appendix d. street trees - sustrans

Appendix D. Street Trees


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Connect2 and Greenway Design Guide


Ever since the publication in 1898 of Ebenezer Howard’s seminal book ‘Garden Cities of Tomorrow’, the notion of trees and vegetation as vital amenities in the urban environment, has been expounded by utopian thinkers and urban planners alike. There is now little debate over the value of trees to the town and the city in improving the urban landscape, providing the aesthetic qualities required in softening the hard edges of the built environment and assisting in limiting or buffering harmful emissions of air and noise pollution.

In the context of Greenway construction, and more specifically greenway links, trees have further benefits in that tree lines can provide a buffer between the footway and the carriageway and can help give guidance to routes. The planting of trees will also help to ensure that walking and cycling will be a more pleasant activity, even next to a busy road, continuing the greenway aesthetic onto the trafficked highway.

Street trees have an innumerable number of benefits. They brighten up an area and can help break up the often oppressive nature of continuous concrete landscapes. Trees and landscaped vegetation provide a sense of place and pride for a community, more so than other measures, such as art and sculpture, which can be more divisive, as trees are rarely a source of debate and controversy, unless they are being chopped down. Even without any external or planned influence, any new residential development rapidly ‘greens’ as the new occupiers rapidly embark upon their own private tree planting programme. There is a large body of evidence that people feel better in green and leafy surroundings: trees can help improve both a person’s physical and emotional well being. Studies have shown that within three minutes of entering greener surroundings muscle tensions relax and the pulse rate is lowered. Street trees also help clean our air. A report by the UK Environment Accounts in 1998 suggested that 24,000 people die prematurely a year as a result of air pollution.

Trees act as a filter to trap toxic particles including lead and absorb gases such as carbon monoxide and sulphur-dioxide. Noise pollution is an increasingly serious form of pollution and trees can help filter out sound and provide a barrier against the drone of a busy road – with mature trees the leaves will scatter the sound whilst the trunk reflects the sound.

Furthermore trees reduce wind speed, provide shelter and reduce air turbulence around buildings, all of which can serve to reduce heating and cooling costs.

Trees also provide habitats for a wide range of wildlife. They can increase the biodiversity of an area, helping to bring the countryside to the doorstep.

Selection of Trees

Trees have to be carefully selected to be in proportion with the scale of the streets and spaces in which they are planted. The planting should complement the buildings and should not obscure key visual links. The species and the shape of the trees chosen should seek to improve the visual quality of the street.

Species selection is crucial when planning such tree planting. In consultation with a Tree Officer, consideration should be given to the selection of a suitable tree for a particular location, giving consideration to future growth, both above and below ground; potential leaf and fruit fall problems; soil type; and potential tolerance to conditions and nearby buildings and footway surfaces. Size is also a factor to be considered, for example, large trees will need pruning in the early years in order to avoid interfering with sightlines. Trees that are too small will not ultimately fill the space defined for them and therefore have limited landscape impact.

However, for residential road schemes smaller species may be entirely appropriate. Consideration should always be given to the characteristics of the species chosen and its interaction with its immediate surroundings.

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The right tree planted in the right place not only enhances a road scheme for the duration of its lifetime, it also pre-empts problems traditionally associated with trees and roads.

Always use the expertise of landscape architects and/or arboriculturists to select the most suitable species for the local conditions. A good guide for this process can be found in the British Trust for Conservation Volunteers (BTCV) handbook on ‘Tree Planting and Aftercare’. As a standard guide, trees with 12-14cm girth and 2m clear stem to canopy (to maintain sightlines under the canopy) are an appropriate size for general planting in urban areas. However in areas prone to vandalism or areas requiring instant and greater impact, larger size trees (18-20cm or 20-22cm girth) could be appropriate.

Spacing

The required minimum spacing of trees is dependant on the chosen species and should be advised by a landscape architect or arboriculturist. The minimum spacing of the most popular species is generally around 5 – 6 m.

When including trees in a footway or on the edge of a carriageway, the remaining footway width should be taken into consideration (in the situation with a mature tree!). An equally important consideration is the size of the canopy of the tree. The clearance between the canopy and adjacent buildings as well as the carriageway (consider high vehicles like buses and HGV’s) has to be carefully considered in selecting the type of tree and required lateral clearance. In urban areas the minimum lateral clearance of the tree trunk to the carriageway is generally 450mm. However the canopy size is very often more important in determining the location.

Tree Pits

Tree pits should be constructed as large as possible given the constraints of the site. They must provide adequate rooting space and soil, i.e. 1400mm2 excavation if possible and a minimum of 900mm deep.

Specifications need to take into account simple aeration and irrigation tubes, with a simple drainage layer in the base. A simple irrigation system is the use of a 60mm diameter flexible perforated hose about 60-70cm long, one end leading directly to the root ball and the other protruding slightly above soil level.

If a tree pit is not surfaced, it effectively reduces the width of the footway and could cause a trip-hazard for pedestrians. Soil or aggregate material can furthermore be spilled on the surrounding surface and litter can be trapped in the tree pit.

Different options can be used for the treatment of the top layer of the tree pit:

Unsurfaced tree pit with gravel top layer. Note how gravel is spilled on surrounding surface.


Porous resin bond paving. This paving can be laid right up to the trunk of the tree whilst allowing rain water to access the roots. The paving allows the trunk to grow without causing damage to the surface, whilst keeping the need for maintenance low. When the tree trunk grows in diameter, the material crumbles around the trunk, leaving the remainder of this top layer intact. Although the material is porous, additional treatment to ensure aeration and irrigation are required.

Tree grilles can be used to cover the top layer of the tree pit; the size of the tree pit can be adjusted over time by fitting replacing tree grilles. Note that tree grilles will trap litter.

A treatment of the top layer with compacted earth can be used with good results. If the top layer is more or less level with the surrounding surface, the chances of trapping litter are reduced. Note that drainage and aeration will need to be assured

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Root Damage and Possible Measures

Tree roots can cause disturbance to footways and paths possibly resulting in a trip hazard for pedestrians and an uncomfortable or even hazardous obstacle for cyclists. In most cases resolving the matter is simply a matter of lifting the surfacing and undertaking minor pruning works to the roots growing directly beneath the surface. Consideration should be given to laying a horizontal permeable root barrier of an appropriate material over the retained roots, which would help prevent subsequent root incursion into the upper layers of the path base following reinstatement. Damage to the carriageway due to root growth close to the surface is less likely, due to the very compacted nature of the sub base.


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In newly built situations the use of root directors as illustrated in the following diagrams can prevent future damage to footways and paths.

Standard tree planting in unprotected tree pit

Tree planted with root director


Highway Works, Trees and the Law

Tree planting

Under sections 64 and 96 of the 1980 Act highway authorities are given powers to plant trees and shrubs in the highway, including central reservations and roundabouts, with the proviso under section 96(6) that no tree planted under this section shall be allowed to remain in a position where:1. it hinders the reasonable use of the

highway by anyone entitled to use it; or

2. it is a nuisance or injurious to the owner or occupier of premises adjacent to the highway.

There is also provision under section 96(7) for compensation to be paid if damage is caused to the property of any person by anything done in the exercise of the powers under section 96. Under section 142 of the 1980 Act the highway authority may grant licences to anyone to plant trees in the highway.

Tree work

Under section 154 of the 1980 Act the highway authority may require the cutting back of trees which overhang the highway and, in their opinion:

1. endanger or obstruct the passage and repassage of vehicles or pedestrians, or

2. obstruct or interfere with the view of drivers of vehicles or the light from a public lamp.

They also have powers under the same section to require the cutting back or felling of any trees which they believe are dead, diseased, damaged or insecurely rooted, and by reason of their condition are likely to cause danger by falling on the highway.

If using section 154 powers the highway authority must serve a notice on the owner or occupier of the land first, requiring them to undertake remedial work within 14 days of receipt of the notice. The owner may lodge an appeal against the notice at the Magistrates Court.

The highway authority may decide to undertake the works themselves and to recover their expenses from the owners. Consultation between the highway authority and the tree officer is recommended.

Other Relevant Legislation

Hedgerows Regulations 1997

B.20 If improvement works involve the grubbing out or destruction of a stretch of hedgerow, prior notice may have to be given to the local planning authority. If the hedgerow is important, according to criteria set out in the Regulations, the planning authority may issue a notice requiring the hedgerow to be retained. For the purposes of these Regulations, trees within the hedge are considered to be a part of that hedgerow.

If the authority decides to prohibit the removal of an ‘important’ hedgerow, it must let you know within 6 weeks.

To qualify as ‘important’, the hedgerow must be at least 30 years old and at least 20m long (although shorter hedges can be included if linked to other hedgerows) and meet at least one of eight criteria. These relate to the hedgerow’s archaeological, historical, wildlife or landscape value.

Removing (part of) a hedgerow (or even part of one) without permission (whether or not it is important) is illegal and can be fined with an unlimited fine and the obligation to replace the hedgerow.

Wild Birds

Under Section 1 of the Wildlife and Countryside Act of 1981, it is an offence to intentionally take, damage or destroy the nest of any wild bird while it is in use or being built.

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appendix d. street trees - sustrans

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