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How do we measure hedge structure?

Measuring the height and width of a hedge can give us a good indication about how intensively a hedge is being managed. How big a hedge is has a surprisingly large impact on how valuable it can be for wildlife. 

The base canopy of the hedge is the height above the ground at which vegetation cover starts. Ideally hedge vegetation should extend to the ground, but in hedges that are over grazed, over trimmed or overgrown, this canopy height can be higher. This measurement shows us how leggy a hedge is getting, helps us understand what point of the management cycle it is in, and guides us to what management action should be taken.

Recording the shape of the hedge gives us a really good idea about the current and historic management of the hedge. There are four main categories of hedge structures to choose from; over trimmed, dense and managed, tall and overgrown and recently rejuvenated. 

Lastly, we record some signs of structural damage. A hard knuckle of wood at the trim line of the hedge is a good indicator that the hedge is being trimmed too frequently at the same height. It is often a sign that the hedge is being held at one point in its life cycle, which will inevitably compromise the health and structure of the hedge over time. We also record how much of a margin the hedge has between the base of the hedge and the point at which you get soil disturbance, if this is too close to the hedge, it can damage the roots of the hedgerow bushes and the hedgerow trees which can weaken and even kill them in the long term.

Measuring any dead structural bushes in the hedge can show us whether the hedge structure and connectivity is in immediate threat. Structural bushes in hedgerows can die for a number of reasons; repeated over trimming, soil disturbance too close to the hedge roots, heavy spray drift and now increasingly tree disease.

Have a look at our survey guidelines to learn more about how to survey hedgerows.

Why is hedge structure important?

The structure of a hedge can have a huge impact on how useful that hedge is for wildlife. It can also give us a good indication about how well that hedge is being managed.

From the herbaceous vegetation at the bottom, to the woody shrubs that make up the structure and the trees that tower above the hedge canopy, all the parts that make up a hedge play a role for wildlife.

On top of providing physically more habitat volume, larger hedges can provide a more complex habitat, one that offers more niches and so can be home to a wider range of wildlife. Larger, more mature hedgerows are more similar to some types of woodland, and so can also offer a safer corridor for animals to travel. Dormice, for example, have been found to live in tall hedgerows in densities comparable to woodland, but they are not found in short intensively managed hedges.

  • Larger hedges support a greater abundance of small mammals
  • Larger hedges support more plant eating insects, which benefit many other species up the food chain.
  • Tall hedges support a greater diversity of bird species, as well as the numbers of nesting pairs, possibly through the reduced chances of nest predation.
  • Tall hedges help support a greater diversity of butterflies.
  • Wide hedges provide a better refuge opportunity for small mammals from potential predators.
  • Dense vegetation at the base of a hedge increases the number of nesting birds it can support.
  • The bottom layer of vegetation in a hedge is also important to insects, small mammals and reptiles.

Is there a perfect hedge structure?

There is no definitive answer to this question, as different hedge structures are beneficial to different plants and animals, and each species may have its own preferred hedge type.  The more variation in the hedge architecture within the network, the more species will be supported.

We know that a diversity of habitats within the hedgerow network supports more wildlife diversity, and that hedgerows are ideally managed in in a cycle to ensure their structure in the future. Both of these ideas neatly lead us to rotational management of our hedges to benefit the health of the hedge and maximise the amount of wildlife within them.

Handily, rotational management and longer cutting cycles means that overall we have less hedge management to do each year; e.g. f you are managing your ‘dense and well managed’ hedges on a 2 or 3 year cutting cycle, this may mean you cut 1/3 of all your hedges each year.

There are a few key structural features that are always beneficial to wildlife:

  • Taller wider hedges are better for wildlife than short thin ones
  • Vegetation at the base is very important generally; thinning base vegetation is a sign the hedge will need rejuvenation and/or protection
  • Hedgerow trees will increase the number and the variety of species in most hedges.
  • A good margin of undisturbed ground and unsprayed vegetation from the base of the hedge will help protect the future structure of the hedge, as well as immediately benefiting plants, insects, birds and small mammals.

Find out more about how the structure of a hedge is important to wildlife

 

PTES Hedgerow Survey *

Current Hedgerow Statistics

Length of
hedgerow surveyed

547 km

Average number of woody species
reported per 30m

3.6

% hedges in each
main structure category

  • 42.4 Overtrimmed
  • 28.2 Dense and managed
  • 23.5 Tall and overgrown
  • 5.9 Recently rejuvenated

Average trees/100m

19.5

* Statistics contain results from historic assessment results in addition to current online hedgerow surveys.