The Field

Assembling Polycultures from a Qualified Palette

Part 8 of this series detailed the rationale and methodology for extracting qualified native plant species for use in creating polycultures. This month’s post features a discussion of how to successfully combine the species into a low maintenance native polyculture that can take the place of a monoculture groundcover.

The 109 species selected for use in part 8 were sorted to find groupings unified by height, texture, line, color, or form. Two categories of plants were created for each of the main polycultures. The first is very aggressive groupings of lower plants that serve as the primary intermingled groundcover. The second group of plants for each polyculture are accent plants that are unified with the lower grouping by texture, line, color, or form, but also have a strong contrasting element that will show them to best advantage. These are either more transparent scatter plants or more opaque shrubby plants used more like rocks or small hill shapes.

This sorting of plants for the final polycultures is both a qualitative and quantitative exercise. The qualitative, or creative, part involves imagining the character of the overall polyculture based on the character of two or three of the core plants in the underlying matrix. Ideally, these are plants that you have some experience with that can serve as the “anchors” that will help assure the success of the polyculture.

When assembling the ten polycultures for The Botanical Research Institute of Texas (BRIT), I looked for a wide range of core plant characters by putting good pictures of the plants into a large folder for study. The polycultures where then built around the visual characteristics of those plants. This is where the quantitative data is very helpful. Once a decision is made on plant height, form, or texture, for example, the database can be used to suggest more species that will fit those criteria. The species can then be tested by using the database for horticultural compatibility, bloom season, and all the other variables described in part 8. Once a final palette emerges from this process for each polyculture, a mockup is made using Photoshop (see figure 1 above and figure 1 in part 8 of this series) to get as honest a vision as possible of what the polyculture might look like. If the results are positive, the Photoshop mockup can also be used to obtain buy-in from the client as these types of planting designs are very difficult for most people to visualize.

The edge condition is also carefully considered. Putting a more formal edge on a loose grouping of plants is a time tested way to make them more appealing and less threatening to people who are afraid of nature and may believe that snakes or rats are going to breed in the groundcover. When designing with a high priority on ecology, we need to always be aware of nature/culture alternatives just as we are aware of prospect/refuge and sun/shade alternatives. Some people enjoy an immersive experience in native plants. Others enjoy being next to them with a more controlled edge. And still others only want to see them pictorially from a distance. The successful perception of polycultures by a wide diversity of people in a metropolitan environment depends on addressing all three preferences.

All of the polycultures developed have a crisp edge that is a very low polyculture groundcover (usually with a stone or gravel band to separate it from the main polyculture), a low hedge, or a low wall. These edges allow more aesthetic freedom in the center plantings while still exhibiting design control and intentionality to a wide variety of tastes. One of the polycultures is even two mowable turf species that would be at home in any suburban setting.

Ten plant communities, extracted from the qualified plant list, were initially proposed for testing. Six edge communities that can help frame the larger groupings and transition to walkways and other paved surfaces were also proposed. Plant palettes for sun, part sun, and dappled shade conditions are included. The proposed combinations range from highly ornamental and floriferous to more carefully controlled and unified.

Summary of Research Methodology for Finding Aesthetically Qualified Native Urban Polycultures for Ecological Detention Structures

1. Obtain the most comprehensive list of plants available that grow in your area in regularly inundated conditions without human intervention: around ponds, lakes, streams, seeps, draws, and other watercourses and low-lying areas.
   • a. North Texas research used a comprehensive list of 734 species identified around ponds and streams in the LBJ National Grasslands by BRIT.
2. Qualify useful plants for planting design in ecological retention structures using the following criteria:
   • a. native (indigenous to EPA level 3 ecological zone),
   • b. reliably perennial and persistent (if known),
   • c. facultative as to water requirements,
   • d. aesthetic “presence,”
   • e. enough information available for an informed decision.
     • i. a range of photos available to get a good sense of the plant’s texture, line, form, color, and seasonal attributes,
     • ii. horticultural information from both trusted published sources and web based resources as they can be more up to date and comprehensive.
3. Assign key attributes to all plants selected
   • a. A variety of representative photos of each plant
     • i. closer for texture, color, and line,
     • ii. more distant for form
     • iii. overall visual effect and massing
     • iv. seasonal variation and dormant form
   • b. Database fields useful for searching when making plant grouping decisions (see part 8 for complete list of database fields)
4. Filter database fields to select unifying attribute(s) for selected plant polycultures
   • a. form,
   • b. texture,
   • c. line,
   • d. height,
   • e. color,
   • f. and seasonal interest.
   • g. Try to make sure that two or three species are well proven, reliable, persistent, and available. These plants will be the ‘anchors’ that will cover the practical requirements of the planting design as a ground cover.
5. Place photos of plants selected for each polyculture into separate folders for visual study as a group,
   • a. After final selections are made, use Photoshop to mock up the combinations and test the visual appeal.
6. Check key horticultural attributes of each polyculture’s plants to make sure there is some overlap and that they are compatible.
   • a. Light requirements
   • b. Soil tolerance/preference
   • c. Aggressiveness
   • d. Water use and drought tolerance
   • e. Other site specific requirements such as deer resistant plants.
7. Assign as many plants as possible to each polyculture that meet the aesthetic goals of the combinations. A further narrowing can be done later for site specific design and horticultural considerations—for example, selecting for deeper shade or wetter soil conditions.

The polyculture example that follows describes figure 1 above, intended for the campus of BRIT.

Knee high grass polyculture in full sun (KHGS)

This polyculture is intended for part to full sun and periodically inundated conditions as would be found in ecological retention structures in North Texas. The grasses are moderately to very aggressive and are in a mid-range from around 2 to 3 feet. The texture is fine to medium fine with an ascending to upright diagonal line. There is a base of carpeting grasses and several plants are added as accents. The look is relatively controlled since there is a very tight similarity of texture and color. Many of these plants are not found in the nursery industry and will need to be collected in the wild and propagated either in-situ or in the Botanic Garden greenhouses. All the plants listed are verified native to the Dallas-Fort Worth area and perennial.

Base carpeting grasses: (see spreadsheet in Part 8 for detailed information) mixed and placed 12 to 18 inches on center: 1-Paspalum distichum (Knotgrass), 2-Agrostis hyemalis (Ticklegrass), 3-Muhlenbergia reverchonii (Seep Muhly), 4-Panicum obtusum (Vine Mesquite Grass), 5-Pascopyrum smithii (Western Wheatgrass)

Accents: 6-Helenium autumnale (Fall Sneezeweed), 7-Hypericum hypericoides (St. Andrew’s Cross), 8-Lythrum alatum var. lanceolatum (Winged Lythrum), 9-Muhlenbergia lindheimeri (Lindheimer’s Muhly), 10-Zizia aurea (Golden Zizia), 11-Tridens strictus (Longspike Tridens), 12-Dodecatheon meadia (Shooting Star),

Trees/shrubs: 13-Amorpha fruticosa (False Indigo)

Edge polyculture: 14-Mimosa strigillosa (Sensitive Plant), 15-Phyla nodiflora (Texas Frogfruit)

By extracting plants from similar biomes (the LBJ Grassland Ponds), and by using them in relatively large combinations, plant communities were created that provide some, but certainly not all, of the environmental and ecological services that a community created by a restoration process provides. Biodiversity is further enhanced by using a range of polycultures on a given site that in aggregate will cover many of the ecological bases that are eliminated from most landscape designs. Designers are also able to address cultural rules and personal creativity by tailoring the communities to the practical and aesthetic goals set for each project. The expression of region and other attributes of planting design are under the designer’s control without losing the key element of local ecology.

Next month’s post will discuss some of the all-important maintenance issues that will determine the success of aesthetically qualified native polycultures.

Missed parts 1-8 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
Part 8: Case Study—Extracting native polycultures for bio-retention structures at The Botanical Research Institute of Texas

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