Hedge trimming is different from pruning individual specimen shrubs. Since hedges are usually intended to provide privacy, it is important that the shrubs be as full as possible from top to bottom. This requires a trimming strategy in which the sides of the hedge are pruned at an outward-flaring angle, so that the top of the hedgerow is narrower than the bottom. This shape ensures that the lower branches along the sides of the shrubs aren't shaded by the upper branches—so they receive enough sunlight to grow and fill in the body of the hedge.
To maintain straight lines while trimming a hedge, you will need erect guide lines as a reference point. This is best done with strings attached to wooden stakes driven into the ground at the ends of the hedge row. The strings outline the shape you're striving for, creating boundaries to follow as you trim the branches.
When to Trim Hedges
The proper time to trim a hedge varies, depending on its growth rate and the species of shrub. Generally speaking, a non-flowering hedge should be trimmed every 6 to 8 weeks during the growing season, and less frequently as the weather cools. For flowering shrubs, it's important to time the trimming so it doesn't interfere with their bloom cycle. Spring-flowering shrubs are best trimmed immediately after they bloom. Summer-flowering shrubs should be trimmed in the late winter or early in the spring before active new growth is underway.
This technique is appropriate for most mature hedge plants. Newly planted shrubs require a different strategy for the first year or two, until the shrubs reach a more mature stage.
Tools for Trimming Hedges
Trimming a mature hedge requires fairly simple tools. The biggest decision you will need to make is between hand-operated hedge shears, which look like oversized scissors with long handles, and a power or battery-operated hedge shears, which bears a slight resemblance to a mini chainsaw. The choice is entirely up to you. Some homeowners prefer the relaxing peace and quiet offered by manual shears, while other prefer to get the work done quickly with power tools.
Equipment / Tools
- Hand pruner
- Mallet or hammer
- Tape measure
- Stepladder (for tall hedges)
- Line level (optional)
- Carpenter's level
- Hedge trimmer
- Tall stakes or poles
Prepare for Layout Strings
Setting up guidelines for trimming can be more complicated than it sounds, since an untrimmed hedge is oversized, making it difficult to run guide strings without the unruly branches getting in the way. The best way to deal with this is by using hand pruners to cut away the offending branches so that the guide strings can assume a straight line from one end of the hedge to the other. Essentially, the goal is to cut channels through the shrubs so that guide strings can stretch out straight and taut and provide an accurate reference.
You can usually get by with four guide strings—one at the top and one at the bottom of each side of the hedge. On very tall hedges, though, a third interim string on each side might be helpful.
Set the First Pair of Stakes
Drive a stake into the ground at one end corner of the hedge, using a hammer or mallet, The stake location should mark the spot where you want the bottom end corner of the hedge to fall. The stake should be set in slightly from the reach of the longest branches. Angle the stake slightly so it "leans in" toward the top of the hedge. The amount of lean is based on expected sun exposure: On the side that will face direct sun, make the lean relatively slight; on the shady side, make the lean more pronounced.
Pound in a second stake at the opposite corner of the hedge on the same side, angling it as with the first stake. For precision, you can measure the distance from the trunk or main stem of the nearest shrub to make sure the distance is the same for both stakes.
Set the First String
Attach a long string from one stake to the other, near the lowest branches on the shrubs. Make sure the distance above the ground is the same from one side to the other. If the ground is level, the string should also be level; sloped ground will require a sloped string. You can use a line level to adjust the string for level, if necessary.
Set the Upper String
Repeat the process to set an upper guide string marking where you want the upper edge of the hedge to fall. To ensure uniformity from end to end, measure up from the lower string, and position the upper string at the same distance from the ground at each end. If the ground is level, you can be very precise by using a line level to check the orientation of the string and make sure it is perfectly horizontal. On sloped ground, it's best to measure from the ground to ensure uniformity from end to end.
Once the strings are installed, use a hand pruner to remove any branches that are deflecting the strings. The goal is to make sure the strings are straight and taut from end to end.
Set the Remaining Stakes and Strings
Repeat the process to set the other pair of stakes and strings on the other side of the hedge. Check once more to make sure all strings are straight and taut, and, if necessary, prune away any branches that are touching or deflecting the strings.
Hold a long carpenter's level across the hedge, just touching the top strings, and adjust the strings if necessary so that they are level side-to-side. This will ensure that you trim your hedge so the top face is flat and level regardless of the ground slope.
Trim the Top of the Hedge
Start by trimming the top face of the hedge to create a plane that is flush with the top guide strings. Hold the hedge trimmer as level as possible as you trim. Be very careful if you are pruning a tall hedge that requires the use of a stepladder. It is best to position the ladder "into" the hedge, so you are leaning forward into the hedge as you trim.
Trim the Sides and Ends of the Hedge
Trim the sides of the hedge to create planes that are as even as possible with the guide strings. If necessary, use hand pruners to cut through thicker branches. Don't worry too much if there are indentations and thin spots in the hedge. This is normal, and the trimming itself will stimulate new growth that will fill in these areas.
Finally, trim the ends of the hedge. Often, the ends are simply squared up to be vertical, but you can also angle them inward in the same fashion as was used on the sides of the hedge.
Feed and Water the Shrubs
Trimming is stressful for hedge shrubs, and they naturally will respond with new growth. To facilitate this, now is a good time to feed them according to the needs of the species. Generally, a mild feeding of compost or slow-release fertilizer is most appropriate. Water the shrubs thoroughly to help jump-start new growth and help them take up the fertilizer.
The new growth stimulated by trimming will quickly fill in any holes in your hedge. If you are insistent about the look of a formal hedge, you might want to repeat this trimming on a monthly basis, as appropriate for the plant species.
Trimming Newly Planted Hedge Shrubs
After planting a hedge with boxwood or another shrub species, pinch back all new growth during the first year, which will induce low branching. At this stage, it is not important to create perfectly flat sides by using stakes and string as a guide. In this first year, just "eyeball" the trimming pattern as you pinch back all new growth. The tool of choice here is a pair of hand pruners, or even your fingertips. As you cut back the new growth, it will stimulate overall plant growth and give a fuller shape to the shrubs.
Even at this stage, though, take some pains to begin trimming back the upper branches a bit shorter than the lower branches. This will allow sufficient sunlight to reach the bottom branches. You are taking the first step in training the shrubs into the classic hedge shape of an inverted vase or flat-topped pyramid.
This early style of pruning can be kept up for one to two years, depending on the shrub species. At the point where your hedgerow is full-bodied and beginning to assume the inverted vase shape, you can switch to a more precise trimming technique to create a classic hedge with a flat top and sides.