Trees and the Public Realm - Westminster City Council

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Trees and the Public Realm (Draft)
7. SPECIES SELECTION
84. Urban street trees are generally subject to greater stresses than trees that can grow
undisturbed. They are subject to higher summer temperatures, are grown in poor
disturbed soils that are contaminated with road salt in the winter, have restricted root
runs, are subject to mechanical damage to roots, trunks and branches, and if grown in
hard surfaces poor supplies of water. All of these factors shorten the trees’ normal
expected life span, even given some of Westminster’s trees are over 150 years old.
85. This section looks at some of the characteristics that need to be considered when
selecting a species to plant. Sufficient quantities of new trees should be planted
regularly to ensure that losses in appropriate locations are replaced. Felled trees should
also be replaced with another of a suitable species in order to preserve the character of
the area, where is it desirable and appropriate to do so with specific regard to
streetscape design. More detailed information on tree species which are used in
Westminster is provided in Appendix D.
Native vs. non-native species
86. It is most important that tree species are chosen for their adaptability to the prevailing
site conditions rather than a strict adherence to the current mantra that only native
species will do. If a biologically diverse habitat is the design brief then it will be
important to provide the appropriate growing conditions. Parks, cemeteries and
housing estates are the sites most likely to provide the space and conditions for the
creation of native habitat.
87. Native trees, such as birch and field maple may be suitable for street tree planting, but
the choice of native species with suitable characteristics for street tree planting is very
limited. Therefore it is likely that a significant proportion of new tree planting will be nonnative
species.
88. Few evergreen trees are native, and their alien columnar or pyramidal form and minimal
contribution to biodiversity make them unsuitable as street trees.
Size
89. Selecting the right size of tree for the site and one that relates to the space in which it is
to be planted is essential. This is particularly important in residential areas where trees
that tower over and dominate adjacent residential property can create excessive shade,
and block street lighting, causing people to feel apprehensive. It is less of a problem
where the buildings are in commercial use.
90. The planting of large-growing species in Georgian and Victorian times has caused
some problems today. These plantings caused no problems initially, as they were
frequently managed by regular pollarding. However, pollarding as a management
technique is not ideal for aesthetic reasons, as trees can look unnatural.