Sweet Gum, Liquidambar styraciflua, growing as a street tree on Buckingham Road, Norwich. Photographed on 23rd November 2021.

This year’s autumn colours have been spectacular, not least the Sweet Gum (Liquidambar styraciflua) trees near where I live.

Liquidambar styraciflua comes from warm temperate areas of the eastern United States and tropical montane regions of Mexico and Central America . The first Sweet Gum was introduced to Europe in 1681 and Liquidambar styraciflua is now widely planted here as a street tree. It is also grown as an ornamental tree in Australia.

Luckily I don’t have to travel far to see a Sweet Gum because it has been planted in several places in Norwich, including Earlham Cemetery and on several streets near me. The trees on Buckingham Road, just south of Eaton Park, are always especially lovely.

In its native range Liquidambar styraciflua can live for 400 years and reach 45 metres (150 feet) tall, with a trunk some 60 – 90 cm (2 – 3 feet) in diameter. In the UK, mature trees are more likely to reach 12 metres (40 feet) in height and can spread to 8 metres (26 feet) across. It can take at least fifty years for a tree to reach its full height. Sweet Gum trees are hardy enough to grow in all of the British Isles and northern Europe (hardiness H6: down to -20 to -15C) and will grow in moist but well–drained or well–drained soil. In moderately alkaline soils the tree will survive but will generally disappoint in autumn. Drought stress is sometimes a problem. The trees on Buckingham Road grow on sandy soil and Norwich is often droughty in summer, but so far the trees have done well.

In its native range, Sweet Gum trees grow in moist-to-wet woods, tidal swamps, swampy bottomlands, streambanks, clearings and old fields, and mesic upland forests and forest edges. They grow best on the rich, moist soils of river bottomlands.

Sweet Gum is a deciduous tree and its leaves have five points (sometimes as many as seven) and give it superficial resemblance to a maple (Acer). But maples and Sweet Gum are not close relatives: Liquidambar is the only genus in the family Altingiaceae (note 1).

Sweet Gum leaves

Look closer: Sweet Gum leaves grow alternately on the branches, not in pairs like maples. They are glossy, leathery and dark green and if you crush one it releases a delicious, gummy scent. The tree has greyish brown bark which soon cracks into rugged vertical ridges. Smaller branches have corky wings and in warmer places the trees form spiky balls of fruits (note 2).

More detail of autumn leaves, with fruit.

The fruits persist through winter and are sometimes known as gum balls. They drop from the trees in late winter.

This fruit fall isn’t a problem in the UK, but the Missouri Botanical Garden website warns that fallen Sweet Gum fruits “not only create unsightly litter, but also create human safety problems (e.g., turning an ankle by inadvertently stepping on a cluster)”. Sweet Gum seeds are a food source for birds and wildlife, however. The Jersey-Friendly Yards website puts things in perspective: “The seed pods may be considered unsightly litter by some, but the benefits to wildlife far outweigh this minor nuisance.”

Winter: bare branches, but the fruit persist. Photographed in January 2021..

Sweet Gum is especially prized for its lovely, long-lasting autumn leaf colours. These can vary from a deep red-purple, through crimson and scarlet and orange, to lemon yellow or gold. Some trees are of a uniform colour but others have a mix of colours. The deepest red-purple seems to be on the sunniest branches; high humidity can turn leaves more violet. The combination of the leaf shape and colours is very pleasing. The Trees and Shrubs Online website says: “The intricate star shape of each leaf seems to enhance the spectacle, almost as if you are looking into a deep-field image of the universe.”

A deep red Sweet Gum. 23rd November 2021.

Some trees take much longer to produce autumn colour, such as this specimen a few yards further along Buckingham Road from the brightly coloured tree in the first photograph:

This Sweet Gum was still mostly green on 23rd November 2021.

There are many named forms of Liquidambar styraciflua. Wikipedia and especially the Trees and Shrubs Online website list many of them – from cultivars with deep red autumn colours (such as ‘Lane Roberts’) to trees with a fastigiate (upright and non-spreading) growth habit, drought tolerance or variegated leaves. Form ‘Rotundiloba’ is rather odd, with rounded off leaves and no fruit. If you want very corky young twigs, you can choose ‘Corky’, but avoid it if you live in a very snowy area. (The winged stems can accumulate snow and ice.)

Corky wings on Sweet Gum (Liquidambar styraciflua) twigs, in Earlham Cemetery, Norwich.

Liquidambar styraciflua is not only beautiful, but useful. It is one of the most important commercial hardwoods in the Southeastern United States. Its timber has a close grain and a red tinge, and is used for veneer and furniture. Its heartwood is sometimes known as Satin Walnut and its sapwood is sometimes called Hazel Pine. The wood has been used for all manner of objects, often in the form of plywood, from providing flooring in homes and lining the interior of railroad cars (railway carriages) to cigar boxes and chopsticks. The wood can be polished and stained for use as a substitute for cherry, mahogany or walnut.

Resin exudes naturally from trunks of Sweet Gum trees and is harvested commercially in autumn. It is sometimes known as storax, though this name is also used for resins from the tree’s Turkish relative, Oriental Sweetgum (Liquidambar orientalis) and resin from Styrax trees (note 3). Sweet Gum resin can be used to make medicines, perfumes, incenses and soaps. The resin can also be chewed and used as a tooth cleaner.  The Plants for a Future website lists its medicinal properties.

The tree’s scientific names both refer to the resin. Liquidambar is from the Latin words liquidus meaning liquid and ambar meaning amber and styraciflua means “flowing storax”.

Its Nahuatl name is Ocotzocuahuitl, which translates as “‘tree that gives pine resin“.

Other English names include Liquidambar, American Storax, American Sweetgum, Bilsted, White Gum, Red Gum (and Redgum), Star-leaved Gum, Starleaf Gum, Alligator Tree and Alligatorwood. “Alligator” refers to the tree’s small branches and twigs: “The bark attaches itself to these in plates edgewise instead of laterally, and a piece of the leafless branch with the aid of a little imagination readily takes on a reptilian form” (note 4).

Notes

Note 1 – The Altingiaceae is named after Willem Arnold Alting (1724 – 1800), Governor-General of the Dutch East Indies from 1780 to 1797. Liquidambar was previously included in  the Hamamelidaceae, a family that includes Witch Hazel (Hamamelis). Maples are in a different family, the family Aceraceae.

Note 2 – Liquidambar styraciflua is monoecious – it has separate male and female flowers on the same tree. The flowers are produced in spring and are very easy to miss.

Young twigs develop corky wings from their second or third year onwards, though the amount of cork varies from tree to tree. Twigs of Field Maple (Acer campestre) and English Elm (Ulmus procera) develop similar outgrowths. The Trees and Shrubs Online website describes the phenomenon as “another of those fascinating botanical quirks for which no-one, as yet, seems to have come up with a plausible evolutionary explanation“.

Note 3 – Styrax trees are native to Sumatra, Java and Thailand. Their resin is better known as benzoin (sometimes corrupted to “benjamin”). It is used to make perfumes and incenses, and as a flavouring. It is an ingredient in “Friar’s Balsam“, a commercial preparation used to treat colds and skin problems such as blisters.

Note 4 – The name Alligator Wood is also used for the timber of the West Indian tree Guarea glabra (family Meliaceae). This apparently has a musky smell supposedly resembling that of an alligator. I can’t tell you more, not having seen the tree or sniffed an alligator.