Why You Shouldn’t Change Ground Levels Around Established Trees
When undertaking construction, landscaping, or development projects, altering ground levels is often considered for practical or aesthetic reasons. However, if you have an established tree on your site, it’s essential to understand that even minor changes in ground level around its base can cause irreversible damage or death to the tree. This concern is strongly supported by British Standard BS5837:2012 – Trees in relation to design, demolition and construction recommendations, and other arboricultural best practices within the UK.
Understanding Tree Root Systems
The critical reason why ground level changes are harmful lies in the structure and function of a tree’s root system. Contrary to popular belief, the majority of a tree’s roots are located in the top 600mm of soil, where oxygen, moisture, and nutrients are most readily available. These roots extend well beyond the tree’s canopy (known as the “drip line”) and are highly sensitive to changes in soil conditions.
When soil is added (raising levels) over the root zone, it reduces oxygen availability, potentially leading to root suffocation. Compacted fill materials can exacerbate this by physically restricting root growth and water infiltration. Conversely, when soil is removed (lowering levels), it can result in physical root damage and the loss of fine feeder roots, which are critical for nutrient and water uptake. Either change can disrupt the tree’s physiological balance and lead to stress, disease susceptibility, or structural instability.
Guidance from BS5837:2012
BS5837 provides clear recommendations for protecting trees during development. It introduces the concept of the Root Protection Area (RPA) – a notional zone, usually defined as a radius around the tree’s trunk. The idea of the RPA is that this is the minimum amount of root and soil that needs to be retained if you want to retain the tree successfully. The standard advises that no excavation, soil level change, or compaction should occur within the RPA unless absolutely necessary and agreed upon following specialist arboricultural assessment.
Section 5.3.1 of BS5837 specifically notes that changes in soil level, either by cutting or filling, should be avoided within the RPA, as they can be detrimental to tree health. Where alterations are unavoidable, solutions such as no-dig surfacing or engineered systems that maintain aeration and drainage must be considered, and only after consulting a qualified arboriculturist.
Supporting Legislation and Guidance
In addition to BS5837, local authorities across the UK may have Tree Protection Orders (TPOs) or trees within Conservation Areas, both of which provide legal protection. Undertaking works that damage protected trees – including changes in ground levels – without prior consent may constitute a criminal offence.
The National Planning Policy Framework (NPPF) also highlights the importance of integrating existing trees into development design, and prioritises their long-term retention, especially where they make a significant contribution to the character of the area.
Seek advice from a qualified arboriculturist
Altering the ground levels around an existing tree may seem innocuous, but the impacts on root health, oxygenation, and water balance can be catastrophic. UK standards such as BS5837 provide clear and evidence-based guidance: if you wish for your trees to survive and thrive during and after development, ground levels within the Root Protection Area must be preserved.
When in doubt, always seek advice from a qualified arboriculturist early in the design or planning process. Protecting ground levels today ensures the long-term health and safety of trees tomorrow.