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Book:	Hedging
Chapter:	2 Hedges and conservation
Section:	The case for hedges
Metadata:	Details	Buy this book

In the recent past, hedges were seen by many farmers, especially most arable farmers, only in economic terms. From this viewpoint they were seen as expensive to maintain, wasting valuable land, preventing efficient use of large machines, and harbouring pests. Possibly this view is now changing. Well maintained hedges are coming to be seen as an asset for the farmer, to be managed well for the benefit of the farm business, as well as for their wildlife, stock, amenity, and landscape values. However, hedges are continuing be lost at a high rate, because on many farms the economic argument has to win. Many hedges are neglected, so their usefulness and value is minimal.

Hedges versus fences

The advantages and disadvantages of hedges and fences are listed below.

Advantages of hedges

They provide a windbreak for crops and shelter for stock.
They provide shade for stock.
They help prevent soil blow.
They provide a habitat for flowers, birds, insects and other wildlife. Many of the insects, notably beetles, are valuable for control of aphids and other pests.
They provide sport and food: rabbits, hares, partridges and pheasants.
They are permanent. With proper maintenance they can remain stockproof indefinitely. They are assets to a farm, which deserve looking after. If looked after properly, maintenance costs are not high. It is the restoration of neglected hedges, and bringing them back into a proper cycle of maintenance that is likely to be expensive.
Hedges have high landscape, amenity and historic value.
Well-maintained hedges are people-proof and difficult to vandalise.

Disadvantages of hedges

They take up space which could be used for crops or grazing.
They reduce the efficient use of farm machinery.
They require annual or biennial trimming, and periodic laying.
In the period before laying, when allowed to grow up, crops will not grow effectively in their shade.
It may be claimed that they harbour pests, including small mammals, insects and weeds. See page 14.
Depending on the type of hedge and the width of the field margin, they may compete with the crop for nutrients and moisture.
Hedges don't grow well in shady conditions, for example on the north side of a wood.

Advantages of fences

They take up little space.
They cast no shade, and do not compete for nutrients or moisture.
They are quick to install and remove if farming operations change.
They have low maintenance costs.

Disadvantages of fences

They provide no shade or shelter.
They can be broken down by stock, cut by vandals or even stolen.
They need to be replaced periodically.
They have no wildlife, landscape, amenity or sporting value.

Hedgerow trees

Hedgerow trees have a high landscape and wildlife value, especially in areas with little woodland. In many areas, hedgerows are the only remaining location for trees, and may provide a genetic link back to the ancient woodlands. In Northern Ireland, which has little woodland cover, 60% of the total number of broadleaved trees are in hedgerows.

Advantages of hedgerow trees

They provide shade for stock and people.
They have high wildlife, landscape and historic value. Their branching shape, caused by growing in open ground, makes them particularly attractive as landscape features.

Disadvantages of hedgerow trees

Their shade is detrimental to the growth of the hedge beneath, and makes hedge laying more difficult. Shading is more of a problem in hedges running north-south.
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They compete with crops beneath for light, moisture and nutrients. Shallow rooted trees such as ash, elm, poplar and sycamore cause the most problems, whereas deep rooted trees such as oak and lime cause less trouble. Shallow rooted trees are easily damaged by deep cultivation, and by compaction from stock trampling or vehicles. Root damage is thought to be the cause of die-back in ash and other trees in hedgerows and beside roads.
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Hedgerow trees may harbour bird 'pests', such as rooks, jays, magpies and pigeons.
Hedgerow trees have little timber value because of their branching growth, caused by growing in open conditions. The lower trunks of many hedgerow trees are ruined for timber value by having fencing wire and staples embedded in them. Hedgerow trees may be valued by the landowner for firewood.

Some of these disadvantages can be lessened by 'crown lifting', which is removing the lower branches of the trees which are the most troublesome for shade and getting in the way of farm machinery.

For more information on hedgerow trees (see Chapter 5, section Hedgerow trees)

Field margins

The treatment of the field margins is significant in the growth and management of the adjoining hedge.

Advantages of field margins

Increases the wildlife value of the adjacent hedge, and its function as a 'beetle bank' to harbour beneficial insects.
Avoids the problem of unlaid hedge or hedgerow trees casting shade on crops. Lessens the amount by which hedge and trees competes with crop for moisture and nutrients.
Avoids cultivation damage to tree roots.
Leaves space for farm or public access around the edge of the field.

Disadvantages of field margins

Decreases the area of land under cultivation. However, its value as a beetle bank may reduce the expenditure on pesticides, so that the economic return is greater.

Shade

A hedge significantly affects the immediate area with its shade, but only to a distance of one or two times the height of the hedge. Crops are affected, grass less so. Hedges running east-west cast less shade overall than those running north-south, but their north side is shaded most persistently. Individual hedges can be located so that most shade falls on 'unused' land such as streams, roads or tracks. Where field margins are left, shade does not affect the crop.

Shelter

A permeable barrier such as a hedge makes a far better shelter than a solid one such as a wall. The permeable barrier filters and slows the air, and cushions the air which is forced up over the top. A solid barrier pushes all the air over the top and the air drops quickly on the other side. A solid barrier also produces a low pressure area immediately in its lee which causes the wind to eddy, sometimes at speeds higher than the original wind speed.

The optimum permeability is about 40%. A rule of thumb is that, when the hedge is in leaf, you should be able to see movement through it, without being able to see objects distinctly. Clipped hedges may be rather too dense in summer, causing occasional turbulence. Where the sheltered hedge is in cereals the crop may be 'lodged' or flattened by the wind along a zone parallel to the hedge.

A hedge of optimum permeability reduces wind speeds by more than 20%, for a distance of about eight to twelve times the hedge's height, to leeward of the hedge. For a 6' (1.8m) hedge, this is a distance of 48-72' (15-22m). Some shelter is afforded to a distance up to thirty times the hedge height, but while this may affect wildlife such as insects, it is unlikely to make much difference to crops. Some shelter is also afforded to the zone just to windward of the hedge for a distance of up to four times the hedge height, due to a 'pre-cushioning' effect (Pollard, Hooper and Moore, 1974).

The hedge must be at least twenty times as long as it is high for these shelter effects to work well. Otherwise wind eddies around the hedge's ends and reaches more of the field behind it. Since damaging winds can come from any direction, all four sides of the field must be hedged for full protection. To significantly shelter an entire field with, for example, 6' (1.8m) hedges, the field must be in the order of 44m square, or about 0.5 acre (0.2 ha). This only makes sense in special situations such as the Isles of Scilly bulb fields.

The siting of hedges affect their shelter value. Long parallel hedges may work well when the wind is blowing at right angles to them, but act to funnel and increase the speed of parallel winds. In the Scillies, temporary shelter screens are placed at right angles to the parallel hedges to prevent this. Gates and gaps in hedges similarly funnel the wind, as can hedgerow trees, which may make certain parts of a field susceptible to lodging.

A hedge sited along the top of a ridge or on a hillside has its height effect enhanced by the slope of the land behind it, making it a more effective shelter when the wind is in the right quarter. On a flat-topped ridge, however, the cover is only average.

Winds tend to be deflected or funnelled into valleys. Therefore hedges most effectively block these winds when located at right angles to the valley.

The most difficult fields to protect are those which slope into the prevailing wind, which in most cases blows from the southwest. Here the hedge's effective height is reduced.

Hedge shelter affects many climatic factors besides wind. Daytime air temperatures are higher and night temperatures lower, but only by one or two degrees centigrade. Reduced wind speed results in lower evaporation, and soil temperatures and moisture are higher. The relative humidity is also slightly higher.

Hedgerow loss can have more widespread effects. A Danish study of two west-east transects running across the country found that average wind speeds were about six miles an hour less along the well-hedged line than they were in the more open landscape. One of the concerns in Britain is the problem of 'fen blow', during the period when soils are bare of crops. As well as the loss of topsoil, seeds and fertiliser can be lost, or young crops damaged by the scouring effect of blown topsoil The danger is reduced where willow shelter hedges are planted.

Are hedges needed to shelter crops?

After examining the effects of hedges on shelter, Pollard, Hooper and Moore (1974) conclude that 'increases in crop yields due to shelter in this country can be expected to be mainly small or non-existent, as water shortage is not common'. In the decades since, water shortages have become much more of a problem, so the effects on crop yields may have become more significant. Irrigation, which used to be the usual solution, is becoming more difficult and expensive. Shelter hedges have value in protecting orchards from frost.

Are hedges needed to shelter livestock?

Many farmers, particularly on exposed hill farms in the north and west, put great faith in hedges to shelter their herds. Stock obviously do seek shelter in heavy rains and gales, although it is interesting to note that they tend to put their backs to the wind and move away from the storm, even to the extent of moving farther from the shelter. One farmer spent many hours training his cows to turn into the wind to reach a sheltering hedge, rather than end up blocked by another hedge on the exposed crest of a hill. But despite his efforts, shelter's beneficial effects on livestock remain unproven. Studies of liveweight gain and food conversion rates indicate no significant difference between sheltered and exposed stock.

Weeds, insects and diseases

Weeds

Too frequently hedges are seen as places which harbour 'weeds', that seed into the adjacent field. Herbicides are used to eliminate herbaceous growth at the base of the hedge. This has now been shown to be self-defeating, as herbicide use only tends to encourage unwanted aggressive species such as nettle, cleavers, false oat-grass and hogweed. In contrast, unsprayed hedges, regularly trimmed, tend to develop a fairly stable mixed flora, the species of which are dependent on the adjacent land use. Herbicide use creates bare ground, open to invasion by aggressive species.

A further advantage of maintaining a mixed hedge flora is the population of beneficial insects which it will support, as detailed below.

Insects

The value of the hedgerow habitat in supporting populations of beneficial insects is increasingly recognised, in comparison to earlier views which concentrated on the limited range of insect pests, mainly linked with horticultural crops (see below).

The flora of the hedge bottom, bank, ditch and any field margin is more important in supporting insects than the shrubby component of the hedge. Tussocky grasses and leaf litter are important over-wintering habitat for beetles and spiders, which predate on aphids. High quality habitat for these insects can be created simply by growing tussocky grasses on raised banks, or 'beetle banks'.

The numbers of beetles and spiders overwintering on a grass ridge are significantly greater than the number found on grasses at the same level as the rest of the field. This is probably due to the increased drainage. On level ground, a large proportion appear to be killed by being frozen in waterlogged ground. The tussocky grasses such as cock's foot and Yorkshire fog provide insulation, with the predatory insects burrowing deep into the leaf sheaths and leaf litter. When spring comes, there is then already a large population of insects available to keep aphid populations under control. To provide sufficient numbers of predators in the right place, beetle banks need to be located at fairly regular intervals (see below). Their creation in large fields is in effect replacing the hedgerows which have been swept away. A good beetle bank is a more effective habitat for predatory insects than a degraded hedgerow, so it is important for hedgerow survival that they are managed to improve their value for this biological control.

A raised hedge bank with tussocky grasses thus needs to be seen as a valuable economic resource by the arable farmer or horticulturist. The cost of creating and managing a new bank, and the small loss of cropping area involved, is offset by the big savings which can be made by not having to spray insecticide. Grant aid for creation of beetle banks may be available from the Ministry of Agriculture, Fisheries and Food under the Countryside Stewardship Scheme.

Beetle banks can be constructed as part of normal autumn cultivation operations, and sown in autumn or spring with tussocky grasses such as cock's foot, Yorkshire fog and Timothy at a rate of 3g per square metre. In the first year the bank should be cut with a flail three times between March and September, to control annual weeds and promote tillering. Once the perennial sward is established, the bank should only need cutting once every three years. The number of ridges needed depends on field size, shape and the quality of existing boundaries. As a rough guide, one mid-field bank should be sufficient in a field of 40 acres (16 hectares) which has existing good quality boundaries, or if the boundaries are poor, construct two banks which divide the field into three equal sections. Allow a working gap of 25 metres at each end of the bank so the field can still be worked as a unit. Further details are available from MAFF as part of the current grant scheme, and a leaflet on beetle banks is available from the Game Conservancy Trust.

Other beneficial insects of banks and hedgerows are hover-flies, which feed on aphids. The adult hover-flies need pollen from flowering perennials and annuals, and flower-rich field edges provide an important habitat.

Diseases

Diseases such as potato blight and mildews of wheat and oats may occur due to stagnant air near hedges. Hedgerow trees probably cause more problems than well-maintained hedges.

Hedges may be implicated in orchard diseases, since blackthorn, crab apples, hawthorn and dog rose are all members of the Rosaceae family, as are apples, pears, plums and cherries.

Land taking and machine efficiency

The issues of land taking and machine efficiency are closely tied. Aside from simple area occupied, hedges tend to get in the way of efficient use of farmland. This applies particularly to arable land, but also to pasture where access is needed for fertilising, muck-spreading, silage operations and so on.

An average hedgerow 1.5 miles long occupies an acre of land (1 hectare per 6km). Where hedges are on wide banks, as in Devon, a half mile of hedge may occupy an acre (1 hectare per 2 km). Hedges in Great Britain total about 251,000 miles (404,000 km) occupying about 167,000 acres (67,000 hectares), 1998 estimates. To the farmer, this represents land lost to agriculture. To many others, it represents a valuable nature reserve and landscape feature.

By grubbing hedges to enlarge their fields, farmers gain not only more productive land, but also improved machine efficiency. Time wasted in turning at the ends of short rows is reduced. As field size increases, gains achieved by removing more hedgerow becomes progressively less. In a 100 acre (40 hectare) holding with 5 acre (2 hectare) fields, 2.6 acres (1.1 hectares) are gained by removing all the hedges, assuming they are 6' (1.8m) wide. If the same area has 50 acre (20 hectare) fields, 0.8 acres (0.3 hectares) is gained by hedgerow removal. In terms of machine efficiency also, the gains of grubbing decrease progressively as field size increases. The maximum field size worth creating through hedgerow removal was estimated at about 50 acres (20 hectares) (Pollard, Hooper and Moore, 1974), although many fields are much bigger.

When it comes to decisions on the ground, farmers look at field shape as much as size to evaluate land use efficiency. Fifty acre (20 hectare) fields are optimum for machines only if the fields are roughly square. Even those farmers most fond of hedges regret acute angles and inaccessible field corners. Past plantings may have been based on small land transfers or other historical vagaries which are out of keeping with present boundaries. Judicious hedge grubbing can open up cast-off fields and create more regular patterns without destroying the overall 'hedginess' of the farm.

Profits from hedgerow timber?

In the 1980s, hedgerow trees were the source of one-fifth of all home-grown hardwood marketed in Britain. Trees growing in hedgerows, avenues and tiny clumps (less than 0.6 acres or 0.25 hectare) in Britain total about 90 million (Blyth, Evans, Mutch and Sidwell, 1987).

With proper management, good timber can be produced in hedgerows. Because such trees are generally grown on fertile, sheltered farmland sites, the quality of timber produced and the growth rates compare favourably with those from the usual timber growing areas on the uplands and heathlands of Britain.

Trees in hedgerows have a different pattern of growth compared with woodland trees. The open conditions encourage rapid radial growth and the trees reach large diameter sooner than if grown in woodland conditions. However, the open conditions also cause heavy branching, and the development of a large crown. Compared with woodland trees, the proportion of crown wood to stem timber is greater, and in total yield of wood, may add as much again to what is in the trunk.

Hedgerow trees can thus give a good yield in short lengths for firewood, which may make them worth felling for the owner's use or quick profit, but not worth planting for that purpose. Profits from hedgerow timber can be greatly increased by growing suitable species, and most importantly, by managing the trees correctly.

Rapid growth in species such as ash, beech and sycamore produces high quality, uniform wood. To reduce branching and produce a good quality knot-free timber butt, side branches must be pruned before they are about 2" (50mm) thick. Pruning can be done in two stages: to about two metres when the tree is about five metres tall, and if desired, to four or five metres when it is eight to ten metres tall (Blyth, Evans, Mutch and Sidwell, 1987).

The highest value timber can be produced by growing species for hardwood veneers. Such trees need to have a minimum length of 7' (2.1m) of unblemished stem, formed by careful pruning and protection from damage. There is no economic return for at least 70 years, although the value of standing timber would be realised if the farm was sold.

The trunks of hedgerow trees are vulnerable to damage by farm vehicles, bark-stripping by stock, and are notorious for containing embedded fence wire, staples and nails, which can cause damage and injury when being sawn. Trees must never be used as fence posts. An advantage however of hedgerow trees is that extraction costs are usually low, as felling and access for removal is easy. Note that trees in hedgerows suppress the growth of the hedge itself, and make hedge laying more difficult. They also compete with grass and other crops for light and moisture. For further details on planting and management of hedgerow trees, see page 40.