The Whole Hedge

Hedges are more than just lines of shrubs and trees. A hedgerow is defined as a strip of woodland edge habitat made up of row of shrubs or trees over 20 metres long enclosing or separating fields. It includes all woody vegetation growing on any boundary, from stockproof hedgerows to relict hedgerows that no longer serve their purpose as barriers to livestock. They may be set on banks and can have ditches along one or both sides.

The best hedges have wide margins, often referred to as buffer strips or headlands, which are managed differently from the arable or grass crop. They include five components, mature trees (T), a shrub layer (S), the herb layer or base/bank (made up of flowers, grasses and ferns) (HB), the margin (FM) and a ditch (D) in many cases (see Figure 1). It is important that all these different components are considered when deciding how to best manage a hedge.

Figure 1. Hedge components: mature trees (T), shrub layer (S), base/bank (HB), ditch (D) and margin (FM)
Figure 2. Male bullfinch – bullfinch are secretive plump birds feeding on buds & fruit in hedgerows

Hedge Wildlife

Each of the elements of a hedgerow supports its own plant species and each of these supports characteristic animals and fungi using them as a growing substrate, for food, breeding and shelter. Hedges provide significant habitat for many threatened species and are critical for the continued existence of much wildlife in farmed landscapes and are particularly important as biodiversity corridors enhancing landscape connectivity when linked to woodland and scrub by providing safe spaces for plants and animals to move from one suitable habitat to another. Consequently, they are a key pillar of the complex ecological webs that are essential to life on farms.

Species that Depend on Several Structural Components

Bumblebees feed on pollen and nectar from shrubs and trees in the spring and from flowery margins in the summer. They nest and hibernate in the base of the hedge or in tussocky grass margins. When they move around the countryside and between their nests and food supplies, they prefer to follow hedges rather than cross open fields and so benefit from a network of connected hedges.

All these elements need to be in place for the bees to thrive. If they do well then through their pollinating activities shrubs like blackthorn and hawthorn will produce more berries, which will in turn feed birds like finches, thrushes and wintering redwings and fieldfares.

The yellowhammer, like many other farmland birds, nests in the shrub layer or hedge basal vegetation and feeds its young on invertebrates caught in grassy and flower-rich margins. They seek refuge from predators and adverse weather in the shrubs, and use trees as song posts. In the winter, seeds in the field margins form a valuable part of their diet.

Hedgerows in Ireland & Offaly

A 2019 survey determined that there were approximately 689,000 kms of hedgerows in Ireland (about 186,030 hectares of hedgerow) that are 2.7 metres wide on average meaning that they cover about 4% of our land area. At the time of the 2005 hedgerow survey of Offaly, there were 11,543 kms of hedgerows which supported 33 shrub and tree species (19 native) with whitethorn found in 99% of hedges. Most hedges were also growing on top of an earth or stone bank and 40% were unmanaged (i.e. on way to being lost as hedges), 90% were of low quality but interestingly over 25% showed evidence of having been laid in the past. 

Lifecycle of a Hedge

Hedges are not fixed features of the landscape but have a life cycle and without proper maintenance they become degraded. This is typically through overly frequent repeated cutting either at the same height or too low or by lack of cutting which results in the trees emerging and shading out the key shrub and herbaceous layers (called an ‘escaped’ hedge).

Hence, in a recent survey of 600 Irish hedgerows, over 90% were classed as being of low quality and only 1% classed as high quality habitat. Low quality equates to an impoverished ground flora with very low species diversity that can result from shading of emerging trees, herbicide use or close and frequent grazing by livestock. In this case weeds dominate (thistles, nettles and cleavers) and the valuable woodland and meadow flora for which hedgerows acts as a reservoir are absent.  Livestock can also cause poaching at the base of hedges causing bank or wall degradation and where shading has eliminated the shrub layer then animals push through and create gaps. The ‘low quality‘ category is of great concern as many of these poorly managed hedgerows will be on their way to being lost altogether.

Why are hedgerows important?

Hedgerows may often constitute the only biodiversity of significance left in intensively managed landscapes from which all other native species may have been lost and are all that is left of our native woodlands. As a result, they often represent the only ‘natural’ element left in these landscapes. At the other end of the scale older hedges date from centuries ago and are key reservoirs of rare species of plants and animals.

In terms of carbon storage, hedges can sequester between 0.5 and 2.7 tCO2e (carbon dioxide equivalent) per hectare per year up to an average of 58 tonnes per hectare. They provide barriers for livestock and shelter from wind, sun and temperature extremes. They support birds as well as predatory insects which help control pests. They also have an extensive network of roots which absorb overland water runoff and excess nutrients so contributing to flood control and maintenance of water quality. They can also provide firewood and food and are a key component of our landscape giving it its patchwork quilt appearance and conferring a unique sense of character on every farm.

The Management Cycle for ‘Healthy’ Hedgerows

This cycle starts with the planting of a new hedge or the rejuvenation of an existing hedge by laying or coppicing (see Figure 3). Coppicing involves cutting the stems right through (i.e. felling them) close to the ground to form stools from which new stems regrow with great vigour. Over the next 20 years or more, the shrub layer is trimmed at intervals to keep it thick, while being allowed gradually to increase in width and height. When the hedge starts to get gappy at the base, it is allowed to grow up, ready for rejuvenation and the start of a new cycle.

A hedge maintained in this way is a good hedge for the farm and is a healthy hedge for wildlife. The healthy hedge will have:

  • Good density: especially at the hedge bottom providing food and cover,
  • Good size: good width and height to provide livestock shelter and wildlife habitat,
  • Good diversity: of tree, shrub and ground flora species to provide food and shelter for a wide range of wildlife,
  • Good connectivity: with other hedges and semi-natural habitats within the landscape,
  • Well-placed: hedges across slopes (contour planted) to provide extra buffering from erosion and runoff.
Figure 3. The hedgerow management cycle.
Figure 4. Our hedgelayer Richard Markham wielding a billhook to ‘pleach’ a hawthorn stem (the ‘pleacher’) and lay the hedge.

What is hedge laying and why is it beneficial?

One suitable and long-term type of hedgerow management is hedge laying. Hedge laying is an ancient skill, involving a rejuvenative process that keeps the hedgerow young and vigorous indefinitely (see Figure 4). To lay a hedge, the main upright stems called pleachers, are half-cut through near ground level, bent over in the line of the hedge until they are near-horizontal, then fixed in position with vertical stakes of hazel, also in the line of the hedge. To reduce the width of the hedge the side branches of the pleachers (called brush) may be removed or woven into the hedge.

The tops of the stakes may be bound together with interwoven rods, also typically of hazel, to stock-proof the hedge by adding to its sideways strength and to give it a neat finish. While relatively expensive compared to mechanical cutting or coppicing it restores the hedge as an impassable livestock barrier. It lasts thirty years and more negating the need to spend on fencing.

Hedge laying is also a vital management tool for the biodiversity of hedgerows as it increases the density of the base— that is the woody and leafy density along with new growth which are beneficial for shelter, connectivity, and food resources so increasing insects, other invertebrates, birds, pollinators and bats.

Increasing habitat heterogeneity by carrying out hedge laying in small sections (no more than 10% of a farm’s hedges a year) can further increase biodiversity. This approach maintains tall sections which are preferred by bats and many bird species while allowing herbaceous plants, insects and other species of farmland birds to flourish in shorter, laid sections.

Dr. Douglas McMillan – Project Manager, Farm Carbon EIPBSc, CEnv, MIEMA, MSB, MIBI, MAEE, Grad IOSH, Fellow IITD, FSLSenior Laboratory, Energy, EHS, Quality & Ecology Consultant