The diagram indicates the basic profile to which hedges can be trimmed. The choice depends on the various factors discussed below.
Shelter or windbreak requirements
The general effect of hedges on wind are discussed on pages 12-14. The A-shaped or topped A hedge makes the most effective windbreak because it allows wind to pass over it with less turbulence than an upright shape. In arable fields, a tall hedge of over about 6’ may be of less value than a lower hedge, because the resulting wind turbulence causes cereal to ‘lodge’ or lay flat.
Effect on long-term growth
Hedgers express conflicting opinions based on their experiences with local conditions. Some claim the rectangular hedge regrows best because a large top area is exposed to sunlight. Others claim the opposite, that the tight, dense top inhibits growth from the centre. The flat top also increases the amount of shading to the side away from the sun, which lessens growth. The flat top causes trimmings to get lodged in the top, which eventually drop through into the hedge bottom. Again there is conflicting opinion, as some claim this makes a useful mulch which feeds the hedge, whereas others see it as a source of fungal infection which can cause the stems to rot.
The A-shaped hedge exposes both surfaces to maximum sunlight, while allowing trimmings and snowfall to slip off. Regrowth is best if the more shaded side is trimmed to a more gradual batter, allowing low branches to get as much light as possible. Rectangular hedges which show signs of gapping on the shady side can sometimes be brought back to good condition by trimming to an A-shape. Against the A-shape is the argument that trimming ‘nibbles the shoots’ on the near or ditch side of the Midlands type hedge, due to its asymmetrical cross-section. A-shaped hedges tend to be the choice in areas which have heavy snowfall, and are relatively uncommon in the south.
Chamfered, topped A and rounded designs try in different ways to combat the problems of the two more common styles. The chamfered shape increases the height at which shoots are cut on the Midlands type hedge, while preserving some of the light-catching and snow-slipping qualities of the A-shape. The rounded form provides the greatest strength against snow since it is streamlined, yet is not as vulnerably thin at the top as the A-shaped hedge.
Ease of trimming
The cost and speed of trimming is determined by the number of passes which the flail needs to make. Cutting heads of 3’ (910mm) are common, with 4’ (1.2m) heads also available. Many hedges are trimmed to a 3’ height so that each side can be trimmed in one pass. In order to keep costs the same while improving the hedge, an alternative system is to allow the hedge to grow taller, but only trim it every other year instead of annually, using two passes on each side. The A-shaped hedge has the advantage over a tall rectangular hedge that no top cut is needed. Chamfered hedges require a minimum of five passes, and rounded hedges are only suitable for trimming by hand.
Effects on wildlife
Taller and wider hedges provide much greater safety for nesting birds than do smaller hedges, with many species preferring to nest above 4-5’ height. It is easier to achieve height with an A-shaped hedge, so avoiding the problem of making a high top cut. However, the tops of thin A- shapes will not be attractive to nesting birds, and a tall rectangular hedge of 6’ or more, with a similar width, is preferable. Modern flails mounted on long arms can reach up to almost 20’, as well as reaching over a hedge to cut on the other side, so hedge size no longer needs to be dictated by the size of the machine. game birds differ in their requirements, tending to prefer shorter, narrower hedges with plenty of grassy cover at the base.
For many other types of wildlife, the frequency of trimming and its effect on flowering and fruiting is more significant than the shape of the hedge. The centre of a wide, dense hedge provides cover for small mammals, and may provide overwintering habitats for amphibians and reptiles given suitable breeding habitats nearby, but otherwise may be a rather dark and sterile environment. For invertebrates, the perennial grasses and herbs on the banks and margins of the hedge tend to be more valuable than the hedge shrubs themselves.
Stock control
As most hedges are also fenced as necessary against stock, the ability of the hedge to contain stock is no longer so important. Traditionally, hedges against sheep were trimmed at 3’ (.9m) to 3’6” (1.05m), with hedges against cattle trimmed to a height of 4’ (1.2m) to 4’6” (1.35m). The A-shape, because it tapers at the top, needs to be higher than rectangular hedges in order to be cattle proof, as cattle are likely to try and push through something they can see through.
