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Sourwood

Oxydendrum arboreum. Family Ericaceae. Tree.

Also called Sorrel tree, Lily-of-the-valley tree.

Sourwood is a slow-growing deciduous tree native to eastern North America, grown for its drooping sprays of white summer flowers and brilliant red, orange and purple autumn foliage. Its narrow, often irregular crown suits a woodland edge or specimen position, provided the soil is acidic, well drained and not allowed to dry out.

Hardiness

USDA zones 5a to 9b

UK hardiness: H6 (hardy, -15 to -20°C)

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Grows outdoors in the shaded zones. Select a zone to see what else grows there.

At a glance

Sun
Full sun to partial shade; autumn colour is usually strongest in sun. Give afternoon shade in hot climates and shelter from drying winds.
Water
Keep the root zone evenly moist, especially during establishment and summer drought; avoid waterlogged soil.
Soil
Acidic, humus-rich, well-drained soil, ideally pH 4.5-6.0. Unsuitable for chalky or alkaline ground and poorly drained, compacted sites.
Height
20-30 ft (6-9 m) in gardens; occasionally taller
Spread
10-20 ft (3-6 m)
Flowers
Summer; small, creamy-white, bell-shaped flowers in long, drooping sprays.
Foliage
Glossy green, finely toothed, elongated leaves turn scarlet, orange or purple in autumn. Pale seed capsules can remain after leaf fall.
Growth
Slow
Care needed
Medium

Care

  • Plant a small container-grown tree in its permanent position; established sourwoods transplant poorly.
  • Water deeply during dry spells for the first two or three growing seasons, then during prolonged drought.
  • Spread 2-3 in (5-8 cm) of leaf mould or composted bark over the root zone, keeping it clear of the trunk.
  • Keep grass and digging away from its shallow roots.
  • Feed only if growth or a soil test indicates a need; avoid heavy nitrogen applications.

Pruning. Prune sparingly in late winter or early spring, removing dead, damaged or crossing branches. Establish a sound framework while young rather than cutting back an older tree heavily.

Through the year

Spring
Plant when the soil is workable, watering thoroughly and mulching the root zone.
Late spring
Check new foliage for yellowing between green veins; test soil pH before applying treatments.
Summer
Check soil moisture during hot weather and water deeply before the root zone becomes dry.
Autumn
Renew mulch and use fallen leaves to make leaf mould.
Late winter
Remove damaged or crossing branches with minimal pruning.

In the garden

  • Use as a specimen against a dark evergreen backdrop to show off white flowers and autumn colour.
  • Plant at an open woodland edge with rhododendrons, mountain laurels and ferns.
  • Place near a terrace or path where its hanging flower sprays can be seen closely, leaving room for its mature crown.
  • Include in an eastern North American native planting for summer flowers and seasonal foliage.

Garden styles it suits: Woodland, Wildlife garden, Pollinator garden, Autumn colour and Traditional.

Plant it with: Rhododendrons, Azaleas, Mountain laurel, Highbush blueberry, Foamflower and Christmas fern.

Wildlife: Flowers provide nectar for bees and other pollinating insects. Sourwood is an important regional honey plant in its native range.

Problems to watch for

  • Yellow leaves with green veins usually indicate iron chlorosis in alkaline soil: test the soil and address the pH; a strongly alkaline site is unsuitable long term.
  • Leaf scorch and premature leaf fall can follow drought or drying winds: water deeply, mulch and provide shelter.
  • Root rot and decline occur in waterlogged soil: improve drainage before planting and avoid low, persistently wet ground.
  • Leaf spots may occur in humid weather: collect affected fallen leaves and avoid wetting foliage; minor spotting rarely needs treatment.

Safety

Not known to be toxic to people, cats, dogs or horses. Safety information for substantial ingestion is limited; do not encourage pets or children to eat the leaves, bark or seeds.

Easily confused with

  • Japanese pieris (Pieris japonica)

    Also has hanging white bell-shaped flowers, but is an evergreen shrub that usually flowers in spring; sourwood is deciduous and flowers in summer.

  • Black gum (Nyssa sylvatica)

    Has similarly vivid autumn colour, but its leaves usually have smooth edges and its flowers are inconspicuous; sourwood has finely toothed leaves and conspicuous white flower sprays.

The name

Oxydendrum means 'sour tree' in Greek, referring to the acidic taste of the leaves, while arboreum means tree-like.

Propagation. Usually grown from seed, surface-sown on a moist, acidic medium with light available for germination. Softwood cuttings are possible but can be difficult to root and establish.

Checked against open data

Accepted botanical name
Oxydendrum arboreum
In the US and Canada
Native in the lower 48 states.
Native range
Alabama, Florida, Georgia, Indiana, Kentucky, Louisiana, Maryland, Mississippi, New Jersey, New York, North Carolina and Ohio.
Measured
Soil pH 4 to 6.5. Average height 960 cm.

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

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