Beyond the Border Part 3: How to Select Plants for Landscape Settings

This is the third post in a series I’m doing on perennials and grasses in the larger landscape. I’ve made the claim that perennials and grasses—possibly the most dynamic and interesting plants a designer can use—ought to be used more often in our built landscapes. Imagine our public landscapes, yards, and office parks cloaked in a rich tapestry of sustainable and beautiful perennials and grasses inspired by the patterns and palettes of our native vegetation.

My last post talked about a compositional strategy of massing plants in order to reduce their maintenance and increase their legibility. This post will focus on the second strategy: how to choose the right plants. Plant selection is absolutely critical to the long term success of a planting, and to be honest, it’s not easy. Here are some general strategies that can help.

What Plants Do I Choose?

Choosing the right plant for the right spot is hard enough. With every site, there is a dizzying list of cultural requirements (exposure, slope, soil, climate) that one must consider. In a larger landscape setting, there are additional design factors to consider.

1. Filler Plants to Create Volume

Aster oblongifolius creates volume against a drive.
Design by Ching-Fang Chen

Large perennial and grass beds are most attractive when they create volume against a void, such as a lawn, path, or street. Let me share a secret with you. Great perennial planting in landscape settings is not about perfectly balanced flower colors—though color matters. It’s not really about creating great photogenic combinations—though combinations add style to a composition. Half the battle in creating herbaceous plantings that endure is to simply cover the ground at a relatively uniform height. If you can find perennials and grasses that thickly carpet the ground and range in height from 12-42 inches, you're halfway there.

In his books and interviews, Piet Oudolf talks much about the distinction between structural and filler plants. This distinction is key to his compositions. A structural plant is one whose form is distinctive and architectural, whereas a filler plant has a more amorphous, cloud-like form. Consider the strongly structural flowers of an Echinacea or an Echinops. In a composition, the eye will fall onto these distinct forms. These plants also tend to dry into distinctive seed heads in the fall and winter, creating enduring interest through the year. Filler plants include most ornamental grasses (Switchgrass) and many mounding perennials (Asteromea mongolica). Oudolf recommends using a ratio that heavily emphasizes structural plants to filler plants, around 70/30, mostly to make sure the composition has strong form throughout the year.

My advice is to reverse that ratio. Of course, Oudolf’s work is undeniably beautiful and masterful. But I have two problems with using that many structural plants in landscape settings. First, structural plants tend not to cover the ground as well as filler plants. So when they’re not maintained well, it creates gaps where weeds can fill in. Second, since structural plants are more about a distinctive profile of a flower or leaf, they tend to have shorter bursts of interest. Yes, they look great in winter, but there are moments in early spring, for example, when they’re wiry or just not as full.

Beyond the Border Part 2: Massing Matters

The same rules that create impact and drama in art can be applied to perennial planting.

My last post set up my proposition that perennials and grasses—the most dynamic plants a gardener can use—ought to not only be used more often, but used in as a larger percentage of our built landscapes.  It’s time to liberate perennials from the confines of the British border and embrace a new aesthetic inspired by the patterns and palettes of our native vegetation. 

This post will ground my lofty rhetoric with some practical how-to advice.  How do you design for long term success with plant material that is inherently ephemeral?    To achieve lasting, low-maintenance perennial gardens, there are two areas one must master: composition and plant selection.  This post will focus on the first, and most important, rule of composition: massing.

First, let’s understand the context we’re discussing.  Perennials in a landscape setting (parks, civic landscapes, large residential) are inherently different than a flower border.  They are larger in area, typically set farther away from the viewer, and are not gardened as intensively.  So the rules of composition must address this context.

Massing Matters

More than any other strategy, massing perennials and grasses together is the golden rule for landscape perennials.  Why?  We group several of the same plants together in order to make them more legible and give them visual impact.  A single flower in a half-acre planting disappears; but a block of 100 (residential), 200 (small park), or 500 (large park) has dramatic impact even from a distance.  Massing perennials together draws attention to their ornamental characteristics.   It amplifies their color, form, and texture.   More importantly, it also helps relate the scale of the plantings to the scale of a house, building, or park.  A mass of 20 Echinaceas, for example, can look paltry next to a monumental building. Massing plants together gives the planting proper proportions to their context.

Beyond the Border: How to Use Perennials and Grasses in Landscape Settings

Of all the plants I frequently obsess about, herbaceous perennials and grasses are perhaps my favorite.  Of all the plant categories, they are the most ephemeral, dynamic plants.  One can mark the seasons with these plants; they are harbingers of change.   And as a designer, perennials and grasses are the most expressive plants within my palette.  Rich layers of bold perennial massings can express a site in powerful ways. 

The British border

When it comes to designing with perennials and grasses, however, we have a limited language for their use.  The perennial border—an intricately arranged, delicate frame of flowers—is really the only concept we have for their use.  And while borders can be beautiful, they have limitations.  They are generally high maintenance, fussy, and require a high degree of horticultural knowledge.  As a result, American gardeners and landscapers are often hesitant to use perennials and grasses because we associate them with British-styled borders.  But it does not have to be this way.

Let me propose an alternative.  Instead of limiting our landscapes to two distinctly British genres (the manor lawn and the perennial border), let us take the border and explode it out of its box.  Let’s blanket our landscapes in bold massings of perennials and grasses.  Let’s convert our wall-to-wall carpeting lawns into well-proportioned area rugs surrounded by perennials and grasses.  Let’s drape office parks and civic landscapes in vibrant tapestries of flowers, ferns, and sedges.