What does it mean when someone tells you to plant something “on center”?
Karen Finklestein, Aurora, CO
Contributing editor Barbara Ashmun responds: For gardeners, the term “on center” means arranging several plants to allow exactly the same distance from the center of one plant to the center of the next. This usually applies to a mass planting of one kind of perennial or ground cover to achieve a big sweep of greenery and flower color. Eventually the plants join, seamlessly flowing together, with no soil showing.
To accomplish this, first determine the ultimate spread of a plant. For example, the mature clump of a pink bishop’s hat (Epimedium X rubrum) spreads about a foot across, so placing the center of each plant one foot apart from each other when planting a row will create a unified effect.
Triangulating creates an even fuller picture. Imagine a second row of bishop’s hat, planted a foot behind the front row, with plants staggered so that triangles are formed between two adjacent front-row plants and one back-row plant, midway between them. The center of each plant is again placed one foot apart from its companions.
Any kind of plant can be placed on center and triangulated to achieve a uniform, organized look. For example, I planted three red ‘Flower Carpet’ roses 5 feet on center, knowing that they would spread at least that much, and triangulated them with two up front and one behind. Two years later, a mass of red roses blazes from early summer through frost.
You could do the same with a grove of trees. To weave them together, with canopies overlapping, space them so that each trunk is on center from its companions by a smaller distance than the mature spread of each canopy. If you want each tree to stand alone, let the distance from trunk to trunk be greater than the spread of each canopy.
When plants are young, it’s hard to picture how wide they’ll spread, so it’s especially valuable to do your homework ahead of time, especially with trees that may crowd each other and grow awkwardly. With more forgiving perennials, it’s a lot easier to remove extras in future years.