Informal or formal, shrubs that are planted close together to create a green barrier all need regular pruning. Depending on the species and the season, this can be as often as once a month for eugenia (Syzygium paniculatum) or every couple of months for pittosporum (most species), African boxwood (Myrsine africana), tea tree (Leptospermum species), myrtle (Myrtus communis), and Pyracantha selections. Some hedges require only seasonal treatment, such as boxwood (Buxus species), juniper (Juniperus cultivars), and cypress (Cupressus species). Whatever the plant material, start with sharp tools to give a clean line to the finished living wall.
With time, hedges can lose their original aspect through neglect or accident. Take a look at the hedge and recall what its original purpose was. March is a great month to give hedges a more severe treatment to bring them back in shape.
Even seasoned professionals find it difficult to hold that line: to achieve the perfection of the horizontal and the rigidity of the vertical. One of the best ways to establish that crisp line is to pound a few temporary stakes in the ground and string some twine between them, using a builder’s or string level (the latter are lightweight and inexpensive and just hook onto the string) to find that perfectly horizontal edge. The stakes must be sturdy enough and rigid enough to stretch the string tightly. Then it is just a matter of cutting to that level. Vertical stakes can be used in the same way.
If the hedge is planted on a slope, the choice becomes whether to make it a standard height from the ground—following the slope—or to correct the horizontal to totally level in spite of how high it is from the soil. This is a matter of design and a functional decision, as well. If the slope is severe, one would probably not keep the hedge level, making it take on unmanageable heights at the bottom of the hill. Sometimes a compromise is necessary to trick the eye into seeing level while actually conforming slightly to the contours of the land.
How much can you cut on an established woody hedge? Quite a bit, actually. In re-establishing an old hedge, it may be enough to peer beneath the current green shoots to find that place where everything began to grow wrong. Larger diameter twigs and branches will indicate where that was, with everything above or outside it noticeably thinner. If that is not the case, or a new level is necessary for other reasons (the hedge is creating too much shade for the new plantings nearby, for example), then it may be necessary to go by stages. Shear back by half or so and allow the inner wood to break out before finishing the reduction. Most of the above-mentioned species will pop right back from corrective surgery. Sunburn may be a temporary result for newly exposed foliage.
Such drastic treatment should be followed by a little TLC. Water carefully, and as soon as new shoots are evident, apply a nitrogen-rich fertilizer to feed the emerging foliage. Soon, the ungainly old wood will be once again clothed in verdant green and as shapely as can be.