Smooth hydrangea

Sun

Partial shade, ideally morning sun and afternoon shade. Full sun is suitable in cooler climates if the soil stays moist; avoid hot, exposed sites.

Water

Keep the root zone evenly moist, especially during establishment and summer drought. Water deeply when the upper soil starts to dry, rather than sprinkling lightly each day.

Soil

Moist, well-drained soil enriched with organic matter. Tolerates clay if drainage is adequate and grows in acidic to mildly alkaline soil; changing soil pH does not turn its flowers blue.

Height

3-5 ft (0.9-1.5 m)

Spread

3-5 ft (0.9-1.5 m), becoming wider where suckers are allowed to grow

Flowers

Early to late summer, sometimes continuing into early autumn. Species flowers are creamy white; cultivars may be white or pink, often ageing green and then tan.

Foliage

Broad, opposite, toothed green leaves with pointed tips; autumn colour is usually yellowish and modest.

Growth

Moderate to fast once established.

Care needed

Medium

• Water deeply during dry spells; do not let newly planted shrubs dry out.
• Apply 2-3 in (5-8 cm) of organic mulch in spring, keeping it away from the stems.
• Top-dress with compost in spring; use a modest amount of balanced shrub fertiliser only if growth is weak.
• Support large-flowered cultivars with a discreet ring before the flower heads become heavy.
• Remove unwanted suckers at their point of origin to limit the clump's spread.

Pruning. In late winter or early spring, remove dead, damaged and weak stems, then shorten remaining stems as needed. It flowers on new growth, but cutting everything to the ground each year can produce long stems that flop under heavy flowers.

Late winter

Remove dead and weak stems before new growth starts; retain a sturdy framework where flopping is a problem.

Spring

Plant while the soil is workable, water thoroughly and mulch the root zone.

Late spring

Install supports around large-flowered plants before stems become tall.

Summer

Check soil moisture during hot weather and water deeply when needed.

Early summer

Take softwood cuttings from healthy, non-flowering shoots.

Autumn

Leave sound dried flower heads for winter structure, and remove unwanted suckers.

• Plant in groups along a woodland edge, with ferns and sedges beneath.
• Use as a low, informal flowering hedge beside a shaded path.
• Repeat through a mixed border with hostas and astilbes for contrasting leaf shapes.
• Use white-flowered forms in a white garden, allowing space for the mature clump.

Garden styles it suits: Woodland, Cottage garden, English country, White garden, Traditional and Wildlife garden.

Plant it with: Hostas, Astilbes, Christmas fern, Japanese forest grass, Coral bells and Solomon's seal.

Wildlife: The small fertile flowers of lacecap forms provide pollen and nectar for visiting insects. Showy mophead cultivars often have fewer accessible fertile flowers and offer less food; the shrub also provides seasonal cover.

• Wilting and scorched leaf edges: check soil moisture, water deeply and mulch; provide afternoon shade in hot climates rather than repeatedly watering already-wet soil.
• Flopping flower stems: avoid excessive nitrogen and severe annual pruning; fit supports before flowering.
• Powdery mildew: improve airflow, avoid crowding and keep roots adequately watered; mild late-season infection rarely needs treatment.
• Leaf spots: remove fallen affected leaves, water at soil level and thin crowded growth; occasional spotting is usually cosmetic.

Toxic if eaten by cats, dogs, horses or people. Leaves, flower buds and other parts contain cyanogenic glycosides; ingestion most often causes digestive upset, while serious poisoning is uncommon but possible. Keep clippings away from grazing animals and seek medical or veterinary advice after ingestion.

• Bigleaf hydrangea (Hydrangea macrophylla)

  Usually has thicker, glossier leaves and stouter shoots. Many forms have pink or blue flowers influenced by soil chemistry, unlike smooth hydrangea.

• Panicle hydrangea (Hydrangea paniculata)

  Has cone-shaped flower clusters rather than flat or rounded heads and usually develops a larger, more woody framework.

• Snowball viburnum (Viburnum opulus 'Roseum')

  Has distinctly three-lobed, maple-like leaves and rounded white flower heads in late spring rather than summer.

Hydrangea comes from Greek words for water and vessel, referring to the seed capsules, while arborescens means becoming tree-like.

Propagation. Take softwood cuttings in early summer, layer a low branch, or lift rooted suckers while dormant. Seed can be used for the species, but seedlings from named cultivars will not reliably match the parent.

Accepted botanical name

Hydrangea arborescens

In the US and Canada

Native in the lower 48 states. Introduced in Canada.

Native range

Alabama, Arkansas, Delaware, District of Columbia, Florida, Georgia, Illinois, Indiana, Kentucky, Louisiana, Maryland and Mississippi.

Sources: GBIF (CC BY 4.0), USDA PLANTS and Trefle (CC BY 4.0). The care and design notes are LandscapeGPT's own.