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.

Find a palette of perennials and grasses that quickly cover and carpet the ground, and then mass them in large groups. Think about perennials or grasses you’ve seen that are vigorous, thick coverers. Ornamental grasses fill this role beautifully, so for landscape settings, consider using them as a higher percentage of the total composition than you would in a border (around 30-50%). Densely-matting perennials such as Aster oblongifolius, Solidago ‘Fireworks’, Asteromea mongolica, Persicaria amplexicaulis, Pycnanthemum muticum, and others should be the basis of your palette. Your base palette should be perennials that have year-round volume, not necessarily year-round blooms.

If your composition uses mostly filler plants, won’t it be just an amorphous blob? The key to creating dynamic visual interest with filler plants is create contrast between massings.

2. Create Contrast between Massings

The second plant selection strategy is to create sharp contrasts between masses of plants. Oudolf’s distinction between structural and filler plants is his way of creating contrast in highly interplanted designs. However, if you use large masses of perennials, the lines between the masses themselves create contrast. It takes some of the effort out of the design process. Instead of trying to contrast every 10 or 12 plants, you have to contrast every 100 or 200.

Since filler plants tend to be more amorphous in form, it’s important to contrast texture or flower color from mass to mass. A fine textured Switchgrass, for example, should be contrasted in the adjacent mass with a coarser textured Persicaria or a shrub rose. Structural perennials should be used, but in smaller pockets that get repeated at the edges of the larger masses. See the diagram.  The 'S' indicate structural perennials. This strategy emphasizes the volume and ground-covering strength of filler perennials, while using structural perennials as accents that get repeated throughout.

3. Incorporate Low Woody Shrubs

Rhus aromatica 'Gro-Low' forms a dense mass

For long-term reliability in large, low-maintenance landscapes, you have one more resource available to you. The use of low shrubs (36” or less) can be a way to add texture, reliability, and interest to your landscape. I’m not talking about those awful carpeting junipers, crimson barberries, or sprawling euonymus that get scattered all over strip mall parking lots. Instead, use a palette of soft, loose, and flowering shrubs that accent perennials. Some of my favorite low shrubs include Caryopteris, shrub roses, Clethra, Aromatic Sumac, Spiraeas, and Itea. Low evergreen shrubs like boxwoods or laurels are can also anchor aspects of the design.

If you use low shrubs, the trick is to choose plants that are loose in form and compact in height. Newer cultivars expand our options to shrubs that once were large and sprawling.

Plant selection is never easy, and to be honest, requires a good bit of trial and error. But if you pay attention to perennials that are vigorous, compact, and have year-round volume, you’ll start to develop a palette and technique that can be applied almost anywhere.

Next post: Beyond Installation—What is required to make perennials and grasses last?
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