There are three ways that hedges have been produced.
Hedge-planting is familiar and well documented; nearly all more ecent hedges have certainly been planted. .
Hedges also arise whenever a ditch, bank, lynchet, or earthen wall is neglected for a few years not too far from a source of tree seed; a hedge will result. Fences turn to hedges by birds sitting and dropping seeds; the fence protects the incipient hedge.
Hedges arise in a third way as the ghosts of woods that have been grubbed out leaving their edges as field boundaries. The marginal trees, often already forming a hedge to protect the wood's interior, may be left as a hedge having woodland, rather than hedgerow, characteristics.
There are three hypotheses to account for the observation that older hedges have more species:
1. A hedge acquires further species as it gets older. Tree and shrub seeds are constantly being brought by chance and birds. They germinate and occasionally get established.
2. In earlier times it was the custom to plant hedges with more species than later. Enclosure Act hedges were generally planted with one species only, usually hawthorn. As with other fashions, it is not easy to determine why, but the large scale and commercial character of the operation encouraged simplicity. Georgian enclosers usually planted hedgerow trees; after the felling of the original trees the regrowth of the stumps gives the hedge a second species. Victorian enclosers often omitted the trees. Pre-Georgian hedges were often planted with two or more species.
3  The older a hedge, the more likely it is to be natural rather than planted, and therefore to be mixed from the start.  Both kinds of natural hedge- ghosts of woodland boundaries and accidental hedges springing from the base of fences- are unlikely to be specifically documented; but conditions for them to arise have probably been much less rare in the past than they are now.