Keep Masses in proportion
Space plants to allow for mature growth. The author lays out a large bed of heaths and heathers in which she interspersed a few juniper and cypress plants.
Proportion is key to determining how many plants to mass in an area. For example, in a spot roughly 100 square feet on the far side of the pond, I planted clusters of four different plants—two varieties of Sedum, silvermound artemisia (Artemisia schmidtiana), and snow-in-summer (Cerastium tomentosum). The effect of these combined forms, textures, and colors is akin to the muted shadings of a tapestry.
I alternated these ground-cover masses with low-growing focal points: standards of Juniperus procumbens ‘Nana’ and weeping trees such as Norway spruce (Picea abies ‘Pendula’), katsura tree (Cercidyphyllum japonicum f. pendulum), and Eastern hemlock (Tsuga canadensis ‘Pendula’). They add height and definition to the planting area’s boundaries.
I’ve also found that smaller gardens, or smaller spaces in large gardens, can usually handle some massing of plants. In a walk-in bed behind the pond— about 10 feet wide by 35 feet long— I planted drifts of colorful perennials that draw the eye through the path. As a general rule, I used not fewer than five pots of the same perennial in a section, and in most cases I used 7 to 13. I made exceptions for big plants, such as Nepeta ‘Six Hills Giant’ and Crambe cordifolia. The number of plants I choose, and how I group them, is ultimately determined by eye. Whenever possible, I opt for a naturalistic, flowing style.