Hydrangea

Light and exposure

The hydrangea does not like scorching full sun, which fades its flowers and wilts its foliage in summer. It prefers partial shade, or a spot that catches the gentle morning sun and stays in shade during the hot afternoon hours. It is a plant of light woodland.

Under cool, often hazy skies, it tolerates a slightly brighter exposure than in a warm region, but always avoid the sun of the hottest hours without any shade at all. A corner that only catches the morning sun, sheltered from the midday sun and drying winds, suits it perfectly.

Watering

Its name says it: hydrangea means water vessel. The hydrangea is thirsty and hates being short of water. It needs soil that stays permanently cool, especially in summer and especially in the first two years. A hydrangea that drops its leaves in the middle of the day is asking for water.

Water generously in dry spells, at the base, preferably in the evening. A good mulch at its base, bark or dead leaves, keeps the soil cool and spaces out the watering. In a damp temperate climate, the rain covers a large part of its needs, but the dry snaps of high summer call for vigilance.

Soil and repotting

The hydrangea likes a soil rich in humus, cool, deep, and rather acid to neutral. It dreads chalky, dry soils, where it yellows (chlorosis) and stagnates. Enrich the soil with ericaceous compost or leaf mould at planting to satisfy it.

It is also the soil that decides the flower colour of the blue-pink varieties. In acid soil, rich in available aluminium, the flowers turn blue; in neutral or chalky soil, they stay pink. A garden soil that is naturally acid therefore pushes the flowers towards blue with no effort at all. To blue them more, you add crushed slate or an aluminium sulphate-based product; to keep the pink, you avoid acidifying. The white varieties, for their part, do not change colour.

Temperature and humidity

The hydrangea is hardy to around minus 15 degrees, which leaves it comfortable across USDA zones 6 to 9, and the plant comes through winter without trouble. The real risk is not the settled winter but the late spring frosts: they can scorch the young shoots and the flower buds breaking out early in the season, and compromise the year’s flowering without killing the plant.

As most hydrangeas flower on the previous year’s wood, a bud frozen in spring means one flower fewer the following summer. Hence the value of not pruning too short and keeping the faded flowers on the plant all winter: they naturally protect the buds below against the frost. In spots with frequent late frosts, a winter fleece over the young shoots on the cold nights of early spring can save the flowering.

Pruning and upkeep

Pruning the hydrangea is simple but done at the right moment: in spring, once winter is over, not in autumn. Keep the faded flowers on the plant all winter, they protect from frost the buds just below, which will give the summer flowers. Above all, do not cut everything down to the ground.

In spring, once the severe frosts have passed, cut the dry flowers just above the first fine pair of plump buds, and remove the dead wood, the spindly branches and the oldest stems at the base to open up and rejuvenate the clump. Pruning too severely or at the wrong time removes the wood that carried the flowers, and the hydrangea then makes leaves but no balls. When in doubt, prune little.

Pests and diseases

The hydrangea is robust, but powdery mildew (Erysiphe) can cover its foliage with a white felting in hot, stuffy weather, and chlorosis makes the leaves yellow between the veins when the soil is too chalky or short of iron. Against powdery mildew, open up the planting; against chlorosis, add ericaceous soil and, if needed, a little iron.

Attacks generally stay light in a temperate climate. Foliage that wilts in full heat is most often not a disease but a simple bout of thirst: water abundantly and mulch. Also watch for leaf spots in very damp weather, of no real consequence for the plant.

Propagation

The hydrangea takes very easily from cuttings, which makes it an ideal shrub to propagate for free. In high summer, take non-flowering stems of the year, about fifteen centimetres long, remove the lower leaves and halve the large upper leaves to limit evaporation. Plant these cuttings in a light mix and keep them shaded, cool and moist.

Rooting takes a few weeks. Another even simpler method is layering: lay a low stem on the ground, cover it with soil leaving the tip sticking out, and it roots by itself. Sever and transplant the young plant the following year. Bring the young cuttings under shelter for their first winter.

Toxicity

All parts of the hydrangea are slightly toxic if eaten and can cause digestive upset. Harmless in the garden, but keep young children away from the leaves and flowers.

Month-by-month care calendar

Green Hub tips

• Never prune in autumn: keep the faded flowers all winter, they protect from frost the buds that will make the summer flowers.
• A hydrangea that wilts its leaves in full heat is thirsty, not diseased: water abundantly and mulch the base.
• The colour follows the pH: an acid soil draws towards blue. Add crushed slate for a true blue, let it be for pink.
• Take cuttings in high summer from non-flowering stems, or layer a low stem: the hydrangea propagates all by itself, for free.