The Field

Beginning the Transition to Native Polycultures

Developing a Plant Palette that Balances Aesthetic Control, Environment, and Ecology

Developing a plant palette for metropolitan areas that moves beyond the native and adapted plant palette is a very challenging and necessarily a very long term proposition. The vast corporate, design, regulatory, and research infrastructure that has evolved to the current state of the art will change very slowly, as it has in the past. As with any innovation, it will first be seen as radical and even eccentric and there will be many stakeholders that will push back hard against the tide of change. There are a number of possible scenarios for moving forward towards a more resilient and ecologically and environmentally supportive landscape palette.

One likely scenario for the transition to a more balanced palette is an incremental approach that gradually introduces native species, varieties, and selections into the infrastructure of the green industry. This would be an evolution of the ‘native and adapted’ palette that has been  emerging since the 1980s, perhaps accelerated by climate change and the ‘new normal’ of warmer conditions with wide swings in rainfall patterns, coupled with increasing water needs from a rapidly growing population. This evolving palette will represent the same basic approach currently used by many designers for the selection of plants. Designers will search for aesthetically pleasing groupings, or drifts, of discrete monocultures that meet the practical, aesthetic, and financial criteria desired, albeit in a more environmentally and ecologically sustainable way.

Reconceptualizing a Plant Palette using Native Polycultures

Reconceptualizing nature in an urban context is no trivial matter. It is one thing to address the visual aesthetic forms of nature and quite another to bring ecological functioning into the design process, particularly in urban or urbanizing areas. This will be a disruptive break with traditional practice that will require the entire green industry to adapt if it is going to be scaled up and have a meaningful impact on regional ecological and environmental imperatives.

Working planting designers must first understand the aesthetics and methodologies for designing with complex intermingled planting designs before they can begin to consider potential candidate plants for the process. Fortunately, Piet Oudolf and Noel Kingsbury have written a seminal book that outlines the concepts and provides many inspirational examples of successful polycultures from throughout the world. Their 2013 book Planting: A New Perspective should be on any serious planting designers bookshelf. There you will learn the basic design vocabulary of designing with intermingled combinations. I like to think of polycultures as slow motion action painting (see figure 2) that utilize a palette of underlying matrix plants (think polyculture groundcovers), accent plants (like “boulders in a stream”), and the important emergent and transparent scatter plants that mover your eye through the design, help unify the polyculture and add structure, character and seasonal interest.

The principal difference between my approach and their approach is that they do not focus on regional ecology by placing a high priority on the use of native plants. Their methodology advocates the native and adapted plant palette as explained in part 4 of this series. Additionally, their highly evolved and artistic methodology may not be practical for many designers who do not have the knowledge or plant focus that they have. My approach celebrates and encourages this level of commitment but it is also very important to move these concepts into the everyday landscape of groundcovers and turf that are such a ubiquitous feature of the metropolitan landscapes of the United States.

Once a decision has been made that the polyculture approach is desirable and practical, the slow task begins of adapting a plant palette and a planting design process that moves in this direction. This does not need to be as daunting a task as it might first appear. Everyone interested in plants has seen many examples of plants encroaching into each other’s territory. The main leap here is to carefully consider if this is a good thing or a bad thing before automatically separating them back into discrete monocultures. If you begin with groundcovers and keep the rest of the planting design intact (particularly the structure planting), this will be much easier to implement.

Do the plants have some unifying and/or complimentary characteristics?

I look for plants that provide some sense of unity by looking for form, line, texture, height, and color. This can start very simple and evolve over time as more species are added. For example, just mixing Liriope Muscari with Hemerocallis Stella d’oro is a baby step in this direction. The two plants have similar height, texture, grass-like leaves, and blooms that are complementary in color and season. Adding Tulbaghia violacea as a scatter plant increases interest and species diversity within a very carefully controlled overall unity. I mention this combination because it is one that is easy to use, and is a safe small step in the direction of polycultures using non-native plants that are widely available.

As sensitivity is developed to successful combination of plants, more polycultures will reveal themselves both through research and through serendipity. In 1995 I planted two species of plants under some shade trees in my house in Dallas, Texas. Both plants are native to North Texas and very well adapted for the conditions of the site. The plants were Chasmanthium latifolium (Inland Sea Oats) and Tradescantia gigantea (Giant Spiderwort). The Sea Oats spread, as it always will, but I discovered that the two plants together were actually much more attractive than either by themselves. The Spiderwort forms a very irregular groundcover that is only really of interest in the spring and early summer. It can even die back to the ground in summer and early fall if not given enough water. The Sea Oats looks good for most of the year, especially in winter, but lacks presence and stature in the spring and is a little monotonous in large areas. Figure 3 shows the Sea Oats with the blooming Tradescantia in spring. The Sea Oats foliage hides the problematic foliage of the Tradescantia which, in turn, makes it appear that the Oats are blooming. This combination remained in place for over 15 years proving that it is very persistent—an important consideration for native polycultures in landscape development.

Discovering two or three reliable intermingled plant combinations begins the journey to a full-fledged polyculture. The Chasmanthium/Tradescantia combination above became the anchor for a polyculture recently planted at The University of Texas at Arlington, where I teach (see part 1 of this series). Since the two plants look good all year by themselves, the other plants used for an intermingled combination could be a little more experimental. It was then a matter of looking for seasonal interest—especially during the busy fall and spring semesters. Asclepias tuberosa with its leaves that mimic the Chasmanthium was added for summer color, winter interest, and to attract butterflies. Ruellia nudiflora and Dicliptera brachteata were also added for later summer and fall bloom when the Asclepias blooms only sporadically. Elymus Canadensis was placed as a tall scatter plants for late spring into early summer interest.

Finally, the larger woody mass of Ageratina havanensis (unified by texture) was placed into the matrix as a taller accent—like ‘boulders in a stream.’ The Ageratina blooms for three or four weeks in late fall with very fragrant white flowers and is a welcome book-end to the many spring blooming trees, and shrubs that appear throughout our campus. The entire polyculture is enclosed with a low hedge of Symphoricarpos orbiculatus (Coralberry) to provide seasonal interest, a more architectonic edge for the wild mass of vegetation, and to keep pedestrians, bicycles, and utility vehicles off of the planting bed in this high use area. All of the plants in the polyculture are native to North Texas. The seasonal table in figure 5 outlines the seasonal plant characteristics of the UT-Arlington polyculture garden.

Any native groundcover that has been used successfully can be the basis for beginning the development of an aesthetically qualified native polyculture for your area. Their character can range from low and delicate to very large and exuberant. I am currently testing 13 polycultures in the North Texas area at UT-Arlington, The Botanical Research Institute of Texas, and at my home in Arlington, Texas. The matrix “anchors” that were the starting point for these polycultures include Chasmanthium latifolium, Conoclinium coelestinum (pictured in figure 4 above blooming blue), native Carex sp., native Juncus sp., Marsilea macropoda, Bouteloua curtipendula, Phyla nudiflora, Scutellaria ovata, Dicliptera Bracteata, Symphoricarpus orbiculatus, Calyptocarpus vialis, and others. I encourage the readers of this post to develop native polycultures suitable for development conditions in your area and to report the results here and to other interested parties in your area.

The next post will feature a discussion of a more complex and detailed methodology that was used to design aesthetically qualified native polycultures for bioswales and ecological detention structures in North Texas. This detailed methodology can be replicated to assemble native polycultures for almost any development condition.

Missed parts 1-6 of this series? See below for the links to previous installments, and stay tuned for the next post:

Future Viable Plant Palettes for Metropolitan Areas

Part 1: Aesthetics, Environment, and Ecology in the Creation of Plant Palettes
Part 2: Fine Gardening
Part 3: The National Green Industry ‘Utility’ Plant Palette
Part 4: Contemporary Native and Adapted Plant Palette
Part 5: Case Study—Lessons from the Bush Presidential Center
Part 6: Native Plant Turf Polycultures

David Hopman, ASLA, PLA, is an Associate Professor of Landscape Architecture at The University of Texas at Arlington, a registered landscape architect, a research associate at The Botanical Research Institute of Texas (BRIT), and co-chair of the ASLA Planting Design PPN.