California is known as a place where “you can grow anything.” But of the many thousands of plant species and cultivars available, only a small percentage are appropriate to a Bay Area landscape design. How do we determine which lucky few will make it into our planting plan?

We wrote previously about how we first screen plant candidates based on fundamentals such as life cycle, cellular structure, and foliage persistence. If you haven’t already read that post, start there to understand what we mean by terms like “perennial,” “woody,” and “evergreen.”

Beyond those attributes, we consider numerous other physical factors to ascertain whether, and how, a plant will fit within our landscape plan: what is its growth habit? Its overall form? What color is its foliage, and its flowers, and in what seasons?

Despite the many different shapes plants can take, based on the way they grow we can condense them into four main categories: trees, shrubs, vines, and ground cover. Very simply, “trees” are defined as woody perennials that usually have a single main stem (trunk) arising from the ground; some species develop multiple trunks, but usually fewer than three.

“Shrubs” are also woody perennials, but have more than a few main stems. We think of shrubs as being shorter than trees, but height isn’t really part of the definition, and there are many tall shrubs and dwarf trees that would be exceptions to the rule anyhow.

A “vine” may be either woody or herbaceous, and is defined by its long stems that trail along the ground or cling to supporting plants or structures via tendrils, adhesive pads, thorns, aerial roots, or simply by twining around an object. Like trees and shrubs, vines may be either deciduous or evergreen.

“Ground cover” is the most difficult to define, as the word can refer to either the plant’s growth habit (low and spreading), size (usually less than two feet tall), or function (anything that covers the ground)—which would include shrubs and vines as well as grasses and other herbs. Our firm tends to define groundcover as low-growing herbaceous species that are not vines, leaving woody spreaders like Arctostaphylos uva-ursi classified as shrubs.

In this way, we categorize most of what are commonly called just “perennials”—such as Achillea, Echinacea, Rudbeckia, grasses, and ferns—as groundcover. However, we do make exceptions to include woody creepers like Mahonia repens that spread by underground roots; and our categorization would also include herbaceous perennials like Zingiber zerumbet that don’t “cover the ground” particularly efficiently. It’s an art, not a science.

We also look at the form a plant will ultimately take. This is even more subjective than growth habit, with words like “upright,” “narrow,” “spreading,” “rounded,” “conical,” and “vase-shaped” that have meaning only relative to each other. A mature redwood tree certainly spreads, but compared to a mature oak tree it is decidedly upright. A single-trunk birch tree may be considered narrow, but a multi-trunk birch tree may splay into a fan shape. We use form in our designs to create contrast, such as the narrow pencil shape of an Italian cypress amidst rounded Lavender shrubs.

Another means of creating either contrast or harmony is through color. While many of our clients show us idea books brimming with flowers, we consider not only the color of a plant’s blooms but also—because flowers come and go—the color of its foliage. We see not just “green,” but a wide spectrum ranging from bright chartreuse gold (e.g. Coleonema ‘Sunset Gold’) to pale silvery-blue (e.g. Festuca ‘Beyond Blue’), stopping at apple green, Kelly green, and forest green along the way. Burgundy, purple, and tan foliage can provide additional interest, as can leaves that are “variegated” with gold or white colorations.

Additionally, just as we consider when plants will flower so as to group them into pleasing combinations, we consider what colors their leaves will turn in autumn. A Ginkgo tree’s bright gold fall color is beautiful above the purple flowers that Salvia leucantha pumps out then. Even some evergreens, such as Nandina ‘Obsession’, turn shades of red and gold in autumn, contrasting beautifully with “glaucous” neighbors such as Blue Spruce (Picea pungens).

Even once we have considered a plant’s type (perennial, herbaceous or woody, evergreen or deciduous), growth habit (tree, shrub, vine, or groundcover), shape (narrow, spreading, rounded, upright), and colors (both flowers and foliage), we’re not quite ready to add it to our planting plan. One vitally important set of characteristics remains to be evaluated—the plant’s cultural needs—and we’ll discuss those in our next post.