FEATURE
Some solutions It is worth treating the tree pit as the ‘transitional zone’ between the ‘tree nursery soil’ and the ‘real world’. It is the rooting environment that needs to minimise transplant stress and promote healthy root growth to optimise tree establishment. Various degrees of design intervention may be required to achieve this, depending on the nature of the site (greenfield/brownfield), the existing soils, the topography, the hydrology and the other environmental factors such as exposure to wind, sun scorch, etc. Regardless of the level of design input, it is useful to stick to some basic principles for an aerated soil and healthy rooting environment. Keep tree pit design as simple as possible and minimise the amount of disturbance to the soil. Soil types Any soil that can maintain its ability to drain and aerate after soil spreading and tree planting has the potential for re-use. However, in many landscape projects where there are time constraints and inclement weather to factor in, sandy soils provide the greatest flexibility. They remain ‘non-plastic’ at higher moisture contents, and they are less prone to compaction and structural degradation. Heavier clay-based soils can be used provided they are reasonably dry and in a friable, ‘non-plastic’ state when handled. Silty soils are generally not suitable for backfilling tree pits as they have weak structural strength and suffer from ‘selfcompaction’ even when handled carefully. Soil depths Quite simply, do not put topsoil too deep. A topsoil depth of 300mm is usually ample, and certainly 400mm should be the maximum and only provided the soil type will allow it. This applies to planting beds
TREE PIT AT THE OLYMPIC PARK – TREE ROOTBALL SITS ON SUBSOIL AND WILL BE SURROUNDED BY TOPSOIL. OVER 4000 TREES WERE SUCCESSFULLY ESTABLISHED AT THE PARK USING THIS APPROACH
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Pro Landscaper | December 2023
as well as tree pits. The rootball should sit on subsoil, and with bigger rootballs, the subsoil will also sit around the lower portion of the rootball. Small rootballs For trees with a smaller rootball (up to 300-400mm deep) that are being planted into ‘in-situ’, undisturbed ground, it is far better to minimise the size and dimensions of the tree pit to limit the destruction of the soil’s structure. The tree pit should be as shallow as possible, and usually only requires excavation to the depth at which the rootball will sit. If machine dug, it is useful to decompact the soil in the base of the pit where the excavator bucket often causes smearing and compaction. After placing the rootball, the pit can be backfilled with the excavated topsoil, ensuring that any soil ameliorants (e.g. green compost) have been evenly mixed with the backfill topsoil.
Tree pit. Ameliorate topsoil with tree planting compost 300mm
Min. 300mm
Topsoil Tree pit subsoil (site or imported) Break up base of pit
TREE PIT DETAIL FOR LARGE ROOTBALLS
Drainage options may include mounding the pit slightly to help shed water away from the upper rootball, incorporating a mini soakaway in the base of the pit, or connecting the pit to a positive, and hopefully sustainable, drainage outfall (a last resort). Soil investigation, combined with a review of the landscape proposals, is essential to determine which drainage option(s) is most applicable to each tree pit. Very often more than one option can be applied to a landscape scheme to suit the variable size and location of trees.
Shallow tree pit. Ameliorate with tri planting compost 300mm
Topsoil Site subsoil Break up base of pit
TREE PIT DETAIL FOR SMALL ROOTBALLS IN 'IN-SITU' SOIL
Larger rootballs For larger trees, there is a need to excavate a deeper pit to accommodate the rootball. This requires excavation into the subsoil. It is often better to use a higher sand content subsoil or even a quarried sand, to sit the rootball on, and to surround its lower portion. Sands and sandy subsoils will support the weight of the rootball better, and thereby prevent later settlement. A coarser sand with a narrow particle size distribution should maintain adequate porosity even when consolidated by the rootball. Roots happily grow into a sand as it is full of oxygen and water. Tree pit drainage considerations To prevent anaerobism caused by stagnant water, the principle of any tree pit design is to ensure that inputs of water are equal to or less than the outputs.
TIM O’HARE Tim is the principal consultant of Tim O’Hare Associates, the UK’s leading soil and landscape science practice. He has been a professional soil scientist for over 25 years, working predominantly in the commercial landscape, construction and sports agronomy sectors, and covering anything from domestic garden projects to major construction developments. He also advises government departments, national institutions and agencies through the preparation of standards, strategies and codes of practice on soil management and specification. In 2019 Tim was awarded an Honorary Fellowship by The Kew Guild (Association of Alumni of the Royal Botanic Gardens Kew) for his “outstanding contribution to the environment”. toha.co.uk
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