Platanus

This tough genus includes trees that can cope with pollution and poor soils but disease and leaf litter are challenges, says Miranda Kimberley.

P. orientalis - image: Floramedia
P. orientalis - image: Floramedia

The London plane is a majestic tree, planted en masse in the streets, parks and squares of towns and cities, particularly the capital. As many of these trees reach what we believe are their mature years, new diseases are rearing their ugly heads, Massaria platani being the latest, and landscape professionals are wondering what is to become of this iconic tree. But the positive outlook is that research is being done to protect our existing stock and to produce the next generation of trees to be planted.

The genus is a tough one, consisting of trees that have found ways to cope with pollution and poor soil. Ten species are recognised, among them the lovely gnarly Oriental plane (Platanus orientalis) as well as the American sycamore (P. occidentalis). The Oriental plane has been growing in Britain for centuries. A magnificent specimen dating from around 1760 stands near the Orangery in Kew Gardens. It is a tall, wide tree with horizontally lying leaves, making it the perfect shade tree. P. occidentalis is rarely grown over here because it is susceptible to Anthracnose. It prefers its natural habitat in eastern North America alongside streams, rather than the urban environment.

These two species are alleged to be the parents of the best-known plane and urban tree, the hybrid P. × hispanica Award of Garden Merit (AGM), which is also known as P. × acerifolia. Hundreds of them were planted in London during the Industrial Revolution, when smog covered the city, leading to them being penned the "London plane".

The tree actually absorbs pollution, catching particles in the hairs on its leaves and shedding the pollutants coating the bark as it chips off naturally over time to reveal clean creamy and green shades beneath. The roots also tolerate small spaces with soil compaction, making them ideal for roadsides.

Planes are not without their challenges though. They create huge amounts of leaf litter and the fine hairs on the leaves and fruits cause irritation to the throats and skin of people nearby. On streets, the trees are often pollarded to knuckles. Once done, this pruning needs to be undertaken regularly because of the large amount of regrowth.

Anthracnose has been known to be an enemy of planes for many years. It particularly affects P. occidentalis, leading to foliage damage and early leaf fall. Luckily, P. orientalis and several cultivars of P. × hispanica — ‘Bloodgood’, ‘Columbia’ and ‘Liberty’ — are resistant to the disease. But now there are new diseases to worry about, including Massaria platani, a pathogen that has already spread across Europe and is now taking hold in parts of southern Britain. In London, trees in the City Mile, the Royal Parks and the Inns of Court are already affected.

Massaria is a branch-decaying disease caused by a fungus. If it takes hold, it generally infects small branches between 25mm and 100mm in diameter. However, it does have the potential to infect larger branches up to 300mm thick, and because the fungus tends to run along the top side of branches it needs aerial inspections and infected branches removed because this is the only control. Therefore management can be expensive — some managers are opting for inspections three times a year because the disease can develop within six months.

However, the good news is that the disease does not kill the tree. In addition, British tree nurseries are now selecting out those trees that appear resistant to the diseases, and organisations such as the City of London Corporation are collaborating with others, collating information to counter the negative impacts of threats to London’s tree population. Making up the largest proportion of the trees in the Square Mile, the London plane is certainly worth fighting for.

The specialist’s view

Hossein Arshadi, amenity director, Hillier, Hampshire
"Demand for London planes has not gone down despite the arrival of diseases such as Massaria in the UK. P. × hispanica is still a popular amenity tree and robust. At Hillier we have a long-term programme, selecting out characteristics such as significant height, width, colour of bark and disease resistance.

"While it is important to find trees that are disease-resistant, focusing just on this is not necessarily seeing the bigger picture. Trees such as ‘Bloodgood’, which were developed to be resistant to Anthracnose, are prone to frost damage. So we’re looking to develop trees that are good all-rounders. They need to be fully tested in our climate.

"Landscape professionals must be careful where they source their trees. Do they know the place of origin? Could they have come from an area known to be affected by Massaria? Platanus are included in the EU plant passporting scheme and the Food & Environment Research Agency must be informed of imported stock, but it is still worth asking the questions."

In practice

John Robinson, tree consultant and surveyor, Down to Earth Trees, Kent
"From an arborist’s point of view London plane trees are an iconic part of London’s skyline. Historically they have been resilient to many of the pests and diseases that resulted in the decline of many other species of tree.

"They are amazing trees to work on due to their height, open crowns and excellent wood strength, although a climb should be avoided where possible during summer due to the irritant seed hairs. The surge in pathogens affecting Platanus species is worrying from an amenity point of view and from a hazard assessment standpoint, given the very high usage of many of these urban giants."

Species and varieties

P. occidentalis — the buttonwood or American sycamore — is rarely seen in Britain because it is seriously affected by Anthracnose, made worse by cold weather and frost damage in spring. Becomes a significant tree in the USA, with a wide trunk featuring light-grey flaking bark, grey/brown once older. Known as a parent of the London plane. Height: 20-25m (occasionally 40m).

P. orientalis — the Oriental plane — is a large deciduous tree with a short gnarled trunk and widely spreading branches. Its green, sometimes grey, bark flakes off in small pieces leaving yellow beneath. Its maple-like palmate leaves are more deeply incised than those of
P. × hispanica. It produces small inconspicuous flowers followed by dangling, spiky, round clusters of fruits. Height: 30-35m.

P. orientalis f. digitata AGM is a form of the Oriental plane with highly dissected leaves and flaky bark. The leaves are large and bright-green until autumn, when they turn yellow and brown before falling. A prime candidate for pollarding or pleaching because of its interesting foliage. Height: 30-35m.

P. orientalis ‘Cuneata’ — the wedge-leaved plane — is a rare variety of oriental plane. It remains smaller than the species and has low branches that hang down as the tree ages, forming a rounded crown. The leaves are less deeply incised than the species. There are usually three lobes, sometimes five, with sharply serrated points and a central lobe longer than it is broad. Height: 25-30m.

P. orientalis ‘Minaret’ has deeply cut leaves, making it similar in foliage to P. orientalis f. digitata, but its leaves tend to be more symmetrical and its fruits are on shorter stems. The leaves are somewhat smaller than in most planes. They are wider than they are long, with three-to-five lobes. Slower-growing than the species, it forms a compact column-shaped crown with dense branches. Height: 8-12m.

P. × hispanica AGM (H6) (syn. P. × acerifolia) is known as the London plane. Allegedly a hybrid of P. orientalis and P. occidentalis, it was widely planted in towns and cities including London due to its tolerance of air pollution and pruning. It is a large deciduous tree with maple-like palmately lobed leaves and spiky dangling fruits. Its bark flakes, leaving green and creamy yellow wood underneath. This is its way of shedding the pollution. Height: 20-30m (sometimes 35m).

P. × hispanica ‘Augustine Henry’ forms a well-shaped tree, usually with a prominent straight central trunk and regular branching when allowed to grow freely. It can usually be distinguished by its crown shape, winter buds and large leaves.

P. × hispanica ‘Pyramidalis’ is thought to be the commonest variety of London plane in the capital, but nurseries normally supply it as the straight species. It remains narrower and smaller than the species. Only pyramidal in shape when young and then becomes a broad, spreading tree with a wide trunk. Perhaps flakes a little less than other planes, which means the bark becomes more rugged as a result. Height: 15-20m.

Thank you to Floramedia, which supplied the images for this article from its photo library
www.floramedia-picture-library.com


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