WHILE walking around the nursery at Barcham recently I was again pondering as to why so many suitable tree species are rarely used in our urban environments. The question of resilience to climate change and pests and disease remains topical with diversity in tree populations suggested as essential. Yet certain species remain underused with very few justifiable reasons apparent.

Two species immediately come to mind. These are Celtis (both australis and occidentalis)and the cumbersomely named Styphnolobium japonicum, formerly and probably better known as Sophora japonica. If the names of trees were a deterrent to their popularity and use then the latter would score very highly and be consigned to the archives, but I doubt this is the case.

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Certainly, both species present challenges to the tree nursery, being erratic growers in their early years and in need of extensive formative pruning to produce well-balanced and structurally sound crowns. This development needs to be continued into the landscape post planting, but should be standard practice for all newly planted young trees. Sadly this is not universally the case, but there is no harm in having aspirations.

Celtis australis has a natural range extending across southern Europe, north Africa and Asia Minor, reaching heights of 20 metres-plus. It is fully hardy in the UK and produces insignificant flowers in April followed by small fruits, which ripen in October. Its preference and tolerances are wide. It will thrive in light, sandy and heavy loams and will grow successfully in acid, alkaline and neutral soils. It prefers dry or moist soil and can tolerate drought.

Forestry Journal: Koelruetaria seed podsKoelruetaria seed pods

Commonly known as ‘the lotus tree’, for those who like a story or fable to go with their trees, it is recorded that Homer has Ulysses refer to ‘lotus eaters’ and the fruit is described in Tennyson’s poem The Lotus Eaters, but enough of the mythology which doesn’t influence the tree's suitability or otherwise with regard to increased planting in the urban environment.

It is relevant that I have seen these trees growing successfully as both street and parkland trees in many European cities, with Barcelona, Prague, Berlin and Lyon being the most recently visited. On a recent visit to Croatia I saw the tree naturalised and growing successfully on chalk rock faces on the Adriatic coastline. It was introduced to the UK in 1796. So the question remains as to why it isn’t used more and called upon to contribute to the urban tree population diversity now being widely recognised as a necessity if our urban tree populations are to be sustainable and the benefits they provide enhanced.

The same story is true of Celtis occidentalis, which has a wide, natural distribution across the United States resulting in genetic variations between cultivars. The tree has a light grey, rough and corky bark when mature, and grows to 25 metres. While in youth it is broadly pyramidal it matures to form a broad top of ascending branches with often drooping branchlets and has been described as 'not unlike the American Elm’ in appearance. 

Forestry Journal: Ostrya japonica leaves and fruitOstrya japonica leaves and fruit

It is a riverine species but will grow in dry, heavy, sandy and rocky soil types with a wide tolerance of variable levels of pH. It is a deep-rooting species which is described in the literature as being tolerant of flooding, though I cannot verify this from personal experience, thankfully.

I can say that, in addition to the European cities I have already mentioned and a couple of others I have not, I have seen it growing successfully across the USA – most notably in New York and Ithica, the home of Cornell University and Nina Bassuk, who took great delight in walking my colleague Tom Wilson and me through what felt like the entire street tree population of Ithica when we visited the year before last.

Both Celtis australis and Celtis occidentalis are members of the Elm family but are not prone to Dutch Elm disease. To finish this very limited discussion of the merits of Celtis, another story. According to the literature, the common name of Hackberry is a corruption of the Scottish Hagberry which was the common name used for Prunus avium.

Another story can be used to introduce Styphnolobium japonicum, commonly known as the Pagoda tree. It is infamous in Chinese culture where folklore demons are said to have been drawn to it. Even more sinister is the story that the last Minh Emporer, Changzhen, hung himself from a Pagoda tree after peasants stormed the Forbidden City in 1644. 

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Irrespective of the validity of the above, the tree itself will become significant, reaching some 25 metres in ideal conditions. It is usually upright and spreading with a broadly rounded crown at maturity. It is a relatively fast grower with a crown which produces light shade, not dissimilar to that produced by ash. It was this superficial similarity which caused me to discover a fine specimen tucked away in a small public garden in my current home town of Lewes in Sussex. It is this tucked-away element which is frustrating when considering the use of Styphnolobium. Speaking to many tree managers and asking them about the tree, the response is often: "Oh, we have one tucked away in..." Surely it deserves more prominence in our urban centres?

Forestry Journal: StyphnolobiumStyphnolobium

At Kew Gardens in London there is a wonderful example of the species' longevity.

Although now propped and the recipient of some harsh, long since forgotten arboricultural practices (such as bricking up a cavity), the tree continues to thrive. It was planted in 1753 by the famous nursery man James Gordon. The tree was one of the first of the species planted in this country and is now classified as one of Kew’s ‘Old Lions’.

I have been lucky enough to be in two European cities when the tree is in full flower and it is spectacular, with the entire canopy covered with creamy white flowers creating a glazed, almost haunting effect. As the flowers fall to the ground, the colour remains and could be described in some quarters as ‘messy’. I found it beautiful, with the colour remaining in the crown mirrored by a similar colour on the ground, each reflecting the other. I was also fortunate to be in Croatia when the seed pods were present on the tree.

These are also of a sweet lemon yellow and engulf the canopy. Again, the effect was stunning.

There is some dispute as to how old the tree needs to be before it flowers. Some authorities suggest a specimen needs to be some 30–40 years old, while others suggest 10–14 years, which is obviously a considerable discrepancy. It is likely that a combination of perhaps warm nights coupled with high summer heat influences early flowering.

In Europe, the tree is credited with being one of the highest rated in terms of pollution tolerance and suitability for urban situations. It prefers loamy, well-drained soil but, once established, withstands heat and drought well.

Again, there is a challenge for the tree nursery as when young it is a somewhat floppy grower and producing a well-trained dominant central leader can be tricky, but not impossible.

Other species available at Barcham and other nurseries fit into this underused category.

These include Lagerstroemea indica, Ostrya carpinifolia, Koelruetaria paniculata, and Pterocarya fraxinifolia to name but a few. The TDAG species selection guide authored by Andrew Hirons and Henrik Sjoman provides extensive information and guidance on multiple species and is available as a free of charge download at www.tdag.co.uk.

Forestry Journal: Styphnolobium japonicum in full flowerStyphnolobium japonicum in full flower

So, it is likely there will be those of you who have other experiences, many contrary to those expressed above, but again the question of diversity in the urban tree population and that population’s long-term resilience has to be answered. Here are potential contributors to that diversity, which deserve wider consideration and use.