Pruning shears and clippers
image by S.F. Heron
Trimming the top off a plant is an extreme measure to control the upward growth of the plant. Topping off involves shearing away the branches furthest away from the ground. This technique doesn't come highly recommended by gardeners as a method of controlling plant or shrub shape. Unless you're trimming a hedge, slicing the top of a plant off will disturb the plant's natural shape. All plants grow based on light availability and soil conditions, making the most by adapting to the given situation. Always trim the top of a plant without harming its natural growth pattern.
Evaluate the best time of year for each particular plant for trimming. Some flowering shrubs should be trimmed right after they finish blooming. Others thrive when trimmed during the coldest months of the winter. Check with your local nursery to determine the best time to trim each plant.
Plug in extension cords to ensure the cords don't interfere with your movement and that you cannot mistakenly cut into a cord while trimming. Wear protective goggles to keep flying plant debris from your eyes. If you're trimming an evergreen, protect your arms by wearing a long-sleeved shirt.
Evaluate the plant you want to trim. Trimming the top of a plant isn't recommended for anything other than a hedge. If you "top off" a flowering shrub, you'll encourage the plant to concentrate blooms on the top where the most recent cuts have been made. When you clip a plant back, it rejuvenates at the point of clipping. You'll be creating a bushy area on the plant that will make it look top heavy.
Trim the top of any plant other than a hedge with a process called thinning. Thinning involves choosing older, randomly spaced branches and clipping these branches back with a 45-degree cut at the point where the branch joins the parent trunk of the plant. This type of trimming encourages growth in the interior of the plant. The angle of the cut refers to 45-degrees from vertical. This cut allows for quick healing of the wound to prevent insect and disease infestation.
Avoid partial clipping of branches to shape the plant. If you must trim half a branch, be sure to clip back to a bud. Remember that your aim lies in maintaining the plant's natural shape.