When to plant hydrangeas

Spring or autumn: what the RHS says

The RHS growing guide for shrubby hydrangeas says the best time to plant is in spring or autumn. It does not pick one over the other. It does say to plant soon after you buy the plant.

Plants in containers are more flexible. The RHS says containerised plants can be planted all year round, as long as the soil is not frozen, too wet, or excessively dry in summer.

The RHS does not give months for spring and autumn. That is why the chart above shows them as approximate.

What you are likely to buy

The RHS says garden centres usually sell shrubby hydrangeas in 3 litre pots, at 30 cm or more in height. Mail-order plants may arrive as young cuttings in 9 cm or 1 litre pots.

Check the height on the label. Some grow into sizeable shrubs. The RHS plant guide lists *Hydrangea arborescens* 'Annabelle' at 1.5–2.5 m high and wide, while *H. macrophylla* 'Altona' reaches 0.5–1 m high.

Where to plant: light shade and shelter

The RHS growing guide says to ideally plant your shrubby hydrangea in light shade. A sunnier spot works too, if the soil is not too dry in summer.

The RHS plant guide puts it as sun or light shade, in a spot sheltered from cold winds. The new spring growth is prone to frost damage, so it says to avoid frost pockets and scorching wind. The growing guide gives the same reason for avoiding frost pockets and exposed spots: to reduce late frost damage to the new spring growth.

Spacing depends on how vigorous the hydrangea is. The RHS gives 90 cm to 2.4 m.

Soil

The RHS plant guide says hydrangeas like moister soils that are not prone to waterlogging. The growing guide calls them well suited to clay soils, and says to plant in moist soil that drains well.

Improving the soil and mulching with organic matter helps keep it moist, the plant guide says. It suggests garden compost or a manure-based soil conditioner.

How to plant a hydrangea

The RHS method is simple. Water the plant well before planting. Dig a hole as deep as the rootball and three times wider. Set the plant so the surface of its compost is level with the surrounding soil. Avoid planting it too deep.

Firm the soil around the rootball by pressing with the heel of your boot or shoe. Water it in and keep the soil damp in the following weeks.

Mulch with a soil improver, and leave a gap of 10–15 cm around the base to reduce the drying out of the soil.

Watering in the first year

Hydrangeas thrive in evenly moist soil, the RHS says. In their first growing season, from spring to summer, water newly planted hydrangeas when there has been no significant rain for 7–10 days.

Even mature plants benefit from watering in hot, dry spells, and regular mulching reduces moisture stress.

Soil acidity and flower colour

On blue or pink cultivars, the flower colour depends on your soil. The RHS says flowers are blue in acid soil, mauve in slightly acid to neutral soil, and pink in alkaline soil. White, red and green cultivars do not change with pH.

Blue-flowered cultivars stay blue in acidic soil at pH 4.5–5, the growing guide says. If they turn purple or pink, it suggests a hydrangea blueing compound containing aluminium sulphate, used as the maker says. This will not work on very alkaline soil, or where you can see pieces of chalk. There, grow blue hydrangeas in containers of peat-free ericaceous compost. Water blue hydrangeas with rainwater where you can, but use tap water if rainwater runs short, so the plant does not suffer drought stress.

For better red or pink flowers, the RHS applies ground limestone or chalk in winter, at 75–100 g per square metre.

Hydrangeas in pots

The RHS says more compact cultivars can be grown in containers. It names *Hydrangea macrophylla* 'Red Hot Violet' and 'Altona'.

For blue flowers on alkaline soil, use ericaceous compost. Otherwise, the RHS mixes two parts peat-free John Innes No 3 with one part peat-free multipurpose compost.

Move pots to a shadier spot in summer so they dry out less. In winter there is no need to wrap the pot, the RHS says. Raise it on pot feet or bricks so it drains. You can move pots to a more sheltered spot, such as a cold greenhouse or against a sheltered, sunny wall, to protect the buds and new growth.

Late frost and the first spring

The RHS says most hydrangeas are hardy across most of the UK, but the young buds may be damaged by late frosts.

On mophead and lacecap hydrangeas (*H. macrophylla*), the RHS says there is no need to remove the faded flowers straight away. The dead blooms help protect the tender new growth below from frost.

If shoots are badly frosted, prune them back to just above the first undamaged pair of buds on live wood.

On mophead and lacecap hydrangeas (*H. macrophylla*) and *H. serrata*, the RHS says the flowers form on new shoots growing from the previous year's stems. Heavy pruning removes the flowers, so it prunes these lightly in mid-spring.