At least 15 species of tree. Terminal buds ovoid, many-scaled. Leaves alternate, long-stalked; stipules often present and sometimes fused to the base of the petiole, leaving a distinctive scar once shed. The species with a temperate distribution are all deciduous and often produce showy autumn colour; their leaves have 3–7 radial lobes with serrated margins and are aromatic; their crown is typically taller than it is wide, and the mature bark is fissured. The subtropical species have mostly unlobed, evergreen leaves. All species are monoecious, with flowers usually in condensed, globular panicles, yellowish green, male and female in separate inflorescences, subtended by bracts. Male inflorescences carried several to many together in a head or spike; female inflorescence solitary, long-stalked, with 5–40 flowers. Sepals and petals absent. Capsules woody, with 2 valves and many pointed scales formed from persistent staminoides; styles persistent. Seeds many, most of them sterile and minute; one or more of the lower seeds fertile and larger, ellipsoid, membranous-winged. (Ickert-Bond & Wen 2013; Zhang, Zhang & Endress 2003; Grimshaw & Bayton 2009).
Originally understood as a genus of four temperate species within the family Hamamelidaceae, Liquidambar is now widely recognised as comprising a larger group of trees in their own distinct family, Altingiaceae. The temperate forms have been joined by about 11 species that were formerly placed in the genera Altingia and Semiliquidambar; all of these derive from subtropical Southeast Asia and can look very different from the familiar hardy Liquidambar species, with evergreen and often unlobed leaves, but genetic analysis shows them to be nested among these (Ickert-Bond & Wen 2013). Among the newcomers, L. gracilipes (Hemsl.) Ickert-Bond & J. Wen and L. chinensis Champion have recently been offered for sale by specialist nurseries in the United Kingdom, but seem likely to remain collectors’ plants in the milder parts of the country.
The four deciduous Liquidambar that are familiar to northern gardeners form a classic instance of a genus fragmented by tectonic movements and, more recently, by Ice Age extinctions. L. styraciflua in the New World and L. formosana in East Asia have both expanded from their respective mountain refuges in Central America and southern China to become widespread and successful over large ranges; L. acalycina in China remains confined to high-altitude forests, while in Europe L. orientalis has failed to spread out from a few tiny refugia in Turkey and the East Aegean islands. From China, two additional deciduous species have been described, L. edentata Merrill (1927) and L. rosthornii Diels (1900), but remain little more than names (Zhang, Zhang & Endress 2003). In coastal Florida, L. tuberculata Silba was described in 2015 and is accepted by the Atlas of Florida Plants (Atlas of Florida Plants 2021) but not by the World Checklist of Vascular Plants (Govaerts 2019). Photographs taken in Querétaro State in central Mexico in 2010 show a grove of similar trees, whose three-lobed leaves recall the Asiatic hardy Liquidambar much more than they do the common forms of L. styraciflua.
The name Liquidambar is a mongrel of the Latin liquidus (liquid) and the Arabic anbar (amber) (Jacobsen 2006), but is at least more euphonious than such portmanteau words tend to be. The aromatic gum that is tapped from wounds in the trunk of L. orientalis used to provide an important local industry, and the gum of L. styraciflua has also long been used medicinally.
The radiating lobes of a deciduous Liquidambar leaf can suggest some kind of maple, especially in brilliant autumn colour, but they are carried alternately rather than in opposite pairs. As always, colour is best expressed by trees growing in acid soils; Liquidambar species can cope with some lime, but most gardeners will find little reason to grow one if it fails to colour. All tolerate waterlogged soils and are slightly drought-sensitive.
The flowers are wind-pollinated, though still visited by bees, and unshowy (Plants for a Future 2021). The hard, spiky fruit (‘gum-balls’) are distinctive but can present a major littering problem when summers are long and warm enough for them to be produced in abundance.
None of the hardier species fruit reliably in the short summers of northwestern Europe – although climate change is likely to increase fruiting – and all enjoy warmth during the growing season; even then, the wood is brittle, however, and trees can be susceptible to damage from strong winds when fully laden with leaves. The leaves emerge relatively late and they are variably vulnerable to late spring frosts. In the United Kingdom, the species are best raised from imported seed but may not germinate until their second spring (Bean 1981). They root deeply and are best planted when very young, straight from pots (Plants for a Future 2021), but cultivation in Air-Pot® containers significantly broadens a grower’s options.
|1a||Leaves evergeen, unlobed:||2|
|1b||Leaves deciduous, palmately lobed:||3|
|2a||Leaves serrated; seed-head with c. 25 fruit:||Liquidambar chinensis|
|2b||Leaves entire; seed-head with 5 - 8 fruit:||Liquidambar gracilipes|
|3a||Mature leaves always with 3 lobes; bark grey, smooth then cracking on maturity:||4|
|3b||Mature leaves usually with 5 or 7 lobes; bark brownish grey, ruggedly fissured from an early age:||5|
|4a||Buds silky-hairy, petiole at least 8 cm long, fruit-balls with (many) more than 23 fruit and with long bristles between the capsules:||Liquidambar formosana|
|4b||Buds hairless, petiole less than 8 cm long, fruit-balls with (many) fewer than 27 fruit per ball and no long bristles between the capsules:||Liquidambar acalycina|
|5a||Twigs without corky wings; leaf-lobes usually with large lobules as well as teeth; axillary tufts usually absent; fruit-balls with narrow style bases (up to 18 mm wide), areas between fruits appearing as a smooth rim:||Liquidambar orientalis|
|5b||Twigs often with corky wings; leaf-lobes usually without lobules; axillary tufts present; fruit-balls with broad style bases (up to 30 mm wide); areas between adjacent fruits appearing braided:||Liquidambar styraciflua|