Q: I really like the arborvitae hedge between my neighbor’s driveway and my front lawn. The problem is that it is getting so big that my neighbor may lose a parking space. How can I prune it back from the driveway without ruining it? – Bill from Salem, Oregon
A: Like many other types of hedges that get overgrown, arborvitaes can be pruned back ‘aggressively’. Technically, they should not be ruined, but may be quite unsightly for a while. If pruned aggressively soon, they may have enough time to partially grow out of it before autumn. However, because it is already summer, it may be better to only shear them to control them for now, and then prune them aggressively at the end of next winter. The neighbor can help decide.
They should be pruned back about six inches or more past the edge of the pavement, so that the new growth and foliage will have room to fill in before it reaches the edge of the pavement. They should also be pruned back slightly more near the top than down low so that they seem to lean inward. This allows more sunlight to reach the lower stems, and is more important on any side of a hedge that faces north
Pruning the tops down will allow even more light in, and stimulate more growth where it is needed, lower down instead of on top. (Incidentally, many municipalities enforce height limits on hedges that obstruct views from driveways or at intersections.) As with pruning the sides, the top should be pruned down six inches or more below where the top should actually be.
As new growth develops, it should be confined. When it starts to extend past the edge of the pavement, or above the intended ‘ceiling’, it is time to get shorn back within bounds. It does not need to be shorn back as aggressively as it was for renovation, but should be shorn a few inches back within its boundaries so that it has room to grow before needing to be shorn again. Afterward, a hedge should be shorn back to the same dimensions every time it gets shorn.
Otherwise, it is too easy for a hedge to creep out of bounds a fraction of an inch at a time. The most common problem with shorn hedges is that, by getting shorn a little less every time, they eventually become obtrusively wide on top, and shade out their own lower growth. This happens partly because they grow more up high than they do down low, but also because the people shearing them allow it to happen.
Ideally, plants used for a hedge should naturally be proportionate to the particular application, so that they do not need to be shorn very aggressively, or may not need to be shorn at all. Arborvitaes, as well as hop bush, euonymus, myrtle and some types of pittosporum, are actually better as informal, unshorn hedges where space allows, but need to be shorn where narrow hedges are needed. Dwarf types of escallonia and Indian hawthorn are too low to function as screens, but make nice small hedges to define borders.
Certain shrubbery that might otherwise get too large can be practical if the refined appearance of a shorn hedge is preferred anyway. Photinia and privets that can naturally grow into small trees make classic, tall hedges. Although photinia is deprived of its abundant white bloom by shearing, it develops more of its showy red new foliage. Bottlebrush is one of the few blooming shrubs that can still bloom as a shorn hedge if not shorn too frequently. Boxwoods do not get too large, but are a classic small shorn hedge.
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