Tree to 20(–30) m, usually crooked when young. Bark grey brown, soon deeply furrowed with close, acute ridges. Shoots red brown, smooth or puberulent; buds reddish, tiny and rounded and set above the large, pale triangular scar of each leaf. Leaves 6–23 × 2–8 cm, elliptic; base cuneate to rounded, apex acute to acuminate, glabrous or with bristly hairs near veins; margin very finely serrated, rarely entire, sometimes ciliate; autumn colour usually red; petiole c. 1 cm. Inflorescence a terminal panicle opening in mid-summer, formed of drooping racemes or secondary panicles, with 15–50 flowers on downy stalks; 2 bracteoles, medial or distal. Flowers white, in June–July (United States), corolla 4–7 mm long, cylindrical or slightly urn-shaped, slightly fragrant, with 5 pubescent lanceolate sepals connate for half their length, 5 petals, connate for most of their length; lobes much shorter than tube. Ten stamens. Fruit a dry, ovoid 5-celled capsule 4–8 mm long, containing 25–100 narrowly oblong seeds, ripening in autumn and persisting through winter. (Flora of North America 2021; Bean 1976).
Distribution United States Alabama, Florida, Georgia, Indiana, Kentucky, Louisiana, Maryland, Mississippi, New Jersey, New York, North Carolina, Ohio, Pennsylvania, South Carolina, Tennessee, Virginia, West Virginia.
Habitat Woodlands on free-draining or gravelly acidic soils; on cliff faces, along streams and around the margins of swamps and pine heaths; to 1700 m asl.
USDA Hardiness Zone 5
RHS Hardiness Rating H6
Conservation status Least concern (LC)
Although the sole survivor of its genus and tribe, Oxydendrum arboreum is far from being a ‘living fossil’. It is widespread – albeit exclusively on acid soils – throughout the southeastern states and can be a pioneer coloniser of road embankments, where it may form pure stands (Dirr 2009); it may also naturalise when planted further north in New Jersey and New York states (Flora of North America 2021). The pleasantly acidic foliage can be chewed (though not ingested) and was once brewed into various medicinal teas (Plants for a Future 2021); the active principle (oxalic acid) is the same as in the unrelated European herbs Sorrel (Rumex spp.) and Wood Sorrel (Oxalis acetosella), and de Candolle’s generic name derives from the classical Greek oxys (sour) and dendron (tree) (Bernheim Arboretum and Research Forest 2021). A black dye was once derived from the dried foliage (Michaux 1819). Honey from bees feeding from the flowers of wild trees is also prized, and this and the tree are celebrated each August during the Sourwood Festival at North Carolina’s Black Mountain.
Sorrel Tree is showy both in its white Pieris-like flowers – a display lasting 3 or 4 weeks in early summer in the American South (Dirr 2009), but towards the end of summer in cooler climates – and again in autumn when it is one of the first trees to turn from green to a rich pinkish red (Dirr 2009). The golden sprays of ripening seed enhance this autumn spectacle, and the seeds can hang on through winter, faded to silvery grey. Through winter, the deeply but quite crisply ridged and fissured bark is also eye-catching and looks as though it belongs to a much more imposing tree. Without flowers or seeds, Oxydendrum is most likely to be confused with three taxa from the same general areas that share a similar bark and are comparable in autumn colour: the leaves of Nyssa sylvatica can be identical in shape but are never serrated, while those of Sassafras albidum sometimes show large lobes, and those of Diospyros virginiana are thicker and shinier; this last tree’s bark cracks into shorter, more oblong ridges.
Young Sorrel Trees tend to be crooked and branchy, with dense, domed masses of foliage. This, combined with the generally slow growth and the vague similarity in appearance to that of the East Asian species of Pieris, can suggest that the plant will remain a shrub, and less scrupulous nurseries sometimes describe Oxydendrum as bushy (and hence presumably suitable for small gardens). But this species shows a peculiar ability to sort itself out, ending up with a slender, shapely crown on a bole long and clean enough for the characterful bark to be displayed to full advantage (Dirr 2009). This is true even in the cooler conditions and lower light levels of the United Kingdom, where a tree planted in the early 20th century by Lord Swaythling in the shelter of Marlhill Copse, on the outskirts of Southampton, is now 18 m tall, with a minor stem from the base, but is straight and almost conic in its upper crown (Tree Register 2021). These combined ornamental qualities quickly recommended the Sorrel Tree as an ornamental in its native United States, though as a member of the heather family it is strictly calcifuge and roots shallowly, making it vulnerable to summer droughts and intolerant of much competition. Although it is easy to raise from seed, it is a relatively tricky tree for nurseries to grow on (Dirr 2009), which has presumably limited its availability. Like many eastern American trees, it also has a rather delicate root system and is best planted when small. Michael Dirr also warns that it is not a tree for urban and polluted sites (Dirr 2009), though this may be a matter of its poor capacity to cope with compacted soils and dehydrating winds: in England, there is a healthy example in the Fletcher Moss Botanical Gardens on the southern (and historically less polluted) side of Greater Manchester (Tree Register 2021) that flowers regularly, and a 10 m tree is one of many uncommon specimens in Cannizaro Park in Wimbledon, which now serves as perhaps London’s loveliest public park. The species is hardy enough to survive in Orono, Maine (Dirr 2009), though the plant known to Michael Dirr on the university campus here did not flower. In Oregon and Washington states, it can colour well but may need irrigation to see it through the west’s short but regular summer drought (P. Pew and R. Reyes, pers. comms. to Eric Hsu). It is a tree with few natural pests and diseases; Neonectria Canker (Neonectria galligena) can girdle and kill stems and branches, and cool wet conditions can leave trees vulnerable to several leaf-spot fungi (Cristulaiella depraedens, C. moricola and C. pyramidalis), and the Lettered Sphinx Moth (Deidamia inscriptum) can sometimes defoliate whole trees. Unlike the lookalike Nyssa sylvatica, this is never really a long-lived species; 80 years is suggested a likely maximum by one source (Bernheim Arboretum and Research Forest 2021). In ideal conditions, this is obviously exceeded, as it is never very fast-growing, and specimens over 30 m tall have been recorded.
Sorrel Tree was in cultivation in England by 1752, probably in the Chelsea Physic Garden (Johnson 2015), and has more recently established a modest presence in the big woodland gardens of the United Kingdom and Ireland, reaching 20 m in height at Borden Wood in West Sussex – a remarkably favoured private estate where various trees have reached their maximum height in cultivation – and 52 cm dbh on a clean bole at Leonardslee in the same county (Tree Register 2021). As a tree whose natural distribution is centred on America’s Deep South, Oxydendrum does seem to need heat and light to flower and colour well (Williams 2021), but these considerations seem overridden by its limited tolerance for summer drought. (The usual caveat about autumn colours being showier when the soil is acidic is superfluous in this case, as, unlike many trees described vaguely as calcifuge, Sorrel Trees really do hate lime.) The distribution of big examples in the United Kingdom and Ireland is in fact similar to that of some arborescent Rhododendron, with respectable specimens in gardens in high-rainfall areas including Trellissick in Cornwall, Ashbourne House in County Cork, Bodnant in north Wales, Holker Hall in coastal Cumbria and Inveraray Castle among the sea-lochs of western Argyll. The species is also tough enough to have grown to 9 m at Branklyn Garden in Perth, in eastern Scotland, but is absent – or at least very short-lived – in the drier, easternmost counties of England. One peculiarity that can be observed in England’s old woodland ‘wild gardens’, many of which have scarcely been managed for decades, is the tendency for the tree to topple over at no great age, and for the upward-facing branches to grow on and form a new canopy.
Oxydendrum arboreum seems little known as an ornamental in other parts of the temperate world, but is offered by nurseries in France, the Netherlands, Belgium, Germany and Australia. In the coastal, warm-temperate climate of New Plymouth on New Zealand’s North Island, it has made a ‘handsome, wide-spreading tree’ in the garden of Brooklands Zoo.
The only mutation of Oxydendrum that ever seems to have been noticed and propagated is the rather insignificant ‘Albomarginatum’ (q.v.), and only two nursery selections have reached anything like widespread circulation. Three other named clones are ‘Oconee Ripple’ and ‘Oconee Ruffle’, advertised by the New London Nursery in Pennsylvania in 2008, without description (Hatch 2018–2020), and ‘Spiral Twist’, understood to have been rescued from a nearby construction site in 2008 and now growing in the North Carolina Arboretum at Asheville (Hatch 2018–2020). This original plant retains a rather sinuous trunk and a low, spreading habit, features that are shared in some degree by many young Sorrel Trees.
Oxydendrum arboreum 'Albo-marginatum'
A sport with white-speckled leaves, sold in the United States by the Louisiana Nursery (Dirr 2009), but apparently never available in Europe. As the Louisiana Nursery was only established at Baton Rouge in 1983, the name in Latin form is unlikely to be valid.
A selection made by Polly Hill, in the arboretum now named after her on Martha’s Vineyard, in 1990 (Hatch 2018–2020), and first distributed by Briggs Nursery in Washington State. ‘Chameleon’ is floriferous, develops a neat, rather upright habit and was named for its autumn colours, which are remarkably varied and change as the season progresses, with yellow, pink and scarlet tints succeeding each other. For a time, this clone was quite widely sold in the United Kingdom, but it was last listed by the RHS Plant Finder in 2012 (Royal Horticultural Society 2021). The Spring 2020 Newsletter of the Plant Heritage Hampshire and the Isle of Wight Group reports two examples in the United Kingdom’s (easily achieved) National Collection of Oxydendrum at Exbury Garden in Hampshire, one of which is thriving.
A shapely clone with particularly early autumn colour, selected by the West Virginia Association of Nurserymen (Dirr 2009). It remains available in the United States, but has probably never been distributed in Europe.