Trimming a Hedge

The first major step in achieving a smooth hedge leads to the second. An accurate measurement makes the cutting of the hedge much easier. The longer blade on the trimmer helps, too, making it easier to connect the top measurement with the bottom one.

I am the only one who prunes the boxwood (Buxus microphylla ‘Koreana’) hedge at the Morton Arboretum, in Lisle, Illinois. The hedge is the only formal garden at the arboretum, and I enjoy the rigid, neat, and precise trimming it requires. Maybe it’s my background in art that allows me to appreciate its design purity. Or maybe it’s that my shearing system seems to work right every time.

You certainly don’t need a degree in art to want a nice, smooth hedge, and this system, which I’ve used in my nine years of pruning the boxwoods, is one anyone can use to achieve the same clean, formal look our hedges have.

The straight edge that is the hallmark of the formal hedge depends most on what you do before you cut. Accurate measurements and well-placed stakes are the key. I recommend shearing the hedge in an inverted keystone shape, narrower on the top and wider at the bottom. At the arboretum, we shear our boxwood hedge to 24 inches wide across the top and 36 inches wide at the bottom. This slight 6-inch slope is both attractive and healthy, as it allows all branches to get maximum sunlight. The shade that’s created beneath it is an added benefit, because it cuts down on weeding and watering.

It’s better to take off too little than too much. Boxwoods have only a few inches of green foliage to work with, so a gouge will expose the brown branches underneath.

The first step to trimming is getting the template in place. Starting at one end of the hedge, I measure the desired width of the top. Since the outer new leaves tend to grow more vigorously on the southern or sunlit side, I note the location of the shrub’s main trunks before finalizing this measurement, to keep the hedge centered. When I’m sure of the measurement, I mark it with stakes, which are easy to see and to move if adjustment becomes necessary. Metal stakes are preferable, but beware: Most metal stakes fit into a hedge trimmer’s teeth, so work carefully around them to avoid tool damage. Wood, stiff bamboo, or plastic will also work.

I use the inner stakes as guides for placing the outer stakes. I first divide the difference between the top and bottom widths in half. This amount is how far the stake marking the bottom should be from the stake marking the top. For instance, our hedges measure 24 inches across the top and 36 inches at the bottom, which is a difference of 12 inches. So the stakes marking the top and bottom widths of the hedge should be 6 inches apart.

I use the same method to measure and stake the opposite end of the hedge. With my eight stakes in place, I stretch the twine and tie it to the two sets of inner stakes at 24 inches high, the intended height of the hedge. I then tie twine between the two external sets of stakes right above ground level to mark the outer sides of the keystone. I drive the stakes deep into the ground, as the twine needs to be stretched taut to be perfectly straight. More stakes inserted in the middle makes it easier to keep the string stable and the cut even. I use a 3-inch line level strung on the twine to ensure that my cuts stay even.

It's better to take off too little than too much. Boxwoods only have a few inches of green foliage to work with, so a gouge will expose the brown branches underneath.

A few cuts make smooth sides and a flat top

More stakes help keep your string, and your cut, straight. Putting stakes and strings in place takes a bit of time but goes a long way to ensuring a smooth and even cut.

When it’s time to start trimming, I don’t try to do it all in one bite. I shave off a layer of side foliage, using the top twine line and the bottom twine line as a guide. It may make two or three passes to create a smooth surface. I use a light touch when hearing foliage because a year’s growth may only be 3 or 4 inches, giving a total of only 6 inches to work with before bare wood is exposed. But if a cut is made too deep, take heart: Dormant buds will replace the green in a year or two.

To cut a flat top, I use the lines formed by the twine strung between the stakes along the top of the hedge. I never measure from the ground up, since the ground can be uneven. Once I have my guide in place, I just hold the tool firmly, lock my arms at the desired height, and go for a walk. If the hedge top is flat and dense, I rest the tool on top and glide the cut along. I  take a break before the tool gets heavy to avoid a cut that droops. I also watch out for dips in the ground, which can cause dips in my cut. Before putting the finishing touches on the top cut, I evaluate my work from a distance, viewing it from where most see it: a walkway, the street, the entrance, or a window. Then I finish the top cut, keeping these points of reference in mind.

The last thing I do is finish the edges. The hard, sharp edges have less leaf material and, in my opinion, tend to look sparse and ragged. A 45-degree bevel cut rounds this shaggy corner, finishing the hedge with a tighter yet softer appearance. The beveled edge also reduces winter damage by allowing the hedge to shed ice and snow.

High spots in the hedge can be easily trimmed off, but a gouge can be difficult to fix. To repair a gouge, the whole hedge length can be cut deeper to match the lowest area, or the branch arrangement can be rewoven to fluff up and fill the gouged area. But the best practice is to err on the side of leaving too much on rather than taking too much off.

Time your pruning to match the shrub's needs

It would be great if you had to prune a hedge only once a year, but to maintain a clean look, the best practice is to cut often. Early in the growing season, cuts should be light, just to keep the hedge from looking shaggy. This light shearing can be done monthly, beginning in May or June, which is after the burst of vigorous spring growth. Only one accurately measured cut is required per year—in late summer, when new growth is complete. Cutting more often is necessary only if a tighter appearance is desired. Never prune boxwoods in winter, because it can cause freeze-back of foliage and additional stem loss at the cut stubs.

Certain other evergreen hedges can be pruned following these guidelines (see the list). Still others have their own particular pruning needs. For instance, eastern white pine (Pinus strobus) must have its candles cut in half in spring to remain a tight, multibranched bush. Arborvitae (Thuja spp.) needs its flattened branchlets removed to reduce its size. If just parts of the compressed fanlike foliage are sheared it will leave them looking unnatural and deformed. Be sure you know your hedge’s pruning needs before you start to cut.