by GLENN WITHEY and CHARLES PRICE
Choosing Structural Plants
Good borders begin with good foliage
EVERY BORDER SHOULD START WITH A STRONG BACKBONE of good foliage plants. Nothing better illustrates this need than those times of the year when the border isn’t in flower. That’s when you discover whether your overall effect or theme holds together or falls apart. Flowers can contribute moments of beauty and excitement, but in the long run they have to be considered minor players. Fortunately, there are numerous trees, shrubs, and perennials that can create good “bones.”
Rather than discuss specific plants in detail, we think it is more useful to focus on those qualities that make a plant a good candidate for a structural role in the border. That way, you’ll be able to identify useful plants no matter what your climate and conditions. For example, if you garden in the arid Southwest, you might choose a barrel cactus to achieve a certain effect, whereas a gardener in the Middle Atlantic might opt for boxwood, or a gardener in the upper Midwest might pick a tough conifer.
The more limited your space, the more important it becomes to select a tree with multiple seasons of interest, especially if the border is to be viewed throughout the year. Write down a list of desirable qualities—for example, good bark, fall color, and disease resistance. Then do some research before purchasing the plant. (See “Essential Reading,” page 60, for books that can help you in this task.) To get the real story behind how the plant performs in your area, talk to your fellow gardeners.
The tree’s ultimate size is another prime consideration. If, however, the effect you desire can only be achieved with a tree that will grow too large, you can either eventually cut the tree down and start over, or keep the tree and revamp the plantings underneath.
Try to find out how rooty and invasive the tree will become over the years. Some trees have thick, fibrous roots near the surface that resent disturbance—something that’s hard to avoid if they’re surrounded by herbaceous plants. Other trees have deeperquesting root systems, making them easier to garden underneath.
If a tree is already established in the area before the border goes in, ascertain whether it can tolerate soil contouring or mulching. Many native oaks highly resent even an inch of extra soil over their root zone, while most willows shrug off such disturbances.
Be careful with soil amendments. We once killed a Japanese maple by repeatedly spreading cow manure within the plant’s dripline, not realizing that fresh dairy manure, at least in the Pacific Northwest, can cause phytophthora to spread rapidly.
If the tree flowers during the border’s main season of interest, think about whether the fallen petals will make a mess. Large flowers or petals that contain a lot of moisture can be problematic if, for example, they fall onto heavily puckered or cupped hosta leaves. In this situation, a tough-fo-liaged fern or a leathery hosta that has a smooth leaf surface would make a better neighbor (unless you enjoy picking off moldering or self-gluing spent flowers).
With newer gardens getting ever smaller because of escalating property values and expanding building footprints, shrubs play an increasingly important structural role in the border. The main question to ask about a shrub you’re considering is whether it can tolerate competition from other plants. Whereas trees can rise above the competition for the most part, many shrubs can’t. And those that can often grow too tall and wide for the space they’ve been allotted.
One solution is to choose a shrub that’s been trained or grafted as a standard—that is, turned into what is essentially a miniature tree with a single trunk. It’s much easier to grow perennials next to a standard Hydrangea paniculata than it is when the plant is allowed to remain in shrub form. If this gives the border too fussy a look for you, you can try training the shrub yourself so that it has multiple stems and thus a more natural look. Just be careful not to crowd the shrub during its first couple of years, or it may not survive.
Clipped boxwoods or yews are often chosen to serve as the dominant anchors in many a border, and for good reason. First, they are fine-textured, and so are easily kept to a smoothly contoured shape. Second, they are tolerant of conditions ranging from sun to a fair degree of shade—a distinct advantage in the jumbled environment of the border. Except when they are crowded for too long a time by neighboring plants, yews and boxwood won’t drop foliage or branches (thus leaving gaps in their intended outlines). In very cold (or hot) parts of the country where yew and boxwood don’t thrive, the myriad forms of holly make good alternatives.
There are many other evergreen and deciduous shrubs that are somewhat tolerant of varying border conditions, but most have foliage or habits that don’t lend themselves to shearing into maintainable shapes. This needn’t be a problem, of course; it simply means that these plants are more effective when used informally. The rhododendron clan (including evergreen azaleas), camellias, various honeysuckles, leucothoes, pittosporum, pieris, aucubas, and a host of conifers are all likely candidates.
Certain shrubs seem to tolerate shading by their perennial neighbors better than others. Nothing is more discouraging than to discover a dead patch on an otherwise healthy plant at the end of the growing season. Here again, experience is the best teacher. In our own gardens, we often use various forms of phormium, boxwood, yew, rhododendron, and yucca as evergreen anchors. The first three tolerate crowding, while the latter two resent being crowded and will reward us with bare legs or spoiled foliage if neighbors get too close. If we’re concerned about a shrub getting too much shade, we will pull back the perennials from the shrub every two to three weeks to look for signs of decline. Pruning out an errant leaf (or two or three) from the offending perennial will give the shrub a bit of breathing room. If there is only minor damage, a friendly shove or staking of the perennial may allow in enough light to remedy the situation. In more drastic cases, the perennial might need to be removed altogether.
Yet another consideration is when (if at all) the shrub comes into flower. Early-blooming shrubs can become quite boring after they’ve “done their thing.” The same holds true for double-flowered plants. Snowball viburnum, for example, is often a sorry sight by August, when its fertile cousins are displaying brightly colored berries. In general, shrubs that flower later in the season tend to hold onto their foliage with more grace (an observation that applies to perennials, too).
One way to alleviate the late-season visual boredom of an early-blooming shrub is to train an appropriate vine, such as a clematis, over it to provide later color. Of course, you need to make sure that both plants will thrive under similar conditions of light, soil, and moisture, and that they are equally matched in vigor.
Colored foliage produces effects that last much longer than most floral displays. As a rule, bright colors advance visually while darker colors recede. For example, the bright yellow green of Cotinus ‘Golden Spirit’ will appear closer to the viewer than the somber purple foliage of Cotinus ‘Royal Cloak’. White or silver foliage creates a shimmery effect, almost as if one were painting the border with light. Since gold foliage can often burn in full sun, it pays to investigate how the plant will perform in your area before planting it. New cultivars of old favorites can be worth searching out, since they are often more sun tolerant. Remember that having only one plant with colored foliage will create a focal point, whereas the same or a similar color used repeatedly will create continuity.
In harsher climates, the colored stems of some deciduous shrubs (certain willows and dogwoods come to mind), the berries of certain viburnums and Ilex verticillata, and the brighly colored needles of conifers can fulfill the role that evergreen foliage plays in milder areas.
When selecting perennials that are meant to create a long season of interest, don’t rush off to the nursery on the first sunny spring day—you’re looking for structure, and many early bloomers have a long, unattractive downtime. If you’re wedded to a plant that has unattractive foliage later in the season, investigate how it might coexist with companion plants that come on later in the season and could help hide their ugly compatriot. In our own garden, for example, we have myriad primulas for early-spring color, but they have been interplanted among later-emerging perennials such as Cimicifuga ramosa ‘Hillside Black Beauty’ and Thalictrum delavayi, tall-growing plants that provide a scrim to hide the tattered foliage of the primulas without suffocating them.
Some perennials can be rejuvenated by an early shearing, which allows them to send out a new flush of growth and thus extend their structural role. Among this group are lady’s-mantle (Alchemilla mollis), columbines, brunneras, and pulmonarias. Most of the geranium clan that bloom before the end of June respond to this treatment as well. Don’t try it, however, with the superb mid- to lateseason geraniums like ‘Mavis Simpson,’ ‘Ann Folkard’, or ‘Rozanne’, or you’ll end the show prematurely.
Perennials lend themselves particularly well to another important structural role: edging. Many plants fall into this category; some, like the small to medium-size ajugas, are more suited to small borders, while others, such as the bold, leathery bergenias, are better suited to larger-scale plantings.
Whatever their size, edging plants need to have very good foliage and they mustn’t flop (which shouldn’t be confused with spreading gracefully). The effects that edging plants produce are as numerous as their foliar shapes. For example, the spiky or linear foliage of liriope, ophiopogon, acorus, and small ornamental grasses and sedges will produce a much different effect than the broad, rounded leaves of ajugas, hostas, bergenias, or asarums. Even among the grassy-leaved plants there are marked differences—some spread more evenly, like turf grass, while others will remain as distinct clumps no matter how tightly they’re massed. Thus you have the option of creating unbroken sweeps and swaths or discrete colonies of tussocks and bunches. H