3m default minimum width - provide soil volume free of utilities to support sustainable trees
3m is the optimum width that provides sufficient soil volumes of sustainable trees and protects the highway/footway infrastructure from pressure damage. This width also negates the need for supplementary street lighting to the footway behind.
NOTE: Early liaison with the Highways Adoptions team is advised on all landscape proposals associated with any highways to avoid conflicts. In particular, tree positions must be compatible with street lighting /lighting columns. Evidence will be required to demonstrate that agreements have been reached prior to final landscape approvals (commuted sums may apply). Refer also to Transport SPD 2023 Transport Supplementary Planning Document - Appendix A.5 paras 989- 993.
Copper lined root barriers can be installed for additional protection. Verges must include understory planting to provide an air quality buffer and to encourage active travel (subject to highway safety permitting). Liaise in advance with the Highway Adoption team.
Soil requirements
Verges, single pits and beds for tree planting require soil to a depth of 900mm in accordance with BS3882: 2015. The depth of topsoil spread should not normally exceed 300 mm. Suitable (loosened) subsoil in accordance with BS 8601:2013 should provide the remainder of the minimum rooting depth.
Laboratory certification is required for both topsoil and subsoil.
Regardless of whether the soil source is “site won” or imported, an agreed representative number of samples (based on the volume of soil being used) shall be Laboratory tested to evidence compliance with the requirements of BS3882: 2015 (multipurpose topsoil) and BS9601: 2013 (multipurpose subsoil). The subsequent Laboratory Certificates must be submitted to the LPA for approval prior to any filling operations.
If the soil is being imported from an external source, then proof of actual delivery of the certified soil must be evidenced by the submission of the site delivery tickets to the LPA.
Spreading soil
Refer to LCC guidance for Topsoil under Landscape Planning and Development.
Prior to spreading topsoil, the subsoil must be de-compacted to ensure drainage infiltration and soil integration. A method statement is required in accordance with BS BS3882: 2015: Section A.3 “Preparation of the receiving area and spreading topsoil”.
Bark mulch
1.5 m diameter bark mulch circle around trees in grassed areas is required at a depth of 75mm.
Bark mulch to a depth of 75mm is required on all other planting typologies including shrubs, herbaceous and hedging.
The edges to all mulch areas, i.e. planting areas, shall be defined by a “dip” channel. Not only does this demarcate and provide a mowing edge but it also holds water and retains the bark.