Future Viable Plant Palettes for Metropolitan Areas, Part 3

Peter Walker, FASLA, stands in front of his redesign of the UT-Dallas Campus featuring exotic turf and tree species. image: David Hopman

Peter Walker, FASLA, stands in front of his redesign of the UT-Dallas Campus featuring exotic turf and tree species.
image: David Hopman

Part 3: The National Green Industry ‘Utility’ Plant Palette

The next step forward in moving towards a better balance of aesthetics, environment, and ecology has flourished since the latter part of the 20th century with the introduction of better adapted plants by the national horticulture industry. These are the ‘workhorses’ used by landscape architects to cover large areas of ground in landscape development and to provide the structure and spatial definition desired for landscape designs. They are hybridized species of turf, groundcovers, annuals, perennials, shrubs, and trees that are rarely indigenous to the areas where they are planted. The massive scale of the areas in the United States covered by these plants makes them the primary target for the aesthetically qualified native polycultures that are the subject of this series. Turfgrasses alone cover over 63,000 square miles—about the size of the State of Florida—and may be the largest irrigated crop in the United States. [1]

As in part 2 of this series on fine gardening, the priorities of the companies and the plant palettes they produce are revealed by examining the search functions on their websites. These websites show what the companies want their customers to look for and, significantly, what is missing from the thinking that is reflected in the plant palettes produced.

One of the largest companies that grows plants for distribution on a national scale is Monrovia. The database they have created for plant searches features many useful criteria such as size, shape, habit, hardiness, water and light needs, flower and foliage color, landscape use criteria and garden styles, and special features such as deer resistance and ‘North America Native Selection.’ However, using their search engine it is not possible to find plants that are native to a particular biome or region, any plant soil preferences, plant heat tolerance for non-temperate areas, the preferred biome of plants, any associated plants that grow as a community, or even such basic information as whether the plant is an upland plant or a lowland plant.

To test the Monrovia database, I searched for a very common horticultural condition in North Texas: a low water use groundcover for shade that is a ‘North American native selection’ hardy to Zone 8. Despite the thousands of species and varieties in their database, the search yielded no results. Changing the criteria to shrubs also produced no results. Taking away the ‘North America native’ requirement only produced two results for groundcovers. This is an unfortunate result from this large grower that currently has almost 5,000 acres in production and sells plants to nurseries throughout the United States, including most nurseries and big box retailers in North Texas where I live.

Figure 2: Ligustrum quihoui (Chinese Ligustrum) in Crystal Canyon Natural Area, Arlington, Texas, being removed by students from the University of Texas at Arlington image: David Hopman

Figure 2: Ligustrum quihoui (Chinese Ligustrum) in Crystal Canyon Natural Area, Arlington, Texas, being removed by students from the University of Texas at Arlington
image: David Hopman

Another serious problem with the mainstream corporate plant producers is the continued production and promotion of ecologically destructive and invasive exotic species. Some of the plants are capable of taking over a complete biome and supplanting entire categories of native species. The ability of these plants to thrive in undesigned areas varies greatly by region. However, there is little to no attempt to tailor regional plant palettes promoted to nurseries based on this important criteria. Figure 2 shows Waxy Leaf Privet (Ligustrum quihoui) infesting large areas of woodland in North Texas. Hundreds of volunteers spend thousands of hours each year removing this plant that was very popular with home gardens and is still available for sale.

The plant illustrates the fine line between a well-adapted exotic plant and a plant that will escape cultivation and wreak havoc with regional plant communities—a line that can change as a region gets wetter, dryer, warmer, or cooler with the increasing pace of climate change. Changing weather patterns can tip the balance in their favor and quickly destroy the complex plant and other ecological relationships between the indigenous flora and fauna that have taken millennia to establish. The encroaching Nandina shown in figure 3 is currently a minor threat in North Texas. However, warmer temperatures and continued promotion and sale of varieties with viable berries could add this species to Waxy Leaf Privet as a threat to native understory trees, shrubs, and groundcovers in North Texas.

Figure 3: Nandina domestica encroaching into Crystal Canyon Natural Area, Arlington, Texas image: David Hopman

Figure 3: Nandina domestica encroaching into Crystal Canyon Natural Area, Arlington, Texas
image: David Hopman

Figure 4 shows a February 18, 2014 message from the Dallas Arboretum Plant Trials promoting a Honeysuckle that is listed on the Invasives of Texas Database as a serious threat. I was sold this species by accident last year at a specialty nursery. I was looking for the North Texas Native Lonicera albiflora (White Bush Honeysuckle) but was mistakenly sold this plant. As soon as I saw and smelled the blooms, I realized the mistake and removed it. The vast majority of people would have bought the plant and not realized the error, or most likely not considered that it might be an issue. It is another example of the importance of changing the priorities of institutions that should be promoting local plants and ecology away from a narrow focus on aesthetics—a focus that is problematic when scaled up to millions of residents in a given area.

Figure 4: Dallas Arboretum message promoting exotic and invasive Winter Honeysuckle image: David Hopman

Figure 4: Dallas Arboretum message promoting exotic and invasive Winter Honeysuckle
image: David Hopman

A third major problem with the mainstream horticulture industry is the continued promotion of monocultures rather than more diverse groups of plants. A recent consequence in North Texas has manifested in the hybrid Knockout Rose. Knockout has become so popular in recent years that it is often the only Rose variety specified for a large landscape design. Additionally, the traditional gardening practice of mixing roses with companion plants such as Society Garlic, or Artemisia ‘Powis Castle’ has been abandoned. These plants have traditionally been added to help deter garden pests and to help prevent the spread of viral diseases such as the Rose Rosette explained in figure 5. [2]

Figure 5: Photograph of sign at Fort Worth Botanic Garden image: David Hopman

Figure 5: Photograph of sign at Fort Worth Botanic Garden
image: David Hopman

The large monoculture communities promoted by the national green industry are also less resilient than more diverse communities of plants to changes in rainfall patterns, swings in temperature, ice and snow storms, and all the other environmental and horticultural vagaries that challenge plants on a regular basis. A very clear example of this confronts every landscape designer who studies an installed planting design over a period of many years. After five or ten years, a significant percentage of the perennial species installed will have died out for various horticultural reasons (often overwatering) and because it is very hard for designers to find accurate information on the longevity of herbaceous perennials. It is not unusual for the entire landscape palette to be left with only three or four species—approaching monocultures and their problematic aspects. This phenomenon has also been documented in natural communities where severe drought can lead to species extinctions in prairies. [3] In both natural and man-made planting designs, a wider and more diverse plant palette will ensure that even if a significant number of species are lost, there will still be enough variety to maintain a diverse and resilient plant community.

The relative balance of the corporate plant palette developed and promoted since the mid-twentieth century is shown in figure 6. The diagram indicates that environmental factors are better in balance than with the fine gardening palette from part 2 of this series, due to the palette being somewhat better adapted. However, the continued reliance on large amounts of supplemental irrigation in dryer areas and the promotion of synthetic fertilizers and other chemicals keeps this palette from achieving an optimal environmental balance. The ecology portion remains the same as with a fine horticulture palette since native plants are frequently a very minor consideration, and native plant communities and their connection to the local ecosystem are not a feature of this plant palette.

Relative balance of national green industry plant palette image: David Hopman

Relative balance of national green industry plant palette
image: David Hopman

Next month’s post will begin the discussion of the native and adapted plant palette that has become very popular in recent years as a way of saving resources, especially water. Using native and adapted plants is an important step forward in moving towards a future viable plant palette for metropolitan areas. The post will show that despite the big improvements in environmental issues, ecological concerns are often still not a major feature of this plant palette.

References

[1] Bormann, F.H., Balmori, D., Geballe, G.T., 2001. Redesigning the American Lawn: A Search for Environmental Harmony. Yale University Press, New Haven.

[2] Druitt, Liz, 2004. The Organic Rose Garden. Dallas: Taylor Publishing Company.

[3] Tilman, D., El Haddi, A., 1992. Drought and biodiversity in grasslands. Oecologia 89,
257–264.

Missed parts 1 or 2 of this series? See below for the links to previous installments, and stay tuned for the next post in the series:

Future Viable Plant Palettes for Metropolitan Areas

Part 1: Aesthetics, Environment, and Ecology in the Creation of Plant Palettes
Part 2: Fine Gardening

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

4 Responses to “Future Viable Plant Palettes for Metropolitan Areas, Part 3”

  1. James Says:

    I find this series totally off the mark, even offensive, in its assumption of the role as arbiter of “fine gardens” in any context of discussion. To call Butchart Gardens a “fine garden” is, to many designers, a falsehood and a travesty. No doubt some important points are being made, but the whole basis of this series is called into question by the attitudes and assumptions of the writers.

  2. Bob Oberdorfer Says:

    Contrary to the previous comment, I find this series to be EXACTLY on target! As I inferred in a post to the Field a couple of weeks ago (Ruminations on the ASLA 2015 Annual Meeting, for the Ecology + Restoration PPN), there has long been far too much emphasis on initial appearances of our designs, and not nearly enough on the sustainability or ecological appropriateness of the resulting landscape. Thanks, David, for adding your voice to the small but growing chorus of practitioners who promote a more holistic approach to landscape architecture in any and all contexts; not just those assumed to be in the environmental restoration realm! Hopefully from your influence as an educator we will begin to see a new generation of designers who “get” the bigger picture.

    • Bejamin Vogt Says:

      Yes, I too am enjoying this series and look forward to seeing where it goes. I especially like this third installment that calls out the plant production industry; I’ve long lamented the inadequate information on plant tags, for example, which is too simplistic and probably not even geared toward my ecoregion. I long for the days when local nurseries are sources of relevant, local information, and refuse to sell plants-du-jour that might not work in 5-10 years for various reasons. I also enjoyed this idea of using more plants to ensure a designs integrity and longevity in the face of abuse from landscape managers and climate.

  3. Future Viable Plant Palettes for Metropolitan Areas, Part 4 | The Field Says:

    […] Future Viable Plant Palettes for Metropolitan Areas, Part 3 […]


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