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.
Larger massings of sedges and perennials in a biofiltration garden for an office park.  The lines between the species create a pleasing composition.  Design by Ed Hamm for Rhodeside & Harwell. Photo by Thomas Rainer.
The European ‘New Wave’ style made popular by designers like Piet Oudolf, Noel Kingsbury, Michael King, and other celebrated designers tends to interplant perennials in smaller clusters to create highly textured tapestries.  I am personally a huge fan of this look, particularly to see it executed on large public projects like the Highline and Lurie Garden in Chicago (even Oudolf masses plants together for effect).  These designers use repetition rather than massing to give their compositions visual coherence. However, in my experience, the main drawback of a mixed meadow approach is that it requires quite a bit of maintenance, particularly in the heat and humidity of the eastern U.S. 
Massing reduces maintenance.  By placing plants of the same species together, you group them by their cultural requirements.  Everything within that block needs the same care.  It also makes weeds much easier to identify.  In highly mixed plantings, identifying a weed from a young perennial or grass requires a trained gardener—a luxury not available in most landscape settings.  Massing also makes it easy to re-plant if a perennial dies or struggles.  Every project I’ve ever worked on required re-planting anywhere from 3-10% after the first year or two.  With large masses, it’s very clear what needs to be replanted. 
A nicely interplanted moment, but with five species in just a few square feet, the perennials are already competing for light and nutrients.  To keep this composition together would require quite a bit of  maintenance.
Massing plants of the same species together also reduces competition between perennials.  Any time you mix species, the plants compete for light, water, and nutrients.  All of my early experiments with interplanting perennials went poorly as one plant often “ate” another.  The composition grew together, the more aggressive plants eliminated their more demure counterparts, and the end result was a total mess.  On one project where I interplanted about ½ acre of perennials, I went back a year after it was installed and had to un-interplant the entire garden.  The garden had grown together in this awful mess.  Anything that was mixed together was pulled out and separated.  The correction worked.  I’ve visited it several times in year two, three, and four.  The masses are more readable, the maintenance staff can easily identify weed from desired plan, and the garden is more stable. 
Since then I’ve had a good bit more success with interplanting perennials, but it requires much more planning, horticultural knowledge, and care with plant selection than I initially understood.  I’ll discuss some simpler strategies for interplanting later.  Even though I interplant quite a bit now, it is still within the context of larger masses. 
Two images of a garden designed by Wolfgang Oehme that feature large
perennial masses appropriately scaled for the residential setting.
My early mentor, Wolfgang Oehme, of the design firm Oehme, van Sweden & Associates is a huge proponent of large-scale massing.  Wolfgang is one of the great plantsmen of the last century, and his success in using this richly layered style of large perennial masses validates this method.  Since I’ve left OvS, I’ve had an opportunity to experiment with refinements to this style, particularly a method of matrix planting.  It blends all of the advantages of large masses (maintenance, legibility) with a more visually dynamic field.  More on that later.Next post: What type of perennials and grasses do I choose?  Some tips on choosing worthy plants for landscape settings. 
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