clear of the stems. The roots are shallow, so a light daily sprinkle is worse than useless: it trains them into the top few centimetres, where heat reaches them first.

Growing them in pots

Containers suit hydrangeas, with two conditions. The first is volume: a bigleaf under 400mm diameter needs water twice a day in February and will still shed leaf edges. The second is aspect: the morning sun rule gets stricter, because a pot heats through its sides. Use a quality potting mix, feed with a controlled release fertiliser in early spring and again after flowering, and repot or root prune every second or third year.

For a run of pots or a long bed, assorted hydrangeas start at $16.95 in 140mm and run through to 200L. Larger 330mm plants including Tuxedo Pink, Strawberries and Cream and Oriental Lace suit a bed that has to look finished immediately.

When a hydrangea is the wrong plant

It is cheaper to hear this now than in March.

A west-facing courtyard with reflected heat off brick or paving will kill a bigleaf hydrangea, or keep it permanently half-scorched. A garden with no irrigation, or one you visit every second weekend over summer, is not a hydrangea garden: they hold no drought reserve and will not forgive a missed fortnight in a heatwave. Front-line coastal positions with salt-laden wind burn the foliage regardless of aspect. Dry shade under mature trees looks like the right conditions and is not, because the shade arrives with roots attached. In the humid subtropics it is panicle types that struggle: without cool autumn nights their colour stays washed out, so bigleaf in morning sun is the safer choice.

They are also not a screen or a hedge: deciduous, so in cold districts you get bare stems from May through to September, and they never knit into a solid barrier. If that is the job, look at low maintenance evergreens instead. If the position is hot and dry, lavender and similar Mediterranean shrubs will do what you are asking, and our guide to low water gardens in Australia is a better starting point.

Buying and sizing

If you are happy to wait a season, 140mm and 200mm stock is the sensible spend, planted in autumn or early spring. For a bed that has to perform this summer, move up to 300mm or larger. Improve the soil with compost, water in properly, and stake nothing. The full range sits in our hydrangea collection, with more colour across the flowering plants range.

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