Tree species selection for development sites

Species selection on a development site is decided by soil, rooting volume and exposure first, and by how the tree looks a distant fourth. Get the first three wrong and the planting fails within five years, the landscape condition is breached, and the replacement costs more than specifying properly would have done. A landscape strategy that names species without stating soil volume, stock size and provenance is not a specification, it is a wish list.

Matching species to soil, space and exposure

Start with the soil. Test pH and texture rather than assuming, because a site that was agricultural clay before the topsoil strip behaves nothing like the imported subsoil it will be planted into.

Then rooting volume. The working rule used in urban tree design is roughly 0.6 cubic metres of usable soil for every square metre of projected canopy at maturity. A tree intended to reach an 8 metre crown spread needs a soil volume in the tens of cubic metres, which no standard 1 metre square tree pit provides. Either give the tree the volume through a continuous trench, a structural soil or a cell system, or specify a smaller species and stop pretending.

Exposure sorts the rest. Coastal salt, wind funnelling between blocks, reflected heat off south-facing elevations and winter salt spray from the carriageway each rule out species that would thrive 50 metres away.

Climate resilience and diversity

The long-standing planting rule is to plant no more than 10 per cent of any one species, 20 per cent of any one genus and 30 per cent of any one family across a site. It exists because monocultures fail together, as ash dieback demonstrated across the country.

Applied to a site of 60 trees, that means no more than six of any species. Most landscape drawings breach it, usually by repeating one street tree along the whole spine road.

Resilience also means thinking about the climate the tree will mature into rather than the one it is planted into. Species that tolerate summer drought and occasional winter waterlogging have a better future than species that need consistent moisture. Local provenance matters for some purposes, and tolerance matters more for street planting.

Avoid specifying ash for new planting. Be cautious with species known to be under pressure from current pests and diseases, and check the Forestry Commission's current position before finalising a schedule, because the picture changes.

Biosecurity and nursery stock provenance

This is the part that gets left to the contractor and should not be.

Specify stock grown in the United Kingdom or Ireland, with a stated period on the nursery, and require evidence. Imported stock is the principal route by which new pests arrive, and a tree that spent two years in a continental nursery before a few months here does not count as home grown.

Require plant passports for the material, ask for the supplying nursery to hold a recognised plant health certification, and build in a quarantine period where the scale of planting justifies it. On sites within a demarcated area for a notifiable pest, movement restrictions apply to certain species, and a landscape contractor who has not checked can deliver stock that cannot lawfully be planted.

Write all of this into the planting specification rather than a covering note. Contractors price what is specified.

Species that cause problems near buildings

Two separate issues get confused. Direct root damage to structures is uncommon and usually involves drains or shallow foundations. Indirect damage through clay shrinkage is the real risk, and it depends on the tree's water demand, the plasticity of the clay and the foundation depth.

The industry reference tables classify species by water demand and give zones of influence for foundation design on shrinkable soils. Poplar, willow, oak and elm sit at the high end. Where existing trees are involved, the interaction is covered in the note on trees and subsidence claims.

Beyond structures, watch for practical nuisances that generate complaints and maintenance cost:

• Lime cultivars that drop honeydew onto parked cars, where a cultivar selected for low aphid susceptibility is the answer.
• Fruiting crab apples and ornamental pears over parking courts and footways.
• Thorned species beside play areas and cycle routes.
• Heavy leaf fall into a gutter system nobody has budgeted to clear.
• Suckering species in shared amenity grass.

None of these will refuse a planning application. All of them cause a management company to remove the trees in year six.

Stock size and establishment risk

Bigger stock looks finished on day one and fails more often. A semi-mature tree carries a smaller proportion of its root system, takes longer to establish and needs irrigation for several seasons. A light standard planted alongside it frequently overtakes it within a decade.

Specify stock by girth and form using the British Standard nursery stock categories, and state root condition: bare root, rootballed or container grown. Bare root is less costly and establishes well, but it can only be planted in the dormant season, roughly November to March, which has to fit the construction programme.

An illustrative example. A scheme specifies extra heavy standards along a frontage for immediate effect and light standards in the rear open space. Three years later the frontage trees are still struggling and two have died, while the rear planting has closed canopy. The cost difference went into irrigation and replacements rather than into trees.

Where immediate presence is genuinely needed, use a small number of larger specimens in prepared pits with irrigation designed in, and accept the establishment cost as part of the scheme.

Getting the species list approved

Landscape conditions usually ask for a planting schedule before development starts or before occupation. What the officer wants is a document they can check against what gets planted.

Include species and cultivar in full, numbers, stock size by girth, root condition, planting season, soil volume and specification per tree, staking and guarding details, an irrigation regime for the establishment years, and a replacement clause. Cross-reference the tree survey so that retained trees, root protection areas and new planting read as one scheme rather than two.

Where street trees sit in the adopted highway, the highway authority's own approved species list governs, and it is usually shorter than the landscape architect's. Check it before drawing, as the note on street trees and utility trenching sets out.

And name a defects and maintenance period long enough to matter. Twelve months proves nothing about a tree.

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This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.