Hedgerow units, watercourse units, and why area units will not cover them

The statutory biodiversity metric is often described as though it produced a single score. It does not. It produces three, and they do not talk to each other.

• Area units, for habitats measured by extent: grassland, woodland, scrub, heathland, wetland, and so on.
• Hedgerow units, for hedgerows and lines of trees, measured by length.
• Watercourse units, for rivers, streams and ditches, also measured by length.

Each module has to show its own 10% net gain. A surplus of area units does not make up a hedgerow deficit, and no amount of new meadow compensates for a watercourse.

Why this catches people out

Because hedgerows are easy to lose without noticing you have lost them.

An access road cuts a field boundary. A visibility splay takes out twenty metres. A site boundary is rationalised and a hedge that was doing nothing obvious disappears from the layout. None of these feel like biodiversity events at the time. All of them show up in the metric as a hedgerow loss that now needs a 10% gain on top.

Watercourse units are rarer and sharper. Most schemes never touch a watercourse. The ones that do, through a culvert, a realigned ditch or a crossing, tend to need units that comparatively few habitat banks generate.

What the options are

Deliver it on site. New hedgerow planting is cheap in construction terms and takes up a strip rather than a parcel. If there is boundary left, this is usually the sensible first move. Bear in mind the metric rewards condition as well as length. A well-managed native species-rich hedge scores a great deal better than a thin single-species one.

Retain rather than replace. A retained hedgerow keeps its existing distinctiveness and condition in the baseline instead of appearing as a loss. Where a layout can be nudged to keep a boundary, that is almost always cheaper than buying units to replace it.

Buy off-site units of the right type. Hedgerow units and watercourse units are both sold as units, in the same way area units are. The constraint is supply: not every habitat bank has hedgerow in its plan, and fewer still have watercourse enhancement. It is worth asking about availability early rather than assuming it will be there.

The practical advice

Ask your ecologist, at the point the layout is still moving, one question: which of the three modules does this scheme have a deficit in? The answer changes what you need to buy, from whom, and how far ahead you need to arrange it.

If the answer includes hedgerow or watercourse, tell us the numbers and we will tell you what we can supply and when. The earlier that conversation happens, the more room there is to shape the layout around it.