Here’s a brief primer on picking and planting a formal hedge, with tips for how to maintain the shape in the years after, and what to do to deal with problems like holes and uneven areas.
Choosing Plants for a Formal Hedge
A formal hedge is like an expensive television: it has to have high resolution, so the picture (the shape) looks shard, not fuzzy. In plants, the “resolution” is the leaf density and leaf size. A lot of small leaves packed together are what you need for a good formal hedge. This means the best plants, and the only good plants for a formal hedge have small, close leaves (short internodes), and grow slowly. Marie Iannotti has a good list of suggested plants for evergreen formal hedges.
Plant Very Closely
Your hedge plants are meant to grow together, to touch and interweave. They should grow into each other in the first year. Mark out a straight or curved line for the hedge to follow. Bends and corners are possible and will increase your yearly work somewhat. Plant in a single line along this, as closely as your budget allows. If they can be almost touching at planting time, that’s great. You can always remove in later years of excessive denseness. Another option: plant in a zig-zag along the line. This makes a thicker and better-filled hedge but takes more plants (and money). It’s a great option if you are starting with small plants.
Choose a Shape With Sloped Sides
Choose for your hedge to have a smooth geometric shape that is at least slightly wider at the bottom than the top, which lets light hit all surfaces directly (without this, your lower branches will get leggy or die). The simplest options are an elongated Tootsie Roll-like shape or a long wall with slightly sloping sides. These can have turns and bends in them, which will add to the complexity somewhat and create corners that you’ll need to be especially picky and careful about. Plan out the shape that your hedge will take; the next step we’ll shear down to that shape.
Shear Your Formal Hedge to Shape
You can let your plants simply grow with minimal pruning until they start to exceed your chosen size. Depending on your starting size and goal size, this could be right away. Do your first hedge shearing in spring. Our Landscaping Guide created a great picture step-by-step for the first shearing using posts and string for guides, but please modify his method so that the sides of the hedge are not directly vertical. They should slope slightly upwards. Always use some kind of artificial guide to tell you where your desired shape is. String or twine stretched tight between two posts is the easiest way—the string starts out buried in the plant, and you cut to it, along the hedge. A sheet of plywood with the shape cut into it can be moved along the hedge as a template guide, too.
Yearly Maintenance
Shear the edges, as above. Do this at least once a year, more often for fast growers like privet and yew. Healthy plants in full sun need lots of shearing! With hand pruners and loppers, cut out dead wood yearly. Cut out crowded or crossing branches with thinning cuts, back to a hidden location within the hedge. Try not to do major shearing in the hottest or coldest seasons, which would kill additional leaves.
Handling Problems
Remember what I said about making the top narrower than the bottom? It’s a good idea, but probably unnecessary if you have the hedge in full sun and are using a time-tested species like boxwood, privet, and others. It is possible to renovate a formal hedge that has become overgrown or full of holes but is otherwise still healthy. Got a hole? Cut the dead wood back to a living node and pray for regrowth. Exacting gardeners sometimes buy extra plants and plant them elsewhere in the yard, stashing them away as backup transplants in case part of a hedge dies.
References
Turnbull, Cass. Cass Turnbull’s Guide to Pruning, 2 ed., 2006.
