Large tree species in urban areas were often planted because they were pollution tolerant. They were pollarded (where the head of main branches are cut back to promote a more bushy growth of foliage) to prevent them outgrowing their allotted space, casting shade, obstructing electric wires and streetlights or blocking gutters. With cleaner air a wider range of smaller species are less expensive to maintain. There are situations where pollarding may still be appropriate. Adjacent to main roads for example, large trees may be required to provide a tall clear stem.
Pollards are identified by the numerous branches, originating at the same point on the tree. These shoots have emerged from dormant buds under the bark or adventitiously from wound tissue. Initially the new branches are held weakly in place as they grow from under the bark rather than from within the tree. As the wood lays down annual rings the union strengthens often forming a thickened base where the shoot meets the trunk. Over a number of years a noticeably swollen ‘pollard head’ or ‘boll’ forms where new shoots spring up each year. Shoots growing from the pollard are removed while the wood is young close to the base of the new growth.
The London plane, Platanus x hispanica, and common lime, Tilia x europaea are well known to be tolerant of pollarding. Other trees which respond to this treatment include some species of Acer, Alnus, Fraxinus, Liriodendron, Morus, Quercus, Ulmus.