Pruning Evergreen Hedges and Renovating Overgrown Hedges
Pruning Broad-leaved Evergreens
Small-foliaged broad-leaved evergreens, such as dwarf hollies, are usually given their main pruning in the early spring, followed by a grooming later in the season.
Large broad-leaved evergreens, such as camellia and holly, need very little pruning, except for removing awkward branches. Always cut it back to a small side branch that bears leaves, or to a group of buds.
Pruning Narrow-leaved Evergreens
As a group, narrow-leaved evergreens do not need or like as much pruning as deciduous shrubs and trees. A few tolerate frequent pruning or even shearing of their outer young branches. Others must be handled with discretion. Most do not produce new growth easily, or at all, if cut back to older, bare wood.
The best time to prune them is in the spring, just before they put out their new growth, or when the new growth is almost completed. You will be able to spot the new growth for it will be a much lighter and brighter green than the old foliage, and soft to your touch. Plants that grow rapidly can tolerate quite a bit of heading back. Slow-growers should have only slight touching-up.
A wise rule is never to shorten branches beyond the point where they carry green needles. Living leaves are a sign that there are active growth buds at their base available to develop into new shoots. If you prune below them, the branch will probably become a dead stub.
Winter pruning, in order to have evergreen branches to use for Christmas greenery, is all right if you have planned ahead and have specific branches in mind that would have to be removed anyway. It is not the best time from the point of view of the plant, since winter weather may delay the healing of the wound. And you certainly should not whack away at random with your thoughts on getting greenery rather than improving the plant.
Renovating an Existing Hedge
Many deciduous hedges can be renovated with dramatic success. If you have inherited an old hedge, or let the one you started become run-down and bare at its base, you don't have to pull it out of the ground. Within just a short time you can restore it to its former glory. The method is simple though drastic, but better not tell anyone what you are going to do, or they might try to talk you out of it!
Do this only with deciduous plants, evergreens will likely not survive this treatment. Use a heavy duty tool, perhaps a chain saw, and cut the plants back to about 6 inches above the ground. Strong new shoots will burst from the base of the plants. Treat them like new shrubs, and cut them back 4 inches every time they lengthen by 12 inches. Within 2 or 3 years you will have again have a hedge of which you can be proud.