The Field

Case Study: Extracting native polycultures for bio-retention structures at The Botanical Research Institute of Texas

Reconceptualizing a Plant Palette Using Native Polycultures

Part 7 of this series focused on small steps that can be taken by any planting designer that will gradually move their designs in the direction of aesthetically qualified native urban polycultures. This post begins the discussion of a more complex and rigorous approach that I used in North Texas. The complexity of the Dallas/Fort Worth/Arlington area of North Texas is confounding when considering the use of extracted native polycultures as design components. It is a sprawling and rapidly growing metropolitan area of more than seven million people that is larger than the state of Massachusetts.

The problems and opportunities associated with reconceptualizing nature in this non-temperate area clarify an understanding of the issues in other areas where integrating nature may not be quite as complex and problematic. A detailed discussion is presented below that illustrates a research methodology used to develop 10 contrasting native polycultures for ecological retention structures on the campus of The Botanical Research Institute of Texas (BRIT) in Fort Worth, Texas.

Using Research to Define Aesthetically Qualified Native Urban Polycultures in North Texas

In North Texas, as in many other areas of the United States, the information needed to extract a wide range of native polycultures is simply not available. Academics and research institutes have a unique role to play in developing this information as the following description demonstrates. This research is directed at a palette of plants for ecological retention structures (large scale rain gardens), but can also serve as a model that can be adapted for the plant palettes required for many other types of planting design in metropolitan conditions in the Great Plains of the United States and other biomes throughout the world.

The palettes of plants that are the product of the research were defined by myself with the help of several Graduate Research Assistants at The University of Texas at Arlington, notably Kerry Gray-Harrison, Student ASLA. The impetus for the research was a landscape project at BRIT where some of them will be tested in large ecological detention islands between parking bays in the 242 car parking lot at the BRIT headquarters in Fort Worth. BRIT and the Fort Worth Botanic Garden share the parking lot and were struggling to find a plant palette that was more ecologically constituted than the typical ornamental native and adapted palette, featured at the Fort Worth Arboretum, but with more aesthetic appeal for botanic garden visitors than the existing native plant palette on the site.

The research began with a few important assumptions. The first was that for aesthetic (regional character), environmental, and ecological reasons, only plants that are indigenous to North Texas will be used. The second assumption was that annuals will be avoided in order to make the plant combinations more resilient within an urban context as was previously explained in part 5 of this series. The third criterion for the first round of plant selections was that the plants need to be adaptable to both low water use and to regular inundation. Obligate upland and wetland plants were eliminated unless there was very persuasive personal or other evidence that the plants are appropriate and adaptable. The final, and most important, assumption was that aesthetically qualified combinations of plants will be extracted that are designed to grow together in compatible but sometimes unpredictable ways, rather than arranging them as discrete monocultures and maintaining the arrangement over time as the planting matures. The polyculture plant palettes started with an overall carpeting matrix layer. Larger indigenous, aesthetically compatible, and less aggressive plants were added for additional interest as accents and “scatter plants,” as explained in part 7 of this series. The polyculture groupings produced have a range of visual character from relatively subtle, low, and controlled to tall, dramatic, and exuberant.

The ecological detention plant research began with a list of 734 taxa, identified over a 30-year period by scientists at BRIT, growing around the more than 1,200 ponds at the Caddo-LBJ National Grasslands, just northwest of Fort Worth, Texas. I am very fortunate and grateful to have received access, with the help of Robert O’Kennon, to the database of BRIT for this purpose. The Caddo-LBJ National Grasslands comprises 20,250 acres and is the largest publicly accessible undeveloped open space near the Dallas/Fort Worth/Arlington area.

The plants were first qualified for native status using BONAP (explained in post 4), the Illustrated Flora of North Central, the plant database at The Ladybird Johnson Wildflower Center, and other reputable and rigorous sources. Only plants native to North Texas that are reliably perennial were selected. It should be noted that availability was rarely considered and it was assumed that the botanists at BRIT could find wild populations of hard to find plants and the Fort Worth Botanic Garden could help with propagation. Wetland status was then determined using the previous sources as well as the Army Corps of Engineers National Wetland Plant List. Since we were looking for plants for ecological detention structures (rain gardens), the plants that were especially interesting were facultative and facultative wetland. These are plants that tend to grow in Palustrine aquatic system—non-tidal wetlands, seeps, springs, vernal pools, seasonal wetlands, and other low lying areas such as the depressions in natural gilgai prairie structures shown in figure 3. Finally, duration (annual or perennial) was determined for North Texas using primarily The Wildflower Center database and the Flora of North Central Texas. Any plants that are not reliably perennial were eliminated in order to make the palette more reliably persistent.

It was a very high priority for the native plants selected to have an aesthetic presence that can be subtle, but needs to make a significant aesthetic contribution to the new plant groupings. This is where the judgement of an experienced planting designer, preferably a trained landscape architect, is key. We had to be able to assess the aesthetic attributes and document them so that they can be paired and contrasted with other plants to form an aesthetically qualified native polyculture—as was done with the turf polyculture in post 6 of this series. We needed to see good photographs of a species in various stages of development and colonization in order to get a good idea of its potential for the polycultures. The final consideration is critical to the ultimate success of the polycultures after installation. There must be enough horticultural information available to make meaningful decisions about the potential for success in the artificial environment of an ecological retention structure. Many of the plants found by botanists and volunteers at BRIT growing without human intervention have never been produced for sale and horticultural information on them is too sparse to be useful for this effort.

The 109 native plants selected from the original list of 734 based on the criteria above were then carefully analyzed horticulturally and aesthetically by using about 55 database fields. This enabled them to be useful for selecting the plant palette for the new polycultures (figure 5). At this stage, many books on native Texas plants were used as well as web-based resources that included university, state, and federal databases, botanical gardens and arboreta websites, and even plant social media sites. These fields will need to be adjusted for other regions. For example, we did not look at salt tolerance since we are not near a salty body of water and we rarely have ice and snow events in North Texas that result in the streets being salted.

The process of studying the large database of taxa from the Caddo-LBJ National Grasslands was very interesting and revealed compelling plant species that were new to me, even though I have used native plants for planting design since the early 1990s and have been teaching native plants in the UT-Arlington MLA program since 2004. It was a great way to dig into some exciting possibilities that are beyond what is currently offered in the native plant industry.

Next month’s post will outline the remainder of the process of how the 109 plants selected were grouped into a range of polycultures. It will also present in detail several examples of contrasting polycultures that were assembled from the list and show an example of one of the polycultures that was recently planted.

Missed parts 1-7 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
Part 7: Beginning the Transition to Native 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.