Hedgerows for Wildlife

If a short hedge is flail mown every year or two, the familiar 'tidy' box of dense short hedge may result and thicken each year. Often, however, faster growing, scrubbier species such as bracken and bramble regrow rapidly from the base and can swamp the hardwood species, resulting in a patchy line of bushes and scrub.

If left alone, hedges usually get bigger and broader. Eventually, trees usually win the battle and may form sufficient canopy to suppress the lower scrubbier growth – particularly if livestock graze at the base. The result is often a gappy line of trees, or a linear wood with little understorey, depending on how you look at it.

Banks as well as hedges can be vulnerable to livestock. Cattle love to rub their heads in any bare earth along the bank and sheep like to lie up in sheltered cavities along it, causing erosion in the process. Any gap, once formed, can also become a favoured shortcut and deepen to a trench.

What are they for?

Hedges were originally maintained to contain livestock, to provide shelter from wind and rain for stock and crops, and to provide wood and timber for use on the farm. If livestock containment was the priority, this usually meant that the hedge would be layed or plashed every 5 to 10 (or even 20) years, in order to reduce its overall bulk and increase its density.

Even in the days of plentiful skilled labour, only a small part of a farm's hedgerows could be laid in a winter season. The result was that at any one time there would be a succession of hedge forms at different stages of regrowth within a given area. If light wood for fuel, fencing or other uses was needed, some of the hedges might be managed by coppicing every 10-20 years. There might also be an element of heavier timber production from retained old trees.

What makes a good hedge?

The problem in answering this is that the myriad plants and animals that can live in hedges have innumerable different requirements and these often vary at different stages in the life-cycle of a species. A few examples of how wildlife uses hedges are as follows:

Hedges are used for breeding

Some species of bird such as willow warbler or dunnock, and many invertebrates, need good scrubby cover in the lowest metre or so of the hedge. Some require taller bushes (chiffchaff) or trees (woodpigeons, crows etc.). Others might nest nearby (e.g. tree pipits on the ground) but use taller parts of the hedge as song posts.

Hedges with both shrub and taller tree layers have been found to have twice the number of common nesting birds compared to those where either layer is missing. In another study, it was found that the number of bird pairs per 1000 yards of remnant or newly laid hedgerow averaged about 6; in a low, dense hedge produced by regular trimming there might be 9 pairs, whereas mature bushy and broad 'A-shaped' hedges averaged 33 pairs.

Many insects are restricted to one, or a few, species of plants and may need different plants at different stages in their life cycle. Some even need particular stages of plant growth (such as brown hairstreak butterflies, which tend to lay eggs only on 1-2 year old blackthorn growth).

Other animals require large old trees with cavities, cracks and dead wood. This includes hole-nesting birds, bats, and numerous insects ranging from some mosquitoes to longhorn beetles or small predatory wasps. Such wood can also host many fungi along with the specialist insects that feed on and breed in them.

Hedges provide food

Some species of invertebrate, small mammal and bird use hedgerow fruits, nuts and seeds as an important source of winter food. The amount of available food depends on how the hedge is managed. Hard mast (nut and seed) production is suppressed after cutting and doesn't increase significantly for some 5-6 years. Cutting, particularly frequent flail-mowing, also suppresses soft mast (fruit) production.

Hedges provide shelter

Although direct evidence is hard to come by, general observation tells us that many species need the physical shelter of hedgerows to move between other fragmented habitats. This includes some birds, small mammals and amphibia, as well as reptiles such as lizards and grass snakes (particularly if the hedge has an associated ditch). We know from live trapping of small mammals at Denmark Farm that individuals can move hundreds of metres along hedges, and it would be a foolish mouse that headed off across short open grassland to get to the other side.

Physical shelter is especially important in winter. We have found that small mammals can be virtually absent from hay fields in the winter, whilst the hedgerows hold significant numbers. Come summer, the meadows again have large numbers of breeding mammals, many of which will have moved out from the hedgerows, benefiting barn owls and other predators trying to feed their young.

In summer, particularly on breezy days, it can often be seen that insects such as butterflies and dragonflies will be active and feeding along the warmer, sheltered, sides of hedges and absent or grounded further out in the exposed fields.

Hedgerow Plants

In addition to woody plants, hedgerows can be home to a number of species that are largely absent from the fields. Where a hedge is fenced off, there is often an edge of large coarse grasses (such as cocksfoot or false oat-grass) with larger flowering plants (such as hogweed, nettles, meadowsweet etc.) merging into a scrubby growth of brambles, ferns or blackthorn etc. This gradual transition from pasture to hedge can benefit a range of wildlife.

In other cases, the hedge may be on a sunny but grazed earth bank. Such banks tend to warm up faster and be better drained than the surrounding land and often act as a haven for species that have been largely lost from the surrounding fields. Whilst of great value to many invertebrates, they can also be a refuge for less common plants of either grassland or woodland edge, depending on the hedge. For example, some microspecies of dandelion (there are over 200 in all) are only known from hedgebanks! This means that the hedgerow can provide a refuge for plant species as well as habitat for animals.

Managing for wildlife

These differing requirements of plants and animals mean that there can be no simple answer to 'what is the best sort of hedge to aim for?' Any hedge is growing and changing and will be of value in different ways to different species at different stages. However, a few simple management principles, tempered by practical farming needs, can greatly increase the value of hedges to both the farm and the wildlife. The following are some broad guidelines, bearing in mind that a variety of approaches across a site will help to provide a range of conditions for wildlife.

• If a hedge has been degraded by livestock, consider just fencing it off (at least 2 metres out from the centre-line on both sides) to allow the hedge base to broaden and scrub to develop. Then leave it alone until the regrowth is sufficient for other management.
• If you have sunny exposed banks – particularly if stone faced – consider fencing some or all of these closer to the bank. This allows livestock to keep part of it cropped short, providing warm banks for insects and an open aspect for particular plants.
• If you have to plant to fill gaps (or establish a new hedge), only use hedging plants derived from the local area (of 'local provenance') where possible and try to plant several different species in small blocks of each.
• When a hedge is unacceptably big, cut it down by laying and/or coppicing to start the cycle anew. Try to leave some hedges to grow for ten years or more to encourage wildlife that prefers the older stages.
• If the hedge must be trimmed, only do it every few years. Flail mowing might be a practical necessity (and is not necessarily the evil it is often portrayed as) but not every year! If you have to trim a long stretch, try doing only one section or side in any one year and try not to mow any hedge again for at least three years.
• Cut or trim between October and the end of February – allowing time for seeds and berries to be taken by wildlife. Winter cutting also avoids disturbing breeding animals and seeding plants.
• Aim for a broad base narrowing upwards (approximating to an 'A-shape') and allow some larger trees to grow up at intervals.

Finally, if you want to improve your hedgerows, try thinking of the countryside less as fields surrounded by hedges, and more as hedges surrounded by fields!