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Knowledge base

Acceptable Level of Risk

Example of Identifying ‘Tolerable Risk’
Wind load plays a major part in tree failures. A structurally sound building can tolerate wind speeds up to Gale Force 8 on the Beaufort Scale but damage begins to occur at Severe Gale 9. Depending on the target a comparison could be drawn and tree failures due to wind forces of Beaufort No 9 and above may be accepted as a tolerable risk in the same way that structural damage to buildings is accepted (buildings could be made ‘hurricane proof’ but the additional costs and low risk out-weigh the apparent advantages). Therefore it maybe only trees likely to fail during a storm of unexceptional strength need to be identified and remedied to avoid a claim of ‘negligence’.

Planning & Development

British Standard 5837:2012 ‘Trees in Relation to Design, Demolition and Construction − Recommendations’ (BS5837: 2012) Not all aspects may be necessary but a typical sequence of events would be:


  • Tree Survey and evaluation – inventory of all trees potentially affected by development with an assessment of the quality, desirability and likely remaining life expectancy
  • Tree Constraints Plan – indicates the canopy, rooting area and possible shading. This illustrates where retained trees may affect the development design.
  • Arboricultural Implications Assessment – a report that provides information on the likely impacts the development will have on the trees. Consideration can be given to the requirements of retained trees when designing the development.
  • Tree Protection Plan – identifies trees to be felled and trees to be retained. The TPP indicates where protective measures and special techniques will be used to ensure retained trees are adequately protected.


  • Arboricultural Method Statement – The AMS itemizes the sequence of development activity in chronological order and ensures all aspects have been thought through and can be successfully implemented.


  • Site Monitoring – where construction takes place in close proximity to retained trees regular visits may be necessary to ensure work is proceeding as expected and answer questions that may arise during the works. Regular site visits could ensure timely remedial measures are taken to avoid costly mitigation.

Tree Inspections & Reports

It is clear that to take ‘reasonable care’ some form of tree inspection system must be implemented and to demonstrate this has taken place the inspection should be recorded.


When deciding what is ‘reasonable’ a thorough assessment must be made of the possible harm that could be caused should the tree fail. A tree on the side of a busy road is likely to need a relatively high level of inspection by a person with arboricultural qualifications and a thorough understanding of trees and their defects. A tree in a private woodland may be so unlikely to cause harm that the level of inspection could be no more than a quick glance by the owner while out for a walk with the dog. The measures taken to reduce the risk to an acceptable level should be in proportion to the risk posed and the benefits the tree provides


Knowledge and understanding of trees and their defects has increased dramatically in recent years. This has lead to an increase in the level of competence necessary to identify tree failures that are ‘reasonably foreseeable’. The Law does not require tree owners to guarantee the safety of their trees but to disregard foreseeable tree failures as ‘an act of God’ is no longer acceptable.

Tree Management

As trees age there is a greater risk of structural damage to roots, branches and trunks. Mature trees whether they are healthy or have been affected by rot or fungal damage all provide a valuable habitat for wildlife, but can require a more structures approach to maintenance.


Certain fungi are a crucial element to maintaining tree health under stressful conditions. Other fungi when found living on live trees may cause serious health or structural weaknesses while others still may be tolerated for many years before any remedial action may be necessary.


No decision should be made without full consideration of all the facts and with knowledge and understanding of the consequences based on hands-on experience backed up with current scientific knowledge.


A tree does not ever heal but wounds will seal. Damage caused whether intentional or accidental, remains with the tree until it decays. Damage cannot be reversed and should therefore any work likely to affect a tree should only be undertaken once the consequences are understood.

NPTC CS30: Basic maintenance and cross-cutting logs on the ground
NPTC CS31: Fell trees up to 380mm diameter from ground level
NPTC CS 32: Fell trees over 380mm diameter from ground level
TC CS38: Tree climbing and aerial rescue techniques
Additional NPTC CS38 to provide aerial rescue cover
NPTC CS39: Use a chainsaw from a rope and harness
NPTC CS39: Use a chainsaw from a rope and harness
NPTC CS40: undertake pruning operations (reductions and thinning)
NPTC CS41: Dismantle a tree in sections
International Society of Arboriculture Certified Arborist
Waste Carriers License*
Public Liability Insurance Certificate £5 mill
Employers Liability Insurance Certificate £10 mill
Understands and will comply with Wildlife/Habitat legislation
Understands and able to deal with Statutory Protection for Trees
Member of Professional Organisation: Arb. Assoc., RFS, ISA
Verified 12 Monthly Continuous Professional Development Record > 50 hrs
Clear concise written quotation



Cutting a tree rarely benefits the tree and from the perspective of the tree. There is no ‘good’ time to cut but choices can be made that are to the advantage of the tree and the disadvantage of the pathogens. There are many factors to consider for a successful treatment, for example: the genetic ability of a species to tolerate pruning; the age of tree to be treated; previous stresses; existing stresses; the proportion of live canopy to be removed. Apart from urgent safety works, it is likely to be advantageous to consider when the least damaging period of growth may be. This can only loosely be equated to the seasons.


The following is simplified but may assist in deciding when to prune:


End of dormancy/pre-onset of growth Defence systems about to become active again. Wood becoming less hospitable to pathogens End of Winter Low Activity
Onset of Growth (Pre-bud swelling through to bud burst) Onset of Growth (Pre-bud swelling through to bud burst) Early spring Low Activity
Flushing/flowering (Bud burst through to leaf expansion) Energy reserves depleted further Mid Spring Many pathogens becoming active as growing conditions improve
Period of high photosynthetic activity Energy levels are being rapidly restored. All functions active Summer High pathogenic activity but fewer decay fungi producing spores
Cell division/wood formation/storage Energy reserves restored. All tree functions are active and able to respond Summer High pathogenic activity but fewer decay fungi producing spores
End of storage/wood formation Leaf Shedding Cell division defence mechanisms less effective. Wood moisture at its lowest Autumn High proportion of decay fungi are active dispersing spores
Dormant period All systems and functions least active. Cuts provide hospitable environment for pathogens Winter Microorganisms least active but may still be considerable due to our maritime climate


Tree Preservation Orders

Trees that are considered worthy of retention and provide reasonable benefits can be legally protected to ensure detrimental works are not undertaken without good reason. A tree could easily live for several hundred years while


properties change hands every few years, therefore a tree could have many custodians throughout its life, each with different ideas of good management.


The Local Planning Authority make and enforce Tree Preservation Orders (TPOs) but the responsibility for maintaining the tree remains with the legal owner, in much the same way as a listed building. The system may seem draconian to some while others appreciate the controlling mechanism.


Objecting to a new TPO, applying for consent to work on a protected tree or appealing against a LPA decision is far more likely to have a positive outcome if expert advice is taken


Subsidence made simple
Some soils swell and shrink as moisture is absorbed and lost. Whole buildings founded on shrinkable soils may subside as the soil dries in the summer only to rise again as the rains re-wet the soil in the winter. This can continue annually with little detrimental effect to the house. However, if one area beneath the building becomes drier and subsides further than the rest then cracks may appear. Many other factors can be at work contributing to subsidence and a proper investigation should identify the correct cause.


There is a legal duty to ensure that the trees within your control do not cause harm to others. The Law requires ‘reasonable care’ is taken to ensure that ‘reasonably foreseeable’ accidents do not occur. Defining ‘reasonable’ and ‘reasonably foreseeable’ has been the subject of many Civil Court cases.


Knowledge and understanding of trees and their defects has increased dramatically in recent years. This has lead to an increase in the level of competence necessary to identify tree failures that are ‘reasonably foreseeable’. The Law does not require tree owners to guarantee the safety of their trees and to attempt absolute safety would end in an equivalent to Easter Island but to disregard foreseeable tree failures as ‘an act of God’ is no longer acceptable.

Hedges & Boundaries


It would be very easy to step over the line between good pruning of branches and causing ‘harm’ to the tree and specialist advice should be sought. The cutting of any roots without specialist advice should be avoided as the consequences could be catastrophic.


Legislation is in place to ensure high evergreen hedges do not cause unnecessary nuisance to neighbouring property. Local Authorities are responsible for implementing the legislation and act as adjudicators if no agreement can be made between property owners. This last resort can result in the serving of a remedial notice which becomes a local land charge on the property deeds. Prior to Local Authority involvement the complainant must be able to demonstrate they have done all they reasonably could to resolve the dispute.


A common misconception with trees is that the roots below ground mirror the canopy above ground. Roots are often underestimated in terms of strength, form and function. The root-plate provides the majority of the stability while the fine roots, down to the thickness of a human hair, absorb the necessary moisture and essential elements from the soil. Small woody roots provide additional strength to the root-plate. They also transport water and elements from the fine roots and energy to the fine roots to ensure they continue to grow.


Woody roots will live for many years; some may live as long as the tree. The fine root hairs however are replaced regularly. Some may live for a matter of days while others will be replaced following a short dormant period over the winter, in a similar way to the leaves of deciduous trees.


The fine roots, or absorbing roots, are essential in providing the trees water and nutrient needs. They are assisted by root hairs a single cell thick and require very specific soil conditions. The absorbing roots make up a large proportion of the root mass and are easily overlooked.


Woody roots are readily apparent when excavated but their significance are usually underestimated. A single root with a diameter of 45mm could have strength in tension of 8000kg and is likely to be connecting the tree to a large percentage of the absorbing roots.