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 key when you are trimming hedges.

John Sosnowski, horticulturist at the Morton Arboretum in Lisle, Illinois, recommends shearing the hedge in an inverted keystone shape, narrower on the top and wider at the bottom. The 6-inch slope is both attractive and healthy, as it allows all branches to get maximum sunlight. In this video, John demonstrates the procedure of laying out the stakes and string for trimming the boxwoods at the arboretum.

For more on this topic, see John’s article “Trimming a Hedge.”

Step 1: Get a template in place

Starting at one end, I measure out the desired width of the top. When I’m sure of the measurement, I mark it with stakes, which are easy to see and to move if adjustment becomes necessary. I prefer 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 and are a bit less dangerous.

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.

Step 2: Trim in small bites

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.

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.

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.