Botanical condition surveys and the growing season

A botanical survey for habitat condition cannot be done in January, and that single fact sets the earliest date you can submit a planning application subject to biodiversity net gain. Habitat condition is scored against species composition, sward structure and the cover of undesirable species, all of which need the plants to be identifiable and in growth. Miss the window and you are not merely waiting for a report. You are waiting for next spring.

Developers who programme a survey budget without that constraint in it lose six months, and they lose it at the worst point: after the land is under contract and before anything can be submitted.

Why condition cannot be scored out of season

The statutory biodiversity metric scores each habitat parcel for condition against criteria set out per habitat type. For grassland those criteria turn on the number of indicator species present, the cover of herbs against grasses, the sward height and structure, and the proportion of bare ground and undesirable species.

In February a semi-improved neutral grassland is a few centimetres of dead material. A surveyor can tell you it is grassland. They cannot tell you whether it holds nine indicator species or two, and the difference between those two answers changes the baseline unit count substantially, which changes the gain requirement for the whole scheme.

A Biodiversity Net Gain Assessment rests on that baseline. The Environment Act 2021 regime expects the statutory metric to be completed by a competent person and the biodiversity gain plan to be approved by the local planning authority, so an AI-assisted draft is a starting point for that professional rather than a substitute for them.

The growing season window and its edges

The practical botanical survey window in England runs from April to September, and the usable core is narrower than that.

Management is the complication at both edges. A hay meadow cut in July looks poor in August through no fault of the habitat, and a surveyor has to record the management and say what the sward would have been. A horse-grazed paddock in September tells you very little.

Ask the ecologist for the window they need for the habitats present, in writing, before you fix a submission date.

Grassland, wetland and woodland differ

Grassland is the tightest constraint and the one that most often governs the programme, because the condition criteria depend almost entirely on the herb layer.

Woodland splits in two. The canopy and structural criteria can be assessed outside the growing season, but the ground flora criteria cannot, and ancient woodland indicator species need an April to June visit. A woodland survey in November will under-record the ground layer every time.

Wetland and swamp habitats need the main growing season, and water levels matter as much as the calendar. A pond assessed at the end of a dry August is a different proposition from the same pond in May.

Hedgerows are more forgiving. Woody species can be identified from twig, bark and bud by a competent surveyor in winter, though the condition criteria covering gaps, height, width and the presence of a herbaceous margin are easier to score in leaf.

Arable and modified grassland are the easiest, which is tidy, because they also carry the fewest units.

What a winter walkover does establish

A winter visit is not wasted, and on a site under option it is often the sensible first step.

• The habitat types present at classification level, and therefore the shape of the problem.
• Parcel boundaries, extent and area measurement.
• Hedgerows, trees, watercourses, ponds and other linear and point features.
• Obvious constraints: badger setts, bat roost potential in buildings and trees, invasive species.
• Whether the site is likely to need a full multi-season programme at all.

Use it to scope. A Preliminary Ecological Appraisal written from a winter visit should say plainly what it could not assess. The same honesty applies to ground conditions, where winter ground investigation changes what can be established on site. Treat both as scoping exercises that define next spring's programme rather than as the evidence base.

The baseline date and the degradation rule

The pre-development biodiversity value is assessed at a relevant date tied to the application, and the completed metric has to accompany the application rather than follow it by condition. That is why the survey date fixes the submission date and not the other way round.

There is a second rule that catches the unwary. Where activities carried out on or after 30 January 2020 have reduced the biodiversity value of the land, the pre-development value is taken as at immediately before those activities. Clearing scrub, ploughing a meadow or felling a line of trees to reduce the baseline does not work, and a surveyor comparing current conditions with historic aerial imagery will see it.

The practical consequence for a landowner is to stop changing the management of the site once development is in contemplation, and to record what the management actually was.

Repeat visits and multi-season programmes

Habitat condition is one survey. Protected species are several, and they have their own windows. Bat emergence surveys run from May to September, great crested newt environmental DNA sampling sits between mid-April and the end of June, reptile surveys work best in spring and early autumn, and breeding bird surveys need several visits across March to August.

Sequence them in one season where the site allows it. A Protected Species Survey programme that straddles two summers delays the application by a year, and the surveys and any mitigation licence require a suitably licensed ecologist under the Wildlife and Countryside Act 1981 and the habitats regulations, so there is no shortcut.

Budgeting across two financial years is a common consequence, and planning a survey budget across two financial years is worth doing deliberately rather than discovering in March.

Setting the application date from the survey

Work backwards.

1. Identify the habitats present through a winter or early spring walkover.
2. Get the ecologist's required survey window for those habitats in writing.
3. Add the reporting and metric time after the last field visit, typically a few weeks.
4. Add time for the design team to respond, because a baseline that comes back higher than expected changes the layout.
5. Set the submission date after all of that, not before.

Where the programme will not allow a growing season survey, the honest options are to wait, or to submit on a precautionary baseline. Regulators generally expect the precautionary assumption, which means assuming the better condition where it cannot be ruled out, and that raises the gain requirement. Applicants who assume poor condition to lower the target get the metric returned.

One uncertainty worth acknowledging: authorities differ on how much precaution they will accept in a winter baseline, and some will not accept one at all. Ask the ecology officer before committing to that route.

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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