Facilities Management Directorate Grounds Maintenance GROUNDS MAINTENANCE GUIDANCE NOTE ON TREE SELECTION 1.0 INTRODUCTION The University of Reading has many rare and historic trees on land managed by the University of Reading Facilities Management Directorate Grounds Maintenance Section. Grounds Maintenance is conscious of its duty to ensure the continuing amenity and environmental value of the campus. This can only be achieved by the appropriate selection of tree species and planting to the highest standards. The Grounds Maintenance Team is well aware of the potential conflicts that trees can provoke, many of which can be avoided by giving careful consideration to species selection and the sites that they are planted. This Guidance note aims to give practical advice, guidance and references to all those involved with tree planting on University property, with the aim of: Preventing damage to University property or services Reducing the need for future maintenance Reducing future hazards 2.0 SELECTION OF SPECIES This guide does not intend to discuss the amenity value of tree species, as there are already many books on the subject but does hope to highlight considerations that should be made to ensure the most suitable species are selected for the site. 2.1 TOXICITY There are a number of tree species that are toxic if ingested or their sap can cause contact allergic reactions to skin and eyes. The likelihood of serious poisoning occurring is extremely unlikely because trees are generally unpalatable and are unlikely to be eaten in large quantities. Site assessment should be carried out before known toxic species are chosen. Trees which have known to cause poisoning are listed in Table 1 below Table 1 Potentially Toxic Tree Species Species Common Name Toxic Hazard Laburnum sp. Golden Rain Ingested seeds Rhus sp. Sumach Sap irritant and toxic Rhus verniciflua Varnish Tree Sap irritant and toxic Taxus baccata Yew Ingested foliage and seed 1 2.2 PROPENSITY TO BREAKAGE The incidence of mechanical failure differs greatly between tree species and sometimes between cultivars within the same species. Much of the information listed in table 2 is based on observation rather than scientific research. The following list may help to make the designer aware of potential hazard associated with species or cultivars which may make its use next to a foot path or road inappropriate. Table 2 Species with Propensity to Breakage Species Common Name Potential structural Hazard Abies cephalonica Grecian Fir Forms heavy branches in maturity liable to fail in storm conditions. Cedrus libani Cedar of Lebanon Forms heavy branches liable to failure. Cedrus atlantica ‘Glauca Blue atlas Cedar Branch failure at point of attachment. Group’ Failure of trunk in storm conditions. Acer negundo Box Elder Weak wood / frequent failure Acer saccharinum and Cv. Silver Maple Frequent failure of branches especially ‘Pyramidale’ in leaf. Aesculus hippocastanum Horse Chestnut Failure at forks is frequent. Summer branch drop is common. Ailanthus altissima Tree of Heaven Considered to have brittle wood. Fraxinus ‘Raywood’ Raywood Ash Failure at forks from a young age. Liquidambar styraciflua Sweet Gum Top breakage of semi-mature trees. Liriodendron tulipifera Tulip Tree Top breakage of semi-mature trees and branch failure. Paulownia tomentosa Fox Glove tree Brittle wood Populus Sps. Poplar Wood of low-density, breakage of tops and branches common. Robinia pseudoacacia ‘Frisia’ Golden Acacia Branch failure common in breezy conditions, during summer. Salix Sps. Esp. S. fragilis Willow Failure often associated with pollarding. S. fragilis sheds its branches and twigs. Tilia cordata ‘Greenspire’ Small leaved Lime A high incidence of weak fork formation cultivar and of resulting failure. 2.3 POTENTIAL PROBLEMS FROM FRUIT, CONES AND THORNS Tree species should be selected so as to minimise potential future problems from falling fruit, cones, honey dew or thorns. The use of these species should be assessed in areas with a high usage or within areas of hard landscaping to assess their suitability. 2 2.4 ON MATURITY The following factors should be considered when evaluating the true impact on any new tree: (a) Mature habit and ultimate size of the tree. Will the tree selected be suitable for the space allocated? (b) Density of crown / size of foliage. Will the tree selected have such a dense canopy so as to dominate the landscape or shade the building it is meant to enhance? Further information regarding these subjects and information on appropriate tree species are found in a number of publications. 2.5 THE CAMPUS ENVIRONMENT The following biotic agents described in table 3 may have relevance in species selection, positioning of planting and post planting protection. This is by no means an exhaustive list but does highlight recent pest and diseases whose action may question the future use of the affected species. Table 3 Biotic Agents within the Campus Environment Agent Associated Problem Species Affected Students/Public Vandalism/Theft All trees especially Conifers which are taken for Christmas trees Squirrels Bark stripping Many, especially: Acer cappadocicum, Acer platanoides, Carpinus betulus and Fagus sylvatica. Coryneum Canker Dieback/death of tree x Cupressocyparis leylandii Seiridium cardinale Hornet Moth Basal damage of bark/cambium Populus sp. Sesia apiformis Rust of Poplar Leaves die and fall with a loss Some hybrid Populus sps. Melampsoridium betulinum of amenity. Willow Scab Leaves shrivel and die leading Salix sps. Esp. S. babylonica Venturia sps. to complete defoliation of tree. var. pekinensis ‘Tortuosa’ Chestnut Leaf Minor Disfiguration of foliage leading Aesculus hippocastanum Cameraria ohridella to defoliation Bleeding Canker Cambium death to death of Aesculus hippocastanum, tree Weeping Canker Die back of young trees 10-35 Tilia x euchlora years old 2.6 CLIMATE CHANGE Summer droughts are likely to continue to be a problem for tree establishment. On high profile landscaping schemes provision for irrigation should be considered at the design stage. Drought tolerant species should be selected where appropriate. 3 2.7 DESIRABLE SPECIES (a) Trees suitable for structure planting: The list at Table 4 below is based on the highest percentage of recorded species on the University tree inventory therefore indicating the most successful species. It excludes Horse Chestnut (Aesculus hippocastanum) which we would not consider a viable species and Sycamore (Acer pseudoplatanus). It should be noted that Acer, Fraxinus and Tilia species establish well at Whiteknights Table 4 Trees Species Desirable for Structure Planting Species Common Name Acer campestre Field Maple Acer Platanoides* Norway Maple Alnus glutinosa Common Alder Betula pendula Silver Birch Carpinus betulus Common Hornbeam Castanea sativa* Sweet Chestnut Corylus avellana Hazel Crataegus monogyna Common Hawthorn Fagus sylvatica Common Beech Fraxinus excelsior Common Ash Ilex aquifolium Common Holly Pinus nigra* Black Pine Pinus radiata* Monterey Pine Prunus avium Wild Cherry Prunus cerasifera* Cherry Plum Prunus padus Bird Cherry Prunus spinosa Blackthorn Quercus ilex* Holm Oak Quercus robur Common Oak Sorbus aria White beam Sorbus aucuparia Rowan Taxus baccata Common Yew Tilia cordata Small leaved Lime Tilia x europaea* Common Lime Tilia platyphyllos Large leaved Lime * Non Native (b) Trees suitable for specimen planting are listed in table 5 below. The list is based on current successful species and includes species or cultivars that Grounds Maintenance is keen to add to the Whiteknights landscape. 4 Table 5 Tree Species Desirable for Specimen Planting Broadleaf Species Common Name Acer macrophyllum Oregon Maple Acer opalus Italian Maple Acer saccharum Sugar Maple Alnus cordata Italian Alder Alnus rubra Red Alder Arbutus x andrachnoides Hybrid Strawberry Tree Betula albo-sinensis var. septentrionalis Chinese Birch Betula ermanii Erman’s Birch Betula papyrifera Paper Birch Carya cordiformis Bitter Nut Carya laciniosa Shell Bark Hickory Carya ovata Shagbark Hickory Carya tomentosa Mockernut Catalpa speciosa Northern Catalpa Celtis australis Nettle Tree Cornus controversa Table Dogwood Fraxinus americana American Ash Fraxinus angustifolia Narrow leaf Ash Fraxinus excelsior ‘Diversifolia’ One Leaved Ash Fraxinus latifolia Oregon Ash Fraxinus ornus Manna Ash Fraxinus velutina Arizona Ash Gleditsia tricanthos Honey Locust Gymnocladus dioica Kentucky Coffee Tree Ilex x altaclerensis cultivars especially: ‘Balearica’ ‘Camelliifolia’, Highclere Holly ‘Golden King’ and ‘Hodginsii’ Ilex aquifolium cultivars especially: ‘J.C. Van Tol’, ‘Handsworth Common Holly New Silver’, ‘Aurea Marginata’ Ilex x koehneana ‘Chestnut Leaf’ Holly Ilex latifolia Tarajo Ilex perado Azorean Holly Juglans cathayensis Chinese Walnut Juglans nigra Black Walnut Juglans regia and cultivar ‘Laciniata’ Common Walnut Koelreuteria paniculata Golden Rain Tree Ligustrum lucidum Chinese Privet Liquidambar Styraciflua ‘Lane Roberts’ Sweet Gum Magnolia denudata Lily Tree Magnolia salicifolia Willow Leaf Magnolia Malus baccata Siberian Crab Malus transitoria Chinese Crab Morus nigra Black Mulberry Nothofagus procera Southern Beech Nyssa sinensis Chinese Nyssa Nyssa sylvatica Tupelo Parrotia persica + Cultivar ‘Venessa’ Persian Ironwood Paulownia tomentosa Foxglove Tree Platanus x hispanica London Plane Platanus orientalis + insularis ‘Digitalis’ Oriental Plane Prunus sargentii Japanese Cherry Prunus ‘Tai Haku’ The Great White Cherry Pterocarya fraxinifolia