Hedgerows are such a familiar sight to those of us living in Britain that we hardly give them a second glance. If you are an inner-city teacher, they are likely to be more of a novelty, but take a look out of your classroom window - what marks the school boundary? In a surprising number of cases it will be a hedge rather than a fence or wall. Even if it is a boring line of severely clipped privet, you could be viewing it with different eyes: it’s not just a hedge - it’s a resource.

There are many different kinds of hedge, depending on their function and geographical location. They range from enormous barriers of yew or beech lining impressive avenues, to prissy structures of box barely half a metre high, marking the manicured edges of formal knot-gardens or parterres.

The earliest hedgerows were essentially planted for two main reasons: to keep livestock in or out and to mark boundaries. Until very recently, the trend has been to remove any obstacles that restrict farming operations, especially in Eastern England. Many hedges are ancient - at least 800 years old in some cases - making it all the more tragic when landowners decide to grub them up. Centuries of history can disappear within minutes as the machinery moves in and the wildlife moves out, leaving nothing but wide, windswept expanses - wonderful for crops, perhaps, but ecologically sterile.

The classic English landscape suffers, too. Within a twinkling of an eye, those rustic scenes so beloved of Constable are replaced by featureless wildernesses with not a hedge in sight. But all is not lost. Through Agri-environment Schemes, brought in over the past 20 years and partly funded by the European Commission, farmers are compensated for income lost when they establish or improve environmentally beneficial practices, and it now pays them to plant hedges rather than pull them out.

The origins of hedgerows are not always straightforward. First, what exactly is a hedge? Strictly speaking, it is a row of woody plants of a single species, planted closely and cut at intervals to encourage bushy growth. A hedgerow, on the other hand, is comprised of a mixture of species, including some that are allowed to grow into trees. Clearly, a hedgerow makes the better, more varied habitat, but a hedge is useful nonetheless.

Landscape historian Dr Oliver Rackham reckons that older hedges and hedgerows originated in three possible ways. The oldest arose as a direct result of ancient “wildwood” being cleared in the Middle Ages to provide space for agriculture and settlement. Strips of the woodland were probably left to mark field boundaries and later trimmed to prevent their encroachment into the crop-growing area. Such a hedge can be recognised today by a combination of features, all characteristic of ancient managed woodland. For instance, it might be growing along the top of an earthen bank, next to a ditch. This is because woodland was such a precious resource that elaborate “woodbanks” were constructed to mark its boundary and dissuade deer and livestock from entering. Earth from the ditch was piled up to form the bank on the wooded side, and this was then planted with closely spaced shrubs to form a barrier.

There are two other fail-safe signs of a hedge derived from ancient woodland. First, these ancient hedgerows are rarely straight for more than a few metres - the rural medieval residents seem to have been less concerned about geometry than we are. Second, if you look carefully at the hedge-base, you’ll notice plants that normally grow in woods, but never normally in hedgerows: dog’s mercury and wood anemone are two examples.

Even after many hundreds of years, these woodland specialists hang on, despite the disappearance of the forest they once inhabited, giving the name “woodland ghosts” to hedges formed in this way.

Hedges may be lingering memories of woodland long gone, but they can also signify the position of fences erected centuries ago to mark property boundaries. Inevitably, weeds and shrubs grew up on either side and were left uncut. Sometimes this was deliberate, but often it was simply because medieval peasants tended to avoid hard work if it wasn’t necessary. Either way, hedges were the result, and they were subsequently managed to maintain their efficient stock-proofing properties. Some remain to this day, their original backbones - those ancient fences - long since rotted away. As much varied in their species-richness as are the woodland ghosts, and often as sinuous, they can be distinguished by their lack of woodland plants.

The third type of hedge was deliberately planted. Before the 18th-century Enclosure Acts, mixtures of shrubs and trees were used for their multiple benefits. Apart from its main purpose of providing a clear impenetrable boundary, hedges would provide fruits - such as pears, apples, rosehips and blackberries - and wood of different sizes and properties for many household and building purposes. Oak trees were especially grown in hedgerows to provide timber of the correct strength, shape and size for constructing warships. Shipwrights would scour the countryside, carrying templates to hold up against likely looking boughs of trees. The specimens they needed were not usually found in the depths of the forest, but more often in the open, where their shape was not constrained by crowds of other trees competing for light (see box: “Going to war”).

From about 1750 onwards, a marked change in much of Britain’s scenery took place. Dubbed the time of the Great Enclosures, this phase in agricultural history was marked by frenetic parliamentary activity, the result of which was to compel parishes and landowners to demarcate their borders. Before this, in a huge swathe stretching diagonally from north-east England to Hampshire and Dorset, much of the land was communally grazed. By enclosing their plots, those who wished to grow crops could do so without fear of their produce being eaten or trampled. Within a century, this part of Britain - the so-called planned countryside - became criss-crossed with hurriedly planted and geometrically laid-out hedges. More than 321,000km were planted during this time, but most were of a single species, usually hawthorn.

To this day, the distinction between the planned countryside of the r Midlands and the ancient countryside of the West and South-East is very marked. The smaller, irregular fields of the latter are bounded by mixed hedgerows, often of great antiquity, whereas the large, rectangular fields of the planned countryside usually have straighter and narrower hedges, with less biodiversity.

In the modern landscape, hedgerows gradually became anachronistic, especially in the more fertile and gently undulating East of England. With the advent of the tractor and the heavy equipment it could pull, hedges proved to be too restrictive. Gone were the days of keeping out livestock - many farmers were turning to exclusively arable operations. Farms were increasing in size, too, as richer landowners bought up neighbouring properties.

However, the Second World War and its privations marked the beginning of the end for many hedgerows. With the need for more efficient food production than ever before, farmers were actually paid to grub out hedges to make space for crops and reduce the “dead space” in fields of irregular shape, so that tractors with ploughs and other machinery could manoeuvre more easily.

But the removal of hedgerows was catastrophic for wildlife. Unless it has wings, an animal cannot easily get from one suitable habitat to another.

After the war, crop-farming meant increasingly hostile tracts of land, as new deadly pesticides were introduced; if a dormouse, for example, needed to cross from one wooded area to another, it relied on hedgerows to provide a relatively safe passage. Yet within a shockingly short space of time these vital corridors were removed from places where they had stood for centuries.

We now know that many species of plants, insects, animals and birds depend on the sanctuary of hedgerows to live and find their food, whereas the fields to either side are like deserts in comparison. Between 1947 and 1985, an incredible 17,700km of hedges were removed from the landscape of East Anglia alone. The effect that this has had on wildlife can only be guessed.

There is a bright light at the end of this tunnel, however. With the recent introduction of the Environmental Stewardship scheme, farmers are required to involve themselves in looking after nature, rather than subduing it for the sake of profit. Not all farmers were previously “antienvironment”, but now they can all gain by protecting it. Ironically, one of the options offered to farmers, 60 years after they were paid to remove hedges, is to put them back again.

References Trees and Woodland in the British Landscape By Oliver Rackham, Weidenfeld and Nicholson, pound;14.99

* The Environmental Stewardship scheme, involving farming for wildlife, is detailed on the Department for Environment, Food and Rural Affairs website www.defra.gov.ukerdpschemesesdefault.htm

* Nelson: Trees of Trafalgar is a Radio 4 programme in the Nature series, investigating the history of trees used for building Nelson’s ships. To hear it visit www.bbc.co.ukradio4sciencenature_20051024.shtml

What lives in a hedge?

Ancient hedgerows can harbour a huge variety of plants and animals. Even well-groomed school hedges can support a surpising amount of wildlife. In its handy little book, Farm Conservation Guide*, the crop-protection company AgrEvo UK list these hedgerow facts.

*www.fwag.org.ukMainSitepublications.htm

Animals or plants Number of species found in hedgerows

Trees more than 30

Perennial wildflowers 600

Shrubs more than 25

Mammals, reptiles, amphibians more than 20 use hedgerows

Birds more than 65 nest in hedgerows

Insects more than 1,500

Molluscs, worms, centipedes, hundreds of species spiders, and so on

Hedge care

1. Trim every two to three years. This allows sufficient growth for maximum flowering and fruiting and cover for nesting birds.

2. The “A” shape hedge is best for wildlife, at least 1.5-2m high and 2.5m wide at the bottom.

3. Late winter is the best time to cut a hedge. Never cut it during the nesting season (April to July).

4. Allow a few saplings to grow into trees if possible, especially at hedge corners. This will encourage more birds.

5. Plant gaps in the hedge with different species to encourage a variety of wildlife. The following make good hedge plants: hawthorn, blackthorn, field maple, hornbeam, crab apple, hazel and beech.

6. Good trees to include are oak, ash, whitebeam and holly.

7. If you have space, allow a margin of 2m from the base of the hedge to grow wild. Cut it occasionally, but always after June. This allows flowering plants to set their seeds. If possible, cut different parts of the hedge margin at different times, so a variety of plant heights is achieved.

8. Avoid spraying pesticides or fertilisers near the hedge (ideally, don’t use chemicals on school fields, other than for spot-treating weeds such as thistles, dock and nettles).

HOOPER’S RULE

Dr Max Hooper of Cambridge University discovered a rough rule of thumb for calculating the age of ancient hedges. Called Hooper’s Rule, it is simple to apply and holds true for any hedge that has been in place beyond living memory.

First, check that your hedge is old. One way is to ask an elderly person whether it was around when they were young. Another is to check with the town or county council, but a better, more classroom-focused way is to visit an appropriate website. The Ordnance Survey’s Get-a-Map service allows teachers to download copies of historical maps: www.ordnancesurvey.co.ukoswebsitegetamapAlso visit the map archive owned jointly by Ordnance Survey and Landmark Information Group: www.old-maps.co.uk and the British Library’s online map collection www.bl.ukcollectionsmap_os.html

Any hedges that appeared on the First Edition Ordnance Survey maps are at least a century old and are good candidates for testing Hooper’s Rule.

Next, measure a 30-metre length of hedge (use one of the PE or geography department’s tape measures). Then simply count the number of different species of woody plants (trees or shrubs) in the 30-metre length. Don’t include brambles, ivy or any other climber, as Hooper found they gave misleading results. The approximate age of the hedge in centuries is equal to the number of woody species in the 30-metre length. Obviously, you will get a more accurate result if you have a long enough hedge to repeat the process for further 30-metre lengths, then calculate the mean. By this method, a hedge I investigated with Year 9 pupils from Bungay High School this summer was found to be 700 years old.