Sizing replacement planting by canopy area, not numbers
Replacing a 90-year-old oak with one nursery standard replaces about one per cent of the canopy. Sizing replacement by area is the argument that holds up.
One for one replacement is the default a planning officer reaches for and it bears almost no relationship to what is lost. A mature oak with a 15 metre crown spread carries roughly 175 m² of canopy. A nursery standard planted to replace it carries a couple of square metres and will take decades to catch up. Sizing replacement planting by canopy area, and then converting that area into a realistic schedule of stock sizes and numbers, is the way to make compensatory planting mean something.
This is separate from the statutory duty to replace a tree removed under a Tree Preservation Order, which is a specific obligation to plant a replacement of appropriate size and species in the same place. What follows is about compensatory planting negotiated through a planning condition.
Why one for one replacement understates the loss
The benefits trees provide scale with canopy and leaf area, not with stem count: shade, interception of rainfall, particulate capture, visual screening, habitat. A tree count treats a semi-mature field maple and a veteran beech as equivalent units, which no arboriculturist would accept and no resident believes.
Time compounds the gap. A newly planted tree delivers almost nothing in its first decade, and on development sites a proportion of new planting dies before establishment. So a one for one condition discharged on paper in year one can leave a street with less canopy in year ten than it had before the application.
The argument to make to an officer is not that replacement should be avoided. It is that the number should follow from the quantum of canopy lost, which is a measurable figure rather than a negotiating position.
Canopy area as the measure of lost benefit
Take the canopy area of each tree to be removed from the survey carried out to BS 5837:2012, using crown spread measurements rather than estimates, and total it. That total is the loss to be addressed, and it should be stated in the submission as a figure in square metres alongside the tree schedule.
Then be honest about what the replacement will deliver and when. Two framings work in practice: replace the canopy area at maturity, which gives a modest number of large-growing species, or replace it within a defined period such as 15 or 25 years, which drives either more trees or larger stock.
| Approach | Effect on the schedule | Where it suits |
|---|---|---|
| One for one by count | Understates the loss, usually heavily | Nowhere, though it remains the default |
| Canopy area at maturity | Few large-growing trees, long payback | Parks, large gardens, open edges |
| Canopy area within 25 years | More trees or larger stock | Residential streets, urban sites |
| Canopy area plus a condition factor | Accounts for lost veteran features | Veteran or notable trees, where it may not be replaceable at all |
The fourth row is the honest caveat. A veteran tree with deadwood habitat, cavities and a 200-year root system cannot be replaced by planting at all, and an assessment that implies otherwise invites a reasonable objection. Say so, and deal with the harm as harm.
Converting lost canopy into a planting schedule
Work from the species' expected mature crown spread on the soil and exposure of the site, not from a textbook figure. A field maple in a paved urban pit will never reach its parkland spread.
- Total the canopy area to be lost, from measured crown spreads.
- Select species suited to the position, the soil, the rooting volume available and the exposure.
- Take an expected crown spread per species for the chosen time horizon, agreed with the arboriculturist.
- Divide the loss by the expected area per tree to get a number, then sanity-check it against the space available.
- Adjust the stock size upwards where the number cannot be accommodated, or propose off-site planting for the residue.
Show the arithmetic in the submission. An officer presented with a stated canopy loss, a species list with expected spreads and a calculated number will usually accept it, because it is more defensible than the figure they would otherwise pick.
The survey that underpins all of this is the Tree Survey, and the canopy figures need to come from it rather than from a later landscape drawing.
Nursery stock sizes and establishment rates
Larger stock buys canopy sooner and costs more, in both purchase price and aftercare. Extra heavy standards and semi-mature trees need irrigation through establishment, careful handling, root ball protection and usually underground guying, and their failure rate without that care is high.
A balanced schedule often mixes sizes: a few larger specimens where the visual loss is most obvious, with standards and feathered stock elsewhere where time is less pressing. That reads better on a drawing and survives better on site.
Establishment is the step most often skipped. BS 8545 covers the process from nursery to independence and is the right reference to name in a specification, with watering, formative pruning, mulching and stake removal set out as a schedule rather than an aspiration. Winter is also when much of the survey work happens, with the limitations described in winter inspection limits on a tree survey applying equally to assessing the canopy of the trees being removed.
Space available and the realistic planting envelope
A number arrived at by arithmetic has to fit. Map the available planting envelope before you fix the figure: rooting volume clear of foundations and services, distances from windows and boundaries, sight lines at accesses, and the areas committed to drainage features.
Planting trees into drainage features is possible with care, and the constraints are specific to the position within the feature. Where that is part of your envelope, the compatibility questions matter early, and the service clearance issues in utility trenching close to trees on a development site determine how much of the envelope is really plantable.
Where the site cannot take the calculated number, say so in the submission with the envelope drawing attached, and propose the alternative rather than quietly reducing the figure.
Off-site planting and commuted sums
Off-site planting is the usual answer to a residue, delivered through a section 106 obligation or a commuted sum to the authority for planting in a park, a verge or a school. Two conditions make it credible: the location is identified, or at least the area is, and the sum is calculated from a stated cost per tree including three to five years of aftercare rather than from a round number.
Authorities vary in whether they will take money for planting, so ask before you design around it. Where a bond or guarantee is used to secure on-site planting instead, the mechanics in tree bonds, financial guarantees and the planning obligation set out what the authority will be looking for.
An illustrative example. A scheme removes four trees with a combined measured canopy of 310 m². Replacement at a 25-year horizon with species expected to reach about 40 m² each gives eight trees. The site envelope takes five. The submission proposes five on site, including two extra heavy standards on the street frontage, with a commuted sum for three trees off site priced at nursery cost plus five years of aftercare, and the condition secures all of it. The negotiation lasted one round.
Drafting a condition that can be monitored
A condition that says "replacement planting shall be carried out" is unenforceable in practice. Ask for wording that names the approved planting plan and schedule, the stock sizes, the planting season, the establishment regime, and the period and standard of aftercare.
Then include a survival and replacement obligation: any tree that dies, is removed or becomes seriously damaged within a defined period is replaced in the next planting season with the same species and size unless the authority agrees otherwise in writing. Five years is the period most authorities use and it should run from the date of planting rather than from occupation.
Biosecurity belongs in the same condition where stock is being imported or moved between sites, and the practical measures are set out in biosecurity measures for tree works on site.
Survival periods and replacement obligations
The obligation is only as good as the monitoring. Ask for a short annual report for the aftercare period, listing each tree, its condition and any replacements, because that is the document that makes a breach visible and keeps a management company honest.
And set the trigger for planting carefully. Tied to occupation, planting happens in whatever season occupation falls in, which produces a summer planting that dies. Tied to the first planting season following occupation, it happens properly.
This is general guidance, not legal advice. Tree surveys and planting specifications should be prepared by a suitably qualified arboriculturist, and replacement requirements under a Tree Preservation Order are a separate statutory matter, so check the position with your local planning authority.
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Frequently asked questions
Why is one for one tree replacement inadequate?
Because the benefits a tree provides scale with canopy and leaf area rather than stem count. A mature tree with a wide crown carries a canopy area a nursery standard will take decades to approach, and a proportion of new planting dies before establishment. A count treats a semi-mature maple and a veteran beech as equivalent, which they are not.
How do you calculate replacement planting by canopy area?
Total the measured canopy area of the trees to be removed from the BS 5837 survey, choose species suited to the position and available rooting volume, agree an expected crown spread per species for a stated time horizon, then divide the loss by the area per tree. Check the resulting number against the realistic planting envelope on site.
Can a veteran tree be replaced by planting?
Not meaningfully. Deadwood habitat, cavities and a root system developed over centuries cannot be recreated by planting, so an assessment that presents replacement as full mitigation is open to challenge. Say plainly that the loss is not replaceable and address it as harm in the planning balance rather than as something compensated.
How long should a survival and replacement period be?
Five years is the period most authorities use, running from the date of planting rather than from occupation. The condition should require any tree that dies, is removed or becomes seriously damaged within that period to be replaced in the next planting season with the same species and size unless the authority agrees otherwise in writing.
When is a commuted sum for off-site planting appropriate?
Where the calculated number cannot fit the site's planting envelope. For the proposal to be credible, identify the receiving location or at least the area, and calculate the sum from a stated cost per tree including three to five years of aftercare. Ask the authority first, because not all will accept money for planting.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.