Tuesday, July 12, 2011

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?


  1. I just had an ah ha moment when you said to reverse the ratio and have more filler plants and less structural plants. Brilliant!

  2. It was a big aha moment for me, too. I had read Oudolf, but realized that the filler plants were the ones that held up better in the long run. By placing pockets of structural plants among larger filler masses, you get the best of both plant types. Thanks for stopping by!

  3. This is a message that needs to get the attention of more of the American public. I hope you'll be turning this series into a book. I'm wondering if you have any secrets to how to maintain that contrasting line between adjacent masses of filler plants. Vigilance and removal of the plants that grow together, or do you have some other suggestions? Perhaps that's in your next post.

  4. The Oudolf ratio and your ratio are both with merit. Sometimes, our succulent accent or structure plants can overwhelm, though sometimes not. In one way, the desert is fortunate in that the ground does not need to be covered over 1/3 or 1/2 with plants. Negative space here becomes our friend, as it allows the differeing masses to "read"!

  5. Most interesting, and makes sense. I think it might be a form of biomimicry, at least in the prairie/woods/savanna landscape I'm most familiar with: lots of non-flowering grasses interspersed with relatively less flowers--though mostly not masses as you define them. Still, flow punctuated with bursts of color from the set of flowers blooming at that part of the season.

    Have you talked about using plants with similar cultural requirements? If so I missed it.

    1. a brilliant blog entry, though i'm 2 years late in discovering! there is a great list of filler plants in piet oudolf's phenomenal new book "Planting: A New Perspective." also included is a glossary of recommended perennials & grasses, whose cultural requirements & other virtues - such as persistence - are explained and/or ranked. i cannot recommend this book enough!

  6. Great article. As I am still learning and becoming more familiar with perennials, any chance you could put together a larger list of filler and structural plants?

  7. This discussion is so relevant to a project I'm doing right now! Thanks for your very valuable perspective!

  8. Really like your articles on this topic. It is much more eye appealing to use masses as a ground cover than it is to use turf. A lot of planning must be done with this though to make sure the plants are compatable together and compatable for their environment. Really like the idea of the masses becoming established and being able to choke out unwanted weeds. In the long term it'll pay off.

  9. i am a first semester horticulture student. i have some likes and dislikes about covering the landscape with flowers or grasses. i think maybe a comparison of the area you are working with and the ground cover you are thinking about in a picture, to get a rough idea. this might save time and money. my name is charles andress.

  10. I'm late to this party, having just discovered this blog today, but just wanted to say this series of posts is beyond brilliant! I'm an architect with a decent sense of design, and an affinity for gardening, but no formal training. This series of posts is so articulate, and aligns with what I am intuiting are the pitfalls of my current garden (designed by someone else). I can now see more clearly what I need to do to right it. I can hardly wait for spring to start putting some of these ideas into motion.

    I second the suggestion to turn this series into a book.


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