Hedges managed by trimming from the time they are planted and which never have to suffer heavy top-cutting can stay healthy and vigorous for many years. The method of trimming to an A shape described on page 57 , encourages the development of a hedge with a wide base from which vigorous shoots can grow. However, many hedges become damaged by infrequent heavy trimming, by herbicide use or by browsing animals, and suffer a decline as described below.
- At an early stage in the life of hedge shrubs, shoots proliferate from just below the cut ends of the stems. Each shrub species responds slightly differently to trimming, but in general the effect is as shown in the diagram. The cut surfaces develop into knobs of scar tissue as the cuts heal over.
The stems continue to thicken each year, just as in freely-growing trees. - The first stage continues, leading among lateral- branching shrubs such as hawthorn to increasingly interlocked branches. The effect is similar to the natural stunting that occurs where hedges grow in harsh climatic conditions. During these first two stages the hedge improves as a stock barrier due to the interlocking branches and the dense outer growth.
- Eventually, the bottom and inner branches tend to die back as the shrub loses vigour, leading to the development of gaps at the base of the hedge, and a shell-like growth form.
This stage may occur after only twelve or so annual trims, or not for maybe fifty years or more of trimming. About twenty years is average before gaps become serious. growth is likely to remain vigorous for longer in fertile soils, and where damage from other agents does not occur. Even thorns, the most robust of the hedging shrubs, finally thin at the base and centre because the dense outer and upper growth shades out the rest. Once this stage is reached, the hedge should be left to grow on for several years until it is a suitable height for laying. - The hedge gradually loses vigour, signalled by slow regrowth after trimming if continued, and the beginning of die-back in weaker plants. The main stems continue to thicken, but side branches become fewer, with foliage increasingly confined to the top of the hedge. Shoots which have been cut repeatedly at about the same level are unable to put up new growth through the mass of dead stems and scar tissue, so twigs and smaller branches die. gaps may become extensive as weak plants die completely. Many hedges are allowed to get to this stage before being left to grow up for laying, by which time it is almost too late. Trimming should be stopped at the earlier stage, before the plants lose their ability to send up straight, strong shoots, and before individual shrubs die.
- The hedge by now is derelict and dying, with more gap than barrier. Surviving shrubs may spread into the gaps if grazing is excluded. The hedge is probably too far gone to be revived by laying, and renovation and replanting may be necessary (chapter 10).
