Background
Over the next five years Hounslow Highways will be resurfacing many roads and pavements within the borough. These roads will be of a much higher quality and last much longer. The smoother, safer and longer lasting surfaces will make for a better driving and walking experience for everyone.
The borough has a legacy of over 11,000 street trees and Hounslow Highways is committed to maximising their life expectancy within the urban environment. Our trees are actively growing and changing in this hard surfaced environment and will accommodate the tree and design the pavement accordingly.
Your street has been identified for work
Hounslow Highways will inspect each individual tree and its surroundings to assess the most appropriate surface material. This review takes place by a qualified tree specialist with at least ten years’ experience and a project manager who is an experienced, and qualified engineer.
They will thoroughly inspect and assess each tree, considering in detail not only the size and species of the tree, but also its health and condition, its safe useful life expectancy and sustainability and any potential effect on nearby third party buildings and structures.
When examining and evaluating trees, we will also consider the overall visual appearance of the area and character of the trees in the road with a view to retaining as many trees as possible and maximising the life of each tree. Not all of the checks are visible to the naked eye, as the roots are underground and it is therefore vital that experienced professionals inspect each tree.
The tree pit
The area on the pavement surrounding the tree trunk is called a tree pit. Hounslow Highways is using a flexible material - asphalt, to cover this tree pit area. Resin bound materials and topsoil are not considered as suitable surfaces for pedestrian traffic, they do not have the properties required of a pavement and would therefore require extremely high maintenance. The safety of pedestrians would be compromised due to the high likelihood of frequent failure of these materials.
Tree pits can vary in size
Trees will react differently to the environmental urban conditions that surround them, and as a result, each tree will have a different root span particular to that tree and determined by its surroundings.
Therefore a pavement surface that may be used in one road would not necessarily be possible in another due to the localised nature of conditions such as pavement width, tree size and species.
The need for different treatment around trees
Contrary to popular belief, trees are shallow rooted and the majority of a tree’s root system (80 per cent) is in the top 60cm of soil. As a consequence, when laying pavements around trees it is highly probable that a significant number of roots will be encountered, many of which will be important to the structural stability of the tree as well as its long-term health.
These tree-root zones will require a different surface treatment to the rest of the pavement. This means each tree pit will look different in size as every tree is different. Hounslow Highways has to use a flexible asphalt surface to accommodate the changes in size of the tree trunk, canopy and roots over time.
If tree roots are causing a trip hazard, making the pavement unsafe to pedestrians
And the tree is healthy, the first option is to look for an engineering or pruning solution to ensure that the roots are no longer a trip hazard. This could include raising the pavement levels, undertaking some root or crown pruning and using a flexible surface around the base of the tree.
As a result, asphalt has been used as a surface solution by councils around street trees due to its flexibility which adapts to incremental tree root growth without producing a trip hazard. It also has the advantage that it can be laid in various thicknesses which is often necessary around trees due to the variability of tree root depth, position and diameter.
In addition, it is particularly suitable when the level of the pavement needs to be raised too, as it produces a seamlessly graduated profile, which can accommodate the portion of the root system that directly affects the underside of the pavement.
This is extremely difficult to achieve with a paved surface as this leads to an angular profile and the thickness of the block paving in particular would require a pronounced deflection of the pavement to accommodate tree roots.
In the majority of situations, this would lead to a noticeable undulation of the pavement along its length which is not desirable. Both of these options would also be vulnerable to movement from root activity leading to unstable blocks and slabs which would produce trip hazards and necessitate increased maintenance.
From the trees’ perspective, this greater frequency of disturbance to the trees roots is something we wish to avoid as it will have a detrimental effect on the trees’ health, meaning that the life expectancy of the tree will suffer. Conversely the flexibility of asphalt gives us considerable leeway when attempting to provide a long term pavement solution around trees, and allows us to retain more trees in streets that would otherwise have to be removed if we used blocks or paving.
Removing trees
If a pruning or engineering solution as described above is not possible and any root pruning works would make the tree unstable, then it may be necessary to remove the tree. It would be unsustainable to allow the tree to keep damaging the pavement and we would be failing in our duty of care to provide a safe, long lasting pavement for pedestrians.
Nevertheless, Hounslow Highways has a commitment to replant another tree for any that is removed for whatever reason. Where possible we will try and plant another tree in the same road if there is a suitable planting spot. It is not often possible to re-plant a tree in the same place as a tree that has been removed. This is due to residual roots from the previous tree that remain in the ground, acting as a physical restriction on planting and inhibiting growth. If a tree is removed due to fugal disease, these roots could also harbour remnants of fungal decay.
If it is not possible to plant a new tree in the same road we will plant as close as we can, preferably in a nearby street
If there are trees damaging the pavement any remedial work will be done at the same time as the Core Investment Programme works, this provides us with the opportunity to save trees and reduce the amount of disruption to residents.
Find out if a tree outside your house will be pruned
Hounslow Highways are currently working on a programme of pruning trees.
A third of the entire tree stock will be surveyed to assess their condition and to make work recommendations with the express intent to minimise root activity.
Where applicable, works recommendations are assigned to each tree to ensure it does not outgrow its surroundings, e.g. become too big for the street.
These recommendations will form the basis of the next 6-9 months of programmed, cyclical work.
