|Plane trees of London
Cultivation of plane trees
Generally the only selection that is carried out when planting these trees is deciding whether to plant the London plane or the Oriental plane. However, more choices can be made. For street trees, especially where it is known that they will be pruned heavily, the form of the final tree usually does not matter. However, mature unpruned trees can look anything between elegant or quite ugly, and some cultivars, especially the common Pyramidalis, should probably be avoided for its poor crown and mature trunk. Many forms of P. orientalis are also poor choices unless a 'picturesque' effect is wanted.
Trees can be obtained at substantial sizes, and still establish quite well. They will need tending for the first few years after planting however. Although they will establish in dry soils, they may take longer to establish there than in their favoured damp soils.
As with many trees, while pruning as not always needed, it is often carried out to shape the tree to fit into its surroundings. Street trees in particular are commonly pruned to keep them away from houses and buildings and fit them into limited available space. The tree responds well to pruning, and strong regrowth occurs. The nature of the pruning varies. Very often, the 'pollarding' method is used (see below).
In some streets trees have been repeatedly thinned or have had lower branches removed, to try and keep a semblance of their natural form.
A large number of London's parkland plane trees begin branching at between 2 and 4 m above ground with major limbs developing from this height on the trunk. In some cases this may be due to the planting of standard trees with a clear 2 m of stem, followed perhaps by some additional formative pruning to remove the lowest limbs of young trees. Sometimes it can be seen that parkland trees and trees in other extensive public areas have been pollarded at times in the past, even though there seems to have been adequate room for them to grow freely. It may be that they were pruned 'in sympathy' (perhaps not an appropriate expression) with nearby street trees.
Staff carrying out pruning may be affected by breathing problems (see later), though not everyone is affected. This is only likely to be the case when working on trees with young foliage; roughly from May to September. It is standard practice among many staff to put off this work to the winter months as far as possible.
Root pruning is sometimes carried out to deal with subsidence problems, and for roadworks, et.c. The plane seems to cope with this as well as it does with pruning above the ground.
Often the pruning method is the removal of all younger branches to a point or points above head height. This method, known as pollarding, is often used in suburban streets, sometimes on trees other than planes. It is sometimes modified to ensure that some younger growth is left, as this may lead to the tree leafing out quicker. After pollarding is carried out for the first time, it is then repeated, at intervals varying from one year upwards, depending on the room available for the tree to grow into.
British trees are generally grown from cuttings. Traditionally hardwood cuttings have been used as the tree is easy to raise by this method and the stock responds well to the annual removal of the cuttings. It is possible that varieties that are easiest to propagate by this method have been selected for. Standard methods include 1) the insertion of hardwood cuttings of well matured tissue about 25cm long in open ground at or after leaf fall, rooting taking place by the spring, 2) the insertion of similar hardwood cuttings in March in frames with bottom heat, rooting taking place within 3 to 4 weeks.
Older hardwood cuttings, including second year and older wood, can also usually be rooted easily. It is common for prunings used as beansticks to root in the open. Cuttings can also be taken with soft new season growth in summer. I have rooted cuttings taken in June and later, in close conditions under polythene covers, overwintered thereafter in sheltered conditions. A study has shown that the highest percentage takes are obtained from basal soft cuttings, taken in July, and rooted under mist.
Some varieties are easier to root from cuttings than others. Augustine Henry in particular is difficult or impossible to root from cuttings and is often grafted or nurse grafted (grafted on to a temporary rootstock which is later removed). Most of the other acerifolia forms, and especially Pyramidalis, are easily rooted.
Young trees do not normally suffer from problems with root girdling in containers, and even large plants in containers establish quite well given time. Bare-root plants also establish well.
The oriental and western plane trees are usually raised from seed. The seed should be collected from the tree in autumn as it matures, though collection can go on through the winter. It needs no pretreatment other than being kept cool and dry. It should be sown shallowly in the open in spring. While the hybrid planes produce fruits profusely, the percentage of viable seed produced in England is low. Raising the hybrids from seed may theoretically result in the loss of some hybrid vigour. While seed could be raised from the oriental plane, and it produces more good seed than the London plane, it is known that the percentage of good viable seed set in Britain is lower than in warmer climates.
It is possible that that the prevalence of some forms of the plane tree (such as Platanus 'Pyramidalis' and the 'non-descript' forms) in London is due to their easy propagation by traditional cutting methods, and also to their easy cultivation as stock plants.
Planes sold as P. x acerifolia or P. x hispanica are available in all sizes from many sources as a landscape tree. It is however usually not clear what clone is being supplied.
The following varieties are listed in the 1999-2000 Plant Finder; P. x acerifolia, vars. Liberty, Pyramidalis, Suttneri, Tremonia, P. occidentalis, P. orientalis, vars. Cuneata, Digitata, Laciniata, Mirkovec. In addition additional varieties can be obtained by some nurseries which source stock abroad, or propagate to order.
The hybrid planes are very wind-sturdy, at least in Britain. Mitchell claimed (Collins Guide) that planes have never been known to blow down. He was writing before the great storm of 1987 in the south of England, when many large trees were brought down. However, they are undoubtedly among the most wind-resistant of trees for inland areas. Branches can be torn off by high winds from time to time. The occidental plane is said to be comparatively soft wooded, and is prone to branch breakage in its native habitat. Twigs fall off London planes regularly in storms, but this is normal for most deciduous trees.
At some times of the year high winds and changes in humidity can cause large amounts of older bark to be shed at the same time. This can lie in quantities round trees.
Planes are native to regions that have warmer summers than Britain. Within Britain, they do best in the southeast of England because it has the hottest summers. Summer droughts do not seem to be a problem for established trees though they probably slow down growth. Severe winters (in southeast England at least) are not a problem either. Plane trees do not do as well in regions with cooler summers. This is partly due to the effects of plane tree anthracnose (see below), which is much worse in damper and cooler climatic conditions and can overwhelm some trees.
Most reasonably deep and moist soils are suitable for planes, and this includes London clay. It prefers a damp, neutral or slightly acid soil, and growth is poorer or slower in chalk soils.
The London plane, and related forms have been extensively planted in London because of its well known tolerance of polluted air, and also because of its tolerance of compacted soil. These two characteristics made it an ideal tree for London's and other city streets.
The tolerance of the London plane to polluted air follows partly from the glossy foliage of some varieties such as Pyramidalis that is easily washed by rain. It may also be due to the flaking bark that sheds pollution blocked older bark and hence lets the trunk breathe. The leaf hairs on young leaves trap particulate pollution, and as the hairs are shed expose clean mature tissue.
Plane tree anthracnose, Apiognomonia errabunda, (Gnomonia veneta, Gnomonia platani) is a regular problem but rarely causes more than minor damage to most trees. Leaves are disfigured and sometimes shoots and twigs are killed. Parts of London with many trees can be covered by a thin leaf-fall of plane leaves or fragments of infected plane leaves from early June onwards. The disease is encouraged by damp weather during shoot extension in spring and early summer. It is, or used to be, reduced by pollution in cities. It overwinters in the fallen leaves, and also on the bark of the tree. Anthracnose has different effects on different species and cultivars. Platanus orientalis is little affected or is not affected, P. occidentalis and the other American species are very seriously affected and can be killed, and Platanus 'Suttneri' is sometimes seriously affected. However, except for young specimens of the susceptible forms and in nurseries, there is not generally any reason to use chemical controls in the southeast of England.
A more detailed account of anthracnose, especially as it applies in US conditions can be found on the web at this site; ( http://bluehen.ags.udel.edu/deces/pp/pp-23.htm). It is stated here that some cultivars, such as 'Bloodgood' have been selected for resistance to anthracnose and should be planted where the disease is a problem.
Massaria disease which arrived in Britain in recent years, is causing increasing concern, apparently due to increased virulence of this pathogen in hot summers. This is a fungal disease, Splanchnonema platani, that causes dieback of branches, sometimes causing branches to drop of suddenly.
Powdery mildew (Microsphaera platani) is sometimes a problem. It most affects lush new growth and sucker growth and the young growth of pollarded planes is often affected. In some seasons it is common to see numbers of trees with areas of white-grey bloom on affected leaves. It does not do any significant long-term damage to trees. While this disease, with plane tree anthracnose can cause ornamental trees to appear unsightly, there should be no need to use chemical treatments in for either disease, as the trees will recover from any damage. Some treatment may be needed for plants being raised in nurseries however.
Canker stain, (Ceratocystis fimbriata platani), a wilt disease which is a problem in the USA and parts of Europe may become a significant problem in the future in Britain. The disease is related to Dutch Elm Disease
The leaf miner Phyllonorycter platani causes some blotching of leaves, especially the underside. This is usually not an important problem. However, there was quite extensive damage seen during late summer and autumn of 2006, and some trees were badly disfigured.
Planes are fairly resistant to honey fungus.
Many other non-specific pests and diseases can affect planes but are not particularly important.
Street trees may be affected by salt from winter de-icing; if the soil is polluted by the salt, the tree will be affected like any other plant.Further details on many of these diseases can be seen in this Forestry Commission leaflet.
Most species are known to cause bronchial problems, similar to hay-fever with some people. This is due to the hairs and down shed from the young leaves and fruit. These are a problem particularly in spring and early summer. It affects most seriously any people working with or disturbing the foliage, such as people pruning or carrying out other work on the trees. It has also been known to cause problems with people in areas adjacent to the disturbed foliage. Some people are not affected. Pruning work on these trees sometimes has to be halted until late summer till most of the hairs are shed and staff are able to work on it again. The bulk of the irritating hairs come off the young fruits.
Hairs are present both on the leaves and on the fruit to protect the young tissue from sunlight damage. Later in the season other protection develops in the mature leaves (mostly from pigments in the leaf), and the hairs are shed to ensure that it photosynthesises at maximum efficiency. The bronchial reaction is due to irritation of the mucal membranes by the leaf hairs. The victim is usually fully recovered the following day, and it is not believed to cause any long term health problems. Plane trees are sometimes included in listings of poisonous plants because of this reaction.
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