Biosecurity and species choice in a landscape scheme

Plant health has moved from a nursery concern to a planning one. Conditions now routinely require a landscape scheme to be planted in accordance with an approved specification, and that specification increasingly has to state where the stock came from, how long it has been in the country and what happens if a species becomes unavailable. A tree biosecurity planting scheme is no longer an unusual request from a landscape officer.

The reason is straightforward. A scheme planted with a single species across 200 street trees is one pathogen away from losing the whole planting, and the council inherits the replacement cost.

Why species choice is now a planning matter

Two pressures meet. Pest and disease arrivals have accelerated, and several have changed what can safely be planted in parts of England. At the same time, mandatory biodiversity net gain and canopy cover targets have given authorities a reason to care about whether planting survives to maturity, because a 30 year management obligation fails if the trees die in year three.

Landscape officers have responded by asking for evidence of provenance and diversity at application stage, rather than leaving it to a materials condition.

Where the scheme includes a Landscape Strategy, that is where the specification and the reasoning belong. A planting plan with a schedule and no sourcing statement is the version that generates a further information request.

Diversity rules of thumb in specifications

The widely used rule of thumb limits any single species to about 10 per cent of the planting, any single genus to about 20 per cent and any single family to about 30 per cent. It is a heuristic rather than a standard, and it is useful precisely because it is easy to check against a schedule.

Applied to a small scheme it can be unhelpful. A courtyard with six trees cannot meaningfully hit 10 per cent, and forcing six different species into one space produces a muddle rather than a landscape. The sensible application is at the scheme scale for larger developments and at the character area scale within a masterplan.

Diversity also has to respect context. A conservation area avenue of a single species is a designed composition, and replacing a failed tree within it with something different for diversity reasons will draw an objection from the conservation officer. The same tension arises with trees in a historic setting, as our note on trees in the setting of a listed building explains.

Say in the specification which rule has been applied and where an exception has been made, with the reason.

Sourcing, quarantine and nursery provenance

The strongest biosecurity measure available to a specifier is to buy stock that has been grown in the United Kingdom for a stated period.

Common specification wording requires stock to have been grown in the UK for at least the preceding 12 months, or to have been held in a quarantine area on the nursery for a stated period after import, with documentation provided. Some authorities go further for high risk genera and require UK grown throughout.

Ask the nursery for the paperwork rather than an assurance. Plant passports accompany regulated material and record the origin, and nurseries participating in recognised plant health assurance schemes can evidence their procedures. Specify that the documentation is provided to the landscape architect before delivery, not after planting.

Inspect on delivery. Rejecting a load at the gate is possible. Removing planted stock six months later is not, in practice, ever done.

Pests and diseases that change the specification

The list moves, which is why a specification should reference the current national risk register rather than naming a fixed set of threats.

Ash dieback has effectively removed ash from new planting schedules across England and has generated large scale removals of existing trees. Oak processionary moth is subject to statutory control in parts of London and the south east, with movement restrictions on oak. Spruce bark beetle has led to demarcated areas in the south east with restrictions on spruce material. A number of Phytophthora species affect a wide host range including larch, and the bacterium that causes olive quick decline has a very wide host list including several common ornamental shrubs, which is why some specifications now exclude imported host plants entirely.

Two practical consequences follow. Check whether the site falls inside any demarcated area or control zone before the schedule is fixed, since that changes what can be moved onto the site. And avoid specifying a species that is under active threat unless there is a reason to accept the risk.

Where existing trees are being retained through construction, the biosecurity risk runs the other way, and the protection measures in our note on retaining a tree through a construction programme should cover cleaning of plant and materials as well as root protection.

Substitution clauses and approval

Nursery availability changes between permission and planting, often by two or three years on a phased site. A specification with no substitution mechanism forces either a non compliant planting or an application to vary the condition.

Write the mechanism in. Useful wording sets out the specified species, permits substitution with a named alternative from a stated list, requires written agreement from the local planning authority for anything outside that list, and states the criteria any substitute must meet: comparable mature size, comparable form, equivalent or better biodiversity value, and compliance with the diversity rule.

Offer the substitution clause to the officer at application stage. A condition drafted with it included is far easier to discharge than one drafted without it and varied later.

Avoid specifying a single named cultivar with no alternative unless the design depends on it. Cultivar availability is the most volatile part of any schedule.

Monitoring after planting

Conditions usually require replacement of any plant that dies, is removed or becomes seriously diseased within five years of planting, in the next available planting season, with a specimen of the same size and species unless otherwise agreed.

Where biodiversity net gain applies, the obligation runs far longer. The statutory regime expects habitat to be maintained for 30 years, secured through a condition, a planning obligation or a conservation covenant, with monitoring reports at stated intervals. Trees planted as part of that habitat are covered by it, and a Biodiversity Net Gain Assessment has to be prepared by a competent person under the Environment Act 2021 regime, with PlanScope's draft serving as a starting point for that specialist rather than a substitute.

Build the monitoring into the maintenance contract. An establishment period with watering, formative pruning and weed control specified is worth more to survival than any species choice, and it is the item most often cut from a budget.

Wording the condition so substitution is possible

A workable landscape condition does five things: it identifies the approved planting plan and schedule by drawing number, it sets the planting season, it states the establishment and replacement period, it names the sourcing and biosecurity requirements including any quarantine period and documentation, and it permits substitution on the criteria agreed.

Offer that wording to the case officer. Conditions drafted by applicants who have thought about discharge are usually accepted, and they save an argument three years later when the nursery cannot supply what the plan shows.

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

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