We have all watched a crew attack shrubbery with gas-powered hedgers, sculpting it into tidy shapes that resemble nothing Mother Nature ever intended. And while I really liked Edward Scissorhands, no botanist is going to tell you that is good for the plant.
What shearing does to a shrub
Power hedge trimmers damage a shrub in two ways.
Torn wounds. The blades are almost always dull, so they shred the leaves rather than cut them, and the ragged wounds let pests and disease straight in.
Poor plant structure. Shearing forces a dense shell of growth on the outside, which shades out the middle until the interior goes bare and woody. That interior does not come back, and it shows whenever the shrub gets trimmed.
Prune by hand instead
Use sharp, clean hand tools, and cut from the center of the plant. Take out selected branches rather than skimming the surface.
Over a year this takes less time than shearing. A shrub pruned for health is done once a year, not worked over on every visit, and it is not in a constant state of healing.
Remove the four D's any time, and shape once a year. Dead: brittle, with no green under the bark. Diseased: cankers, ooze, discolored wood; cut into healthy tissue and clean the blade. Damaged: broken, split or torn; prune back to a bud on a stem or another branch, or remove the branch entirely. Deranged: sparse, undersized, growing inward through the middle of the plant, or rubbing other branches.
When to use the shears
Shear only when all three are true. Otherwise the plant was chosen for its natural shape, and shearing takes that away.
It is planted in a row or a block, as a screen, a border or an edge. A single shrub standing in a bed is not a hedge.
It is already kept as a formal shape, with a solid face of growth on the outside.
It is a species that takes shearing well, the kind used for formal hedges and topiary: Ligustrum (privet), Pittosporum tobira (mock orange), Pittosporum tenuifolium (kohuhu), Myrtus communis (myrtle), Westringia fruticosa (coast rosemary), Syzygium (Eugenia) and Podocarpus (yew pine).
Shearing a hedge that is meant to be sheared
Taper the sides slightly so the base is wider than the top. A hedge with vertical or top-heavy sides shades out its own base and never recovers. And a shrub with a naked bottom is a sad, sad thing.
Shear lightly as needed in the growing season, using sharp hand shears, rather than hard once a year.
Soft perennials and gray-leaved subshrubs, like Nepeta (catmint), Teucrium (germander) and Artemisia, take a light shear after they bloom. Never into bare wood.
The full rule, with its illustrations, is in Southern California Landscape Maintenance: Ecological Maintenance, Simplified, at rootandbranchdesign.com/book, with Spanish text for crews. The first chapters are free to read at rootandbranchdesign.com/book/sample.
Put down the hedge trimmers and back away slowly.