Delivery – Urban Tree Planting Strategies
The tree planting process
4.1 The practical delivery of tree planting projects may require the co-ordination of a range of stakeholders, specialists and interested parties. It is therefore beneficial to set out a high level process or ‘framework’ for delivery. An outline process plan can be used as the starting point for any tree planting project throughout the 11 settlements and can be amended as necessary. The process plan can also form the basis of a checklist and programme that project partners can work to.
4.2 Proposals for tree planting may come from a range of parties, including Pembrokeshire County Council (PCC) or Town Councils, or community groups. It is recommended that a project or planting list is developed each bare-root planting season (which will likely run from around October up to March at the latest). Sufficient time will be required to make sure all necessary checks are undertaken (such as trial pit excavations or soil testing) and to confirm if any planning or permissions for planting are required. If suitable requests come forward from community groups or residents and funds are available for planting, trees could be added to the planting programme for that year.
4.3 The amount of planning, resources and level of expertise required for tree planting projects will vary considerably depending on the location. For example, requirements for small scale planting in soft landscape (such as a local park or grass verge) will be much less than planting in a hard landscape area where underground conditions are unknown.
4.4 A high level process plan for tree planting projects is shown in figure 4.1 below and includes the following:
- Tree planting projects process plan
- Desk survey to confirm any ownership, planning or environmental restrictions.
- Identify and likely consultation requirements and engage stakeholders early
- Visual survey to scope out number of trees/locations, views, constraints, exposure and existing nearby trees
- Planting in hard landscape
- Undertake ground penetrating radar assessment to help identify underground utilities constraints
- Trial excavation of locations that pass the previous stage (600mm x 600mm minimum trial pit)
- Identify and cost any hard landscaping/civil engineering requirements
- Planting in soft landscape
- Basic soil assessment to inform species selection. Identify soil type, PH any drainage issues
- Confirm a plan maintenance up to establishment is in place (minimum 3 years)
- Successful sites added to annual planting programme
- Ideally, sites should have come forward by August to allow time to secure tree stock for the coming planting season
- Review species options informed by species list.
- Liaise with nurseries to check stock availability and revise planting plans for sites if required
- Secure planting stock. Make arrangements to accept deliveries. Arrang temporary storage if required. Seek opportunities to group orders and save on delivery costs
- Liaise with relevant teams, contractors or community groups to confirm planting and after care arrangements.
- Arrange any hard landscape works, soil amelioration or site preparation well in advance of planning date
- Add new planting to tree records. Monitor trees after planting. Watering and establishment care for 3 years minimum until trees are full independent in the landscape.
Figure 4.1: High level process plan for tree planting projects
Key components for the successful establishment of trees
4.5 The key components for successful establishment of trees can be broadly divided as follows:
- Species selection
- Selecting the right tree for the location, taking into account site constraints.
- The rooting environment
- Providing an appropriate rooting environment, with a sufficient volume of soil of the correct specification.
- Planting and aftercare
- Using good practice throughout the whole process from ordering plants to planting and maintenance
4.6 Figure 4.2 below, highlights the following points for each component:
- The rooting environment
- Rooting volume
- Tree pit specification
- Soil type/specification
- Species selection
- Site constraints
- Strategy Principles
- Planting and aftercare
- Planting practice
- Quality of plants
Figure 4.2: Key components for successful establishment of trees
4.7 There are a wide range of factors that need to be considered when selecting suitable species for any tree planting scheme. It is widely accepted that healthy, large canopy climax trees will generally deliver the greatest benefits. The planting of large trees should therefore be prioritised wherever possible. However, site constraints (such as narrow streets, safety considerations, proximity to structures and utilities) may determine that smaller species or cultivars will need to be selected.
4.8 Whatever the eventual mature size of tree, benefits will only be derived if trees are healthy and ongoing vitality can be guaranteed. Trees that lack vigour and are struggling to survive will not provide the intended benefits. Furthermore, trees in poor health will require more maintenance, are more susceptible to pathogens and will ultimately result in a waste of time and resources. Trees therefore need to be suitable for the environment and site they are being planted, the overarching requirement for species selection will therefore be ‘right tree, right place’. Understandably, aesthetic considerations often influence tree selection to a significant degree, however, this should always be secondary to the trees ability to thrive at the site in question.
Key considerations for species selection
Site characteristics and constraints
4.9 Site characteristics and constraints will need to be assessed for all tree planting schemes. The scale of this task will vary depending on the location and type of planting being proposed. Key site constraints that will need to be considered include:
Below ground / rooting environment
- Soil characteristics (pH, soil type, compaction or drainage issues)
- The presence and location of utilities
- Available rooting volume and surfacing (is the site paved / trafficked)
- Likely pollution run off / flood risk
4.10 The constraints and challenges for the rooting environment posed by planting within hard landscapes are generally greater than planting within soft landscapes (such as parks and open spaces).
- Available growing space (i.e. nearby buildings and utilities)
- Exposure and climate (salt spray, wind exposure, shade etc.)
- Views (i.e. vistas that need to be maintained for safety or aesthetic / landscape or heritage appreciation)
- Surrounding habitat and existing tree population
- Landscape character
- Other site use and access issues
Tree species diversity
4.11 Diverse populations of trees are more resilient to a range of environmental threats, such as pests and diseases, which can proliferate in single species plantings and may potentially threaten entire species of trees. Expected changes in climate will also place challenges to the healthy growth of some trees and potentially exacerbate some pest and disease issues. New tree planting should therefore aim to diversify the species mix at any given site. Whilst single species avenues and streets may still be appropriate, overall species selection throughout a settlement or sub-area should avoid no more than around 20% of the same species overall. As noted under site characteristics and constraints, an assessment of the existing tree population should be undertaken. This should include an assessment of the current tree species mix which can be factored into species selection.
Providing for biodiversity
4.12 Trees provide habitats and food sources for a wide range of wildlife. Larger trees provide space for roosting, perching and nesting (such as birds and bats). Fruits, berries and seeds provide a food source for birds and mammals, and many invertebrates rely on trees both for food (such nectar from flowers) and for other aspects of their life cycle (such as laying eggs / overwintering etc.). Opportunities to plant trees that provide food, shelter and habitat for other wildlife should be prioritised. However, in some locations there may be other site constraints that may limit the use of some trees for the purpose (such as large fruit which may cause a significant maintenance issue that can’t be appropriately managed).
4.13 Native species, having been present alongside other native wildlife for longer, will generally support a wider range of other wildlife than non-native species. However, the exclusive use of native species in all areas should not be considered absolutely essential. There are many non-native tree species with beneficial attributes that may be more resilient to the predicted impacts of climate change. This is especially the case within urban environments, and these species should also be considered when selecting species for tree planting projects, as long as they are not invasive. The focus should be on species which are most appropriate to the locality, and that are most likely to thrive in future conditions. Species which are most closely related to native species will generally be most appropriate.
Species selection for 11 settlements across Pembrokeshire
4.14 Species selection should be informed by a thorough understanding of the planting site and the other key elements discussed above. A set of planting typologies and suggested species lists have been developed which can be drawn upon when developing tree planting schemes. The lists do not set out all possible options and further work would need to be done to determine suitability for any specific site. Final species choice may also end up being influenced by stock availability and project budget. However, the key principle of ‘right tree, right place’ should always be adhered to.
- Trees for paved environments and transport corridors;
- Woodland and shelterbelts;
- Trees for Sustainable Drainage Systems; and
- Trees for coastal locations.
4.15 It is recommended that current guidance from Natural Resources Wales relating to Tree Health in Wales (Opens in new window) is referred to when preparing species lists for tree planting projects. Native species should ideally form the major component of significant woodland planting schemes. The effect of species choice on landscape character should also be considered, especially within the Pembrokeshire Coast National Park (Tree and Woodland Guidance (Supplementary Planning Guidance to Pembrokeshire Coast National Park Local Development Plan 2 – draft approved for public consultation). This document is due to be formally adopted by Pembrokeshire Coast National Park Authority (PCNPA) (following a report of consultations) in 2023.
4.16 Species recommendations for the planting typologies listed above are set out under Tree Planting Zones and Sub-Principles for 11 Settlements.
The rooting environment
4.17 For trees to grow to optimum health and their full potential, they require an appropriate volume of soil with sufficient organic matter, nutrients, suitable pH, the ability to hold enough water and sufficient drainage. Aeration is essential and determines the ability of the tree to access water and nutrients. Soil aeration can be compromised through the following key factors:
- Soil sealing with hard surfaces which prevent exchange of oxygen and other gases between above and below ground environments (mainly an issue in hard landscape environments;
- Poor drainage or high-water tables (which can be due to natural ground conditions and also be artificially created through poor design); and
- Soil compaction through previous use or mismanagement of soils (such as through the use of heavy machinery when soil is wet.
4.18 Other soil characteristics such as soil pH, soil texture and water holding capacity may have a significant bearing on species selection. Further information on specific species requirements will need to be determined where required.
Tree pit (rooting environment) standards for 11 settlements in Pembrokeshire
4.19 It is not possible to provide a simple off the shelf tree planting pit that will apply to all sites. Tree planting pits can range from very simple to highly technical solutions requiring specialist design input and expertise. More complex designs are generally only ever required to address a particular constraint or limitation at a site. The principle for most tree planting with regard to the rooting environment will be to seek the most simple solution, whilst adopting appropriate arboricultural practice. Some projects may only require digging a planting hole, placing the tree and backfilling with the existing soil, ensuring topsoil and sub soil are kept separate. Simple tree pit design should be the starting point although further work and design input may be required in response to specific site constraints. A comprehensive reference text; Trees in Hard Landscapes: A Guide for Delivery. (Trees and Design Action Group, 2014) may be referred to for further guidance on overcoming specific site constraints when planting in hard landscapes.
Trees in soft landscape
4.20 Providing a suitable rooting environment in soft landscape areas will generally be more straight forward than providing a suitable rooting environment in hard landscape (paved) areas. Site assessments will need to identify any specific issues which may impede root growth, including significant drainage issues or soil compaction and soil pH which may significantly restrict healthy plant growth or species choice. Good arboricultural practice should be followed where any soil issues need to be rectified. The soil used to back fill the planting hole should be the material that was excavated. If for any reason this is not possible, the soil used to backfill the planting should be as close as possible in characteristics to the surrounding soil.
Trees in hard landscape
4.21 A key limiting factor in the growth of urban trees is a lack of suitable soil volume for root growth. Proposals for planting in hard landscape areas should ensure that there is sufficient rooting volume for the species being planted for the tree to reach maturity and deliver the intended benefits. This is a challenge, as underground space is often needed for competing uses in the urban areas. The largest possible rooting volume should be sought where possible. If cost is a major consideration for delivering a project within a hard landscape area, it may be more appropriate to reduce the overall number of trees and provide larger rooting volume for a smaller number of trees. Key strategies for ensuring adequate rooting volumes beneath hard landscape areas include:
- Increasing the rooting environment as much as possible through load bearing planting substrates (structural soils) or modular soil cells (generally a high cost solution);
- Creating a continuous trench which enable roots to spread into the space between trees, creating a shared rooting environment. This generally means the overall rooting environment per tree can be reduced; and
- Creating ‘break out zones’ that enable roots to exploit adjoining soil areas that may provide a wider rooting environment.
4.22 New trees can still be planted in areas where soil volumes are limited, however, it should be accepted that such trees will likely not reach their full potential (maturity, size or life expectancy).
Guideline (minimum) soil volumes
4.23 Minimum guideline soil volumes are set out below, which is determined by the eventual size of tree. Guidelines are provided for tree rooting environments made up from a ‘multipurpose’ topsoil that complies with British Standards (BS 3882:2015 Specification for Topsoil). Separate soil volume guidelines are also provided for structural (load bearing) soils, which generally need to be larger due to the higher stone content, having a smaller proportion of mineral soil and organic matter.
Very small trees <5 metres
- Uncompacted loam soils: 6m3 (5m3 if shared); and
- Structural soils: 8m3 (6m3 if shared).
Small trees (5-10m)
- Uncompacted loam soils: 12m3 (9.5m3 if shared); and
- Structural soils: 15m3 (12m3 if shared).
Medium trees (10-15m)
- Uncompacted loam soils: 20m3 (16m3 if shared); and
- Structural soils: 26m3 (20m3 if shared).
Large trees (15-25m)
- Uncompacted loam soils: 28m3 (24m3 if shared); and
- Structural soils: 36m3 (28m3 if shared).
Massive trees (>25m)
- Uncompacted loam soils: 36m3 (30m3 if shared); and
- Structural soils: 45m3 (35m3 if shared).
4.24 Columnar or fastigiate trees will generally require less rooting volume than trees with spreading or wide canopy types. In some circumstances it may be appropriate to reduce the guideline soil volumes by as much as half for trees with a narrow form.
Tree pit opening
4.25 The top surface of tree planting locations should ideally be open, loose textured soil. Open tree pits with exposed soils also provides the opportunity to apply mulches which can maintain healthy soil structure and biological activity, helping to ensure water infiltration and the exchange of oxygen and carbon dioxide.
4.26 It is important that soils over tree roots are protected from compaction and are ideally not subject to pedestrian traffic. There are a range of solutions that may be employed including the use of tree grilles, post and rail tree guards, slightly raised kerbs (ensuring surface water can still be diverted into the tree pit) or simply placing trees to the side of footways and out of the way of pedestrian routes where possible.
4.27 If hard surfacing is unavoidable above tree pits, provision must be made for water infiltration and aeration to the root zone. This will generally determine the use of permeable surface material, gravels or non-permeable surfaces with large diameter aeration vents and tubes.
Incorporating the tree rooting environment as part of Sustainable Drainage Systems (SuDS).
4.28 Wherever possible, new tree pits should contribute towards Sustainable Drainage Systems (SuDS). Trees may be incorporated into a range of different SuDs (such as wetlands or rain gardens) to improve functionality. Individual tree pits may also be designed as a standalone SuDS feature. If designed correctly, the removal of hard landscape can help to accept excess surface water runoff, and rainwater can be diverted into the tree rooting environment to provide a water source and improve tree health. An appropriate and effective drainage system to ensure excess water can be removed to avoid anaerobic conditions. Larger volume tree rooting environments that can accommodate multiple trees will generally function better as part of SuDS schemes due to higher water storage capacity.
4.29 When proposing to plant any trees, especially those along the highway or in hard landscape areas, it is essential that the tree pit and root growth does not cause direct or indirect damage to underground utilities or services. As noted in the process plan above, service investigations will be required to determine the viability of planting proposals. There are a range of design solutions that can be employed to address potential conflict, and relevant utilities companies / highways departments may need to be consulted. Each situation needs to be assessed and resolved on a case by case basis. The cost of designing out some potential conflict may be significant and will need to be considered early in the project. Bespoke design solutions may be required, including the options for ensuring the protection of underground services at each location.
Planting methods and aftercare
4.30 The purchase of healthy trees that are free from pests and diseases is a key component of the successful establishment of new tree planting. Ideally tree stock should be sourced locally and be of known provenance. Biosecurity is a high priority and all up to date measures and requirements to limit the spread of tree pathogens must be adhered to. Tree suppliers have a responsibility to ensure that planting stock and any soil is free from pests and diseases throughout the supply chain. Tree nurseries should be able to provide an audit trail of the transfer and ownership of tree stock from seed to the planting site. Tree stock for new planting should be:
- UK grown and sourced
- Subject to nursery quarantine for a full growing season following importation from oversees (trees should not be imported directly and planted in the landscape)
- From nurseries that are an accredited member of a plant health assurance scheme that follows up to date national guidance and are able to provide a full audit trail for tree stock
4.31 It is beneficial for those overseeing tree planting projects to visit suppliers prior to placing orders. This is advisable for small orders of trees and essential where large numbers of trees are being ordered.
Size and type of tree stock
4.32 Larger tree stock is more costly and will require more intensive aftercare, such as irrigation. Small trees tend to establish quicker than larger tree planting stock and will grow faster due to a more favourable root to shoot ratio. Small trees are much more prone to vandalism and accidental damage within urban environments and therefore larger trees are generally more appropriate and more likely to survive, given appropriate aftercare. The cost of available tree stock size is also a major consideration as larger trees are considerably more expensive and haulage costs will make up a larger percentage of the overall costs. In most situations a ‘happy medium’ will need to be struck, obtaining stock of a reasonable size that is suitable for individual specimen tree planting in hard landscape environments and soft landscape situation such as parks. Smaller stock will generally be suitable for planting hedgerows, woodlands, shelterbelts and community orchard areas.
4.33 Trees are available either as bare root, root balled (root and soil wrapped in hessian sheet), or pot grown. Each has its benefits and limitations. Bare root trees will generally be the stock of choice for larger woodland, shelter belt of hedge planting schemes. Bare root trees do present a logistical challenge as they need to be planted as soon as possible after being received, and it will generally only be practical to store them for very short periods of time. Most trees can be obtained as pot grown plants. Whilst the purchase and transport of pot grown specimens will generally be more expensive, the use of pot grown trees will generally mean more flexibility with when trees can be planted. Pot grown trees can be carefully stored throughout the planting season if needed and if space is available.
4.34 In practice, and depending on availability, this is likely to mean sourcing the follow types and sizes of tree stock for many planting schemes:
Planting in hard landscape:
- Standard (8-10cm girth), or Selected Standard (10-12cm girth) (likely pot grown).
Planting in soft landscape (specimen or individual parkland trees):
- Light Standard (6-8cm girth), or
- Standard (8-10cm girth) (likely pot grown).
Planting in soft landscape (Woodland or shelter belt planting):
- Whips or feathered whips (likely bare root)
- (single stem trees, or with few side branches, just one or several years old).
4.35 For individual specimen trees, parkland trees or trees in hard landscape, sufficient space should be allowed between trees so they can develop full crowns when mature. Other factors may need to be considered when determining spacing such as the amount of shade that may be cast along individual streets. Canopy density and form will also have a bearing on these design decisions. Where views, such as view to the coast needs to be maintained, very low density planting (widely spaced), or planting small groups or trees widely spaced may be more appropriate.
4.36 Woodlands or shelter belts, planted with small whips or feathered trees, will generally be planted closer together. In this scenario, trees will generally need to be planted between 2 and 3 metres apart. Trees within this kind planting will generally benefit from ongoing active woodland management. This may include some thinning as the planting develops to increase the distance between some trees, create open areas and to diversify the structure of the planting.
4.37 The main tree planting season extends from November to March (inclusive). Bare root and root balled trees are generally only available in autumn and early winter and need to be planted whilst dormant. Whilst most trees can be obtained as pot grown specimens, which can in theory be planted at any time of year, successful establishment will be more challenging if planted during summer months. The aim should be to complete all annual tree planting within the main tree planting season.
4.38 Trees need to be planted into prepared soil that is reasonably free from weeds and large stones. Additional organic matter or fertiliser will generally not need to be added to existing soil unless a specific soil issue has been identified and requires rectification. If organic matter is incorporated this should not be incorporated in the base of the planting pit or beneath the root ball. The sides and base of the plating hole may be lightly broken up to aid drainage. Trees should be planted with the base of the stem / root flare at the same level as it developed in the nursery. Soil should be firmed around tree roots / the root ball so there are no air gaps, but without compacting the soil.
4.39 All new trees should be water thoroughly immediately after planting.
4.40 Ideally all tree planting will be mulched with a suitable, weed free organic mulch (such as weed free compost of a suitable pH, or fine bark). This helps to retain moisture, maintain a good soil structure and reduce weed competition. Mulch should be up to 75mm deep (not mounded against trunk) and extend up to 1 metre from the tree stem.
Tree staking and protection
4.41 There are a wide range of systems for providing support to the tree after planting. Tree support ensures that soil remains in contact with the roots of the tree during establishment, reducing excessive ‘rocking’ of the roots in the wind. However, some movement of the stem in the wind is important to encourage the development of structural main stem wood that will support the tree in long term. The most appropriate approach for most applications will be double staking trees (usually timber stakes), as low down on the tree as possible, whilst also keeping roots in contact with the ground. Rubber ties can be used to secure the tree to the stakes. Stakes should be placed either side of the main rootball and measures should be taken to ensure that stakes and ties do not rub on tree stems.
4.42 Other types of tree protection during the establishment period will likely be required in most circumstances. Stem guards (such as weldmesh attached to tree stakes) will generally be the most effective solution in urban areas. Biodegradable tree tubes will generally be the most effective solution for woodland areas, shelter belts of orchard planting.
4.43 All new tree planting will need to be maintained during the establishment phase until it is fully independent in the landscape. As noted in the process plan above, a plan for ongoing maintenance should be confirmed during the planning process of any project. Key annual maintenance operations should include:
- Weed control at the base of trees, maintaining a weed and grass free area extending 1 metre from the main stem. Herbicide use should be reduced as much as is reasonably possible, with weeds controlled by other methods where practical. Weeding should be undertaken for at least the first three years for most planting, although ideally as part of a 60 month maintenance period.
- Top up and maintain a mulch layer extending 1 metre (up to 75mm deep) from the main stem (which will also help to reduce weeding requirements). Mulching can continue once trees are fully established but is most important during the establishment phase.
- Irrigation to ensure healthy growth and development.
- Newly planted trees should be water when planted. Watering should continue throughout the main growing season for at least the first three years until established.
- During establishment, frequency is more important than the volume of water, especially during the first season. If growing healthily from the second season, the frequency can be reduced slightly in favour of less frequent applications of more water. As a benchmark, trees may need to be visited for irrigation up to 12 times throughout the growing season.
- During periods of dry and hot weather, irrigation frequency may need to be increased. As trees develop during the establishment phase the amount of irrigation will need to be increased to account for the addition surface area of roots and leaves.
- Irrigation requirements will vary depending on species, stock size and location. Large, newly planted trees will require larger quantities of water than smaller trees.
- Tree stakes, ties and protection should be checked, adjusted and replaced whenever required. New trees should be inspected at least annually to monitor the success of the scheme and identify any pest and disease, or other issues
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