Sweet Gum, Liquidambar styraciflua

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Sweet Gum, Liquidambar styraciflua

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

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).

Liquidambar styraciflua leaves and fruit

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.”

Liquidambar styraciflua fruit

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.”

Sweet Gum

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:

Liquidambar styraciflua

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

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.

Stinking Fleabane, Dittrichia graveolens

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Stinking Fleabane, Dittrichia graveolens, by the A47 Norwich Southern Bypass. 26th September 2021.

NFN (New For Norfolk)

At the end of September my friend Chris Lansdell told me about a new plant for Norfolk, Stinking Fleabane, Dittrichia graveolens. Louis Parkerson had found a patch of it on the previous Tuesday and reported it on Twitter on 23rd September. Louis had seen similar plants growing by a motorway in the Midlands a few days earlier and he managed to work out what it was. His identification was soon confirmed by other members of the Norfolk Flora Group who were familiar with the plant from their travels.

The Stinking Fleabane was growing by the A47 Norwich Southern Bypass bridge, so just a short bike ride away from home. I decided to go and see it (note 1).

There were actually lots of plants – probably 200 – and they were growing right next to the road. Cars rushed past were just feet away from where I stood to take photographs. Most of the plants had already gone to seed but there were still some flowers left.

I picked a piece of Stinking Fleabane to smell. I found it very aromatic but quite pleasant and I think “stinking” is far too harsh. To me, the plant smelt of camphor (note 2).

Stinking Fleabane, Dittrichia graveolens

Stinking Fleabane, Dittrichia graveolens

Stinking Fleabane, also known as Stinkwort and Stinking Aster, is a member of the daisy family, the Asteraceae. It is one of two species of Dittrichia found in the British Isles, the other being Woody Fleabane, Dittrichia viscosa. Both are introductions (neophytes). Woody Fleabane appears to be declining in Britain but Stinking Fleabane is increasing its range (note 3).

In the British Isles, Stinking Fleabane is a long way from home. It is a native of Southern Europe, North Africa and Western Asia (as far east as Pakistan). In its native region it grows on roadsides, waste and fallow ground, hill slopes and also in damp habitats (note 4). It has been introduced into other parts of the world too: the United States, Australia and other parts of Europe  (Austria, Belgium, the Czech Republic, Germany, the Netherlands, Slovenia, Switzerland, and the United Kingdom. It reached Romania in 2020.) and Africa (Egypt, South Africa).

Dittrichia graveolens is an annual subshrub which grows up to 130cm (52 inches) tall. It has long and narrow leaves and these are pointed at each end with glandular hairs on the surfaces and small teeth along their edges.

Seedy Success

As an annual, Dittrichia graveolens needs to flower and set seed in order to survive. It is very good at this: each plant produces numerous yellow flower heads, each with as many as 16 ray florets and 40 disc florets and a single plant is capable of producing up to 71,000 seeds. It has been suggested that just one plant could produce up to 14,493 adults in the next generation. Scientists in California studying the seed and germination biology of Dittrichia graveolens found that the seeds are only viable for two to three years, with most germinating immediately after rain, from autumn through to spring.

Stinking Fleabane seeds are carried on the wind by means of the fluffy parachute (pappus) typical of many Asteraceae, and they can also float off on water to a new site. Seeds can be also be spread by vehicles, and this is presumably why the plants are growing by the A47.

The plant is thought to have arrived in Australia from Germany as a contaminant of wheat seeds but the seed’s pappus has barbs that allow it to attach to animal fur and the plant has moved elsewhere around the world in sheep’s wool. (The term wool-alien is used to describe plants that are spread in this way.)

At the time of writing, the NBN Atlas shows 45 records of Dittrichia graveolens for the British Isles, not including the recent discovery in Norfolk. In recent years the plant has also been found by the M5 in East Gloucestershire (2020) and by a slip road off the A299 in Kent (2014).   It is easy to imagine Stinking Fleabane spreading further along roadsides and eventually becoming as widespread along major roads as the Danish Scurvygrass, Cochlearia danica, I wrote about in May 2016.

As long as it stays by the roadside, Stinking Fleabane shouldn’t become a problem. But in some other parts of the world, Stinking Fleabane has become a bit too successful for comfort.

Noxious Weed?

Dittrichia graveolens is now regarded as a noxious weed in California and in parts of Australia and in 2013 the United States Department of Agriculture (USDA) produced a report, the “Weed Risk Assessment for Dittrichia graveolens (L.) Greuter (Asteraceae) – Stinkwort“. (There is also useful information with references on the California Invasive Plant Council (Cal-IPC) website.)

The USDA report describes how the plant became one of the the worst weeds in cereals in South Australia just thirty years after it was introduced to the state. By 2001 the plant was under an eradication program in Queensland, though its impact in South Australian cereal fields had lessened as soils became more fertile. Dittrichia graveolens has also spread rapidly in California since it was first discovered in 1984 and also occurs in Connecticut (since the 1930s), New Jersey, New York, and South Carolina.

Grazing animals tend to avoid Stinking Fleabane plants except when they are young, which can lead to infested paddocks with little grazing value. But if there is nothing else to eat, animals will browse on the plants and this can result in tainted meat and milk. Worse, if sheep eat the flower heads, the barbed pappus hairs can puncture the gut walls and cause bacterial infections and death.

Prolonged handling of Dittrichia graveolens can cause allergic reactions and severe dermatitis in some people. A study from 2007 demonstrated how a 56 year old man developed allergic contact dermatitis after handling the plant. Picking a small piece of the plant to sniff (like I did) shouldn’t cause a problem but the advice “So, when pulling this plant out, be sure to wear gloves” is well worth following.

My photographs hopefully give a good idea of what Stinking Fleabane looks like but I also recommend the Italian Schede di botanica website if you’d like to see more pictures of the plant.

Notes

Note 1 – In the late 1980s and early 1990s Vanna and I sometimes went on day trips to see rare birds. We were twitching, but only on a very limited scale. Trips were confined to Norfolk and North Suffolk and relied on friends who could give us a lift in their cars. We never considered driving to Cornwall or flying to Fair Isle to see a rare bird.

One day in 1990 or 1991 we went to Lowestoft to see a Red-eyed Vireo, a North American vagrant. We failed! The bird flew out of the Sycamore tree it had been hiding in and we missed it, which ruined our day. We decided this was ridiculous – we had otherwise had a good day out.

Nowadays we only bother with rare birds if we’re in the area anyway (such as staying at Wells-next-the-Sea in October 2015, when northerly winds brought falls of migrants to Wells Pine Woods).

However, I will travel to see rare plants if they’re not too far away. Plants are not going to fly away and the odds on their survival are usually better than those of a rare bird blown off course and doomed to die many miles from home, without finding a mate.

Note 2 – The smell reminded me of Aztec Sweet Herb, Phyla dulcis, which I grew in a pot for several years.

Note 3 – Neophytes are plants introduced to the British Isles after 1492. Information from Clive Stace, “New Flora of the British Isles“, pages 775 – 776. (Fourth Edition, 2019.)

Note 4 – Information from Marjorie Blamey and Christopher Grey-Wilson, “Mediterranean Wild Flowers“, page 438. (Harper Collins 1993).

The 2013 United States Department of Agriculture (USDA) report, the “Weed Risk Assessment for Dittrichia graveolens (L.) Greuter (Asteraceae) – Stinkwort” gives the following native range: “Southern Europe (Albania, Bosnia and Herzegovina, Bulgaria, Croatia, France, Greece, Italy, Macedonia, Montenegro, Portugal, Serbia, Spain), northern Africa (Algeria, Libya, Morocco, and Tunisia), central Asia (Afghanistan, Cyprus, Iran, Israel, Jordan, Lebanon, Pakistan, Syria, Turkey), and tropical Asia (India)”.

Oak Bracket, Pseudoinonotus dryadeus

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The last few days have seen a sudden change from a very warm and sunny late summer to a much cooler and rather rainy start to autumn. Flowers and insects are on the wane but fungi are starting to appear.

I was cycling near Carleton Rode, south-west of Norwich, a couple of weeks ago and passed an oak tree by the roadside with some distinctive bracket fungi growing at its base and stopped to take a look.

They were the Oak Bracket, Pseudoinonotus dryadeus. 

Oak Bracket, Pseudoinonotus dryadeus

Base of English Oak tree with Oak Brackets, Pseudoinonotus dryadeus

Oak Bracket, Pseudoinonotus dryadeus

Oak Bracket, Pseudoinonotus dryadeus

In its prime, the Oak Bracket is extremely beautiful. Its velvety upper surface is cream to rusty brown with a yellower margin and is pitted with tubes. These ooze an orange-brown liquid that looks like runny honey, though, as the First Nature website points out, “it is not as tasty as honey”. The fungus is not edible – it is described as bitter and tough.

Oak Bracket, Pseudoinonotus dryadeus

Oak Bracket, Pseudoinonotus dryadeus, oozing nicely.

The Oak Bracket is a parasite, mainly on species of oak, although Beech, Birch and Alder and Maple, Elm and Sweet Chestnut are occasional hosts. The fungus causes a white rot in base of the trunk (“butt rot”) and eventually branches can die off and the weakened base of the tree can make it prone to toppling. Spores enter through wounds in the tree’s bark so this is “one of many reasons not to damage tree bases with lawn mowers or other equipment“.

The Oak Bracket grows as an annual bracket without a stem. The ones I saw were in their prime; older specimens become corky and fibrous. The caps become blackened and cracked over winter, and sometimes persist for several years. They apparently smell very unpleasant. (The ones I found just smelt slightly mushroomy. I must go back and have a sniff when they’re older!) (See April 2022 update below.)

Oak Bracket is also known as Warted Oak Polypore, Weeping Polypore and Weeping Conk. The last two names refer to its habit of oozing liquid, but it’s worth knowing that other species of bracket fungus also weep. Two examples are Shaggy Bracket (Inonotus hispidus) , found especially on Ash and Apple trees, and Alder Bracket (Inonotus radiatus), found on Common Alder.

The Oak Bracket’s original scientific name was Boletus dryadeus and prior to 2001 the accepted name was Inonotus dryadeus, which is still used in some field guides (such as Sterry and Hughes) and on some websites (e.g. Wikipedia, Messiah University).

The First Nature website has some good pictures of the Oak Bracket. It also tells us generic name Pseudoinonotus comes from pseudo (false), ino (fibrous), ot (ear; also used in Otidea bufonia, the Toad’s Ear fungus) and –us (making it a Latinised noun). The specific name dryadeus means ‘found with or on oak trees’.

Pseudoinonotus dryadeus is commonest in the southern half of the British Isles. At the time of writing the NBN Atlas lists 292 records, with just three in Scotland. It also occurs elsewhere in temperate parts of Europe and in North America.

The Oak Bracket shouldn’t be confused with the Oak Polypore (Piptoporous quercinus), which is very rare in the British Isles and grows only on veteran oak trees (though there is apparently one old record on a Beech).

Cross section through a Crunchie

Cross section through a Crunchie bar.

When fresh Pseudoinonotus dryadeus also resembles a section through a Crunchie Bar but the taste, difference in size and presence of a chocolate coating on the latter should avoid confusion.

April 2022 Update

I did go back and sniff the Oak Bracket, in December 2021. The fruitbodies were now greyish black. There was no smell at a distance but if I sniffed the surface I could just detect  a not very pleasant nitrogenous smell of rotting fungus, like a cultivated mushroom that had been sweating in the fridge for too long.

I cycled past again on 10th April 2022 and was shocked and saddened to see that the Oak tree had gone. The gales in late February may have toppled it, or a farmer or tree surgeon may have seen the brackets and decided to remove the tree. There were only some thin twigs left, plus a few shattered remains of very woody (and not at all smelly) Oak Bracket on the ground. I took one home as a souvenir.