American arborvitae (Thuja occidentalis), commonly called cedar, is a popular hedge plant throughout the temperate Northern Hemisphere, especially in North America. It’s one of those conifers with scale-like leaves. Unlike needled conifers, which put on all their growth for the year in just a few weeks in late spring, scaled conifers keep growing for much of the summer, until late August or even, in warmer climates, late September. So when should you prune them for the best effect?
(And you will need to prune annually: arborvitaes are trees, not shrubs, and will quickly outlive their usefulness as a hedge unless trimmed regularly.)
There are in fact three theories about when to prune and each has its value.
Technique 1: Twice is Nice
Many gardeners prune their arborvitae hedges twice a year, usually most heavily in early summer, clearing up any winter damage and cutting into the green growth (never cut into the brown, woody part of arborvitae: there are no dormant buds on old wood and that will leave an ugly gap that may never close!) to shape the hedge correctly. Then, since the hedge will grow back to a certain degree over the summer, they give a second, more precise trim in late August or early September.
If you’re an energetic gardener, or have a team of pruners working for you, this probably gives the best overall effect.
Technique 2: Prune in June
Being a laidback gardener, I’ve always found pruning any hedge once a year is enough effort, so I personally adopted the “early summer pruning” technique (that is, before I tore out the hedge).
In late May to mid-June, with new growth starts to appear (exactly when will depend on your local climate), prune to shape the hedge. This allows you to prune out any visible winter damage, for example, and to give the hedge the appropriate pyramidal top (you don’t want a flat top: that can result in excess snow accumulation and subsequent damage). Then, as new growth slowly appears over the summer, it softens the outline of the hedge, giving it a more natural look and nicely covering up the brown tips left by the shears.
Technique 3: Late is Great
Gaétan Paquet, an ecological arborist with 30 years’ experience pruning arborvitae hedges, recommends late summer pruning, from August 15th to 30th.
By pruning at this time, at the tail end of the hedge’s growing season, the hedge will retain a dense, very conical shape through the winter that will prevent the snow from clinging to the twigs and prevent any snow buildup, as even heavy snow will slip off easily.
Another reason to prune at the end of the season is to protect nesting birds. Many native birds nest in late May or June, choosing evergreens so they will be hidden from their predators, and June pruning could disturb them. They’ll have finished well before summer’s end, so there is no risk of alarming them when pruning at that time.
So, one type of hedge, three types of pruning: you choose!