The RSPB gives some good advice on hedge planting and preservation
HERE
If you are planning a wild garden, bear in mind there are two words there “garden” as well as “wild”. Those folk who wish to turn their garden into just a wild area may live to regret it. A wild garden is a garden and needs just as much care as any other garden style. Just leaving it is likely to lead the overgrowth of thuggish and unattractive plants such as nettles and docks- fine in small proportions but not what most people will want. And anyway completely unmanaged wild areas are rare.
The same applies to a hedge in a wild garden. Unless your garden is a field, the hedge will need management. Letting a hedge go completely results in overgrowth of unwanted plants. Ivy being a good example and Elder is another. Letting ivy grow unchecked in a garden hedge results in gaps in the hedge and the longer you leave it the worse it will get. By all means, if you want a natural looking hedge, allow some ivy to grow: it's good for the birds and insects. But get it down occasionally and rotate it. Ivy in a field hedge is presumably subject to natural checks and balances which stop it from becoming too dominant but this will not apply to a smaller garden hedge.
Growing a wild hedge whether or not associated with a wild garden is great for insects, birds and small mammals. Traditionally you would use mainly Hawthorn with the occasional blackthorn and a smattering of other wild plants. A good mix, if planting from scratch, would be 80% Hawthorn, 10 % Blackthorn and the rest made up of a few Holly, Spindle and Field Maple. Plus any other natural tree you fancy, particularly if something grows well locally. Best to avoid planting associated climbers until a new hedge is established and these may arrive naturally anyway. Keep a new hedge mulched and weeded for the first year or two, after that you can encourage weed growth at the base and some leaf litter build up- all good for the wildlife.
Once established prune it in winter allowing about 2 cms of growth a year, with the top slightly narrower than the bottom. Pruning to the same height every year is not good. It results in bare stems at the lower half of the hedge with each tree having a mushroom appearance. After several years of allowing it to grow 2cm you could pick out the occasional plant for some heavy pruning (coppicing) to encourage re growth at the base. This will keep a fine healthy looking hedge. If gaps do appear you could try coppicing on either side of the gap and maybe local re planting. Or, if you know what you are doing, try layering a branch (different from Laying). A really gappy hedge will need all out coppicing or laying depending on the age of the trees. Laying every 15 years or so keeps such a hedge in fine fettle. If you want to maintain a layed wild hedge it’s best first done at about 7 years after first planting. Don’t prune the top in the year before laying to allow the branches to be weaved in but you could prune the sides.