Expanding existing roadways within large urban areas frequently leads to the building of large highway interchanges. Since most are located within expanding urban areas, their aesthetic appearance almost always requires landscape development.
In Texas, these landscapes can pose significant long-term maintenance costs for the Texas Department of Transportation (TxDOT). In these instances, the ideal landscape planting would be one that (1) requires little or no maintenance, (2) is visually acceptable to the public and (3) contributes to improved environmental quality in the area. A technique that is proving that these needs can all be met is being adapted to more and more interchanges within Texas.
It takes a plant community
The technique incorporates the concept of “native plant communities.” This approach is a step beyond the simple use of individual native plant species within the roadside in that it seeks to replicate the natural processes that are commonly found outside the right-of-way. These processes are the results of the structure and interactions of large groups of plants growing as a unit or community.
The structure of the plant groups found in nature creates a mutual dependency. Clusters of plants provide protection from weather and climate extremes for the group as a whole. Larger plants provide protection from winds and sun for smaller, more tender plants. Smaller plants create cover close to the ground and provide important habitat for the small mammals, amphibians, insects, bacteria, fungi and microbes that will recycle the leaves, limbs and roots back into the soil matrix.
The real survival mechanism of most native plants is not their hardiness as individuals but their identity as part of a community of plants.
Surveys conducted by the Texas Transportation Institute (TTI) have shown that the public considers natural landscape to be attractive and relaxing as well as more desirable for its environmental qualities, such as habitat for wildlife. TxDOT is commonly urged to save existing plants, particularly trees, in new roadway construction. While there is a strong preference for the look of a natural landscape with its unconstrained and chaotic arrangement of plants, such areas may be mistaken for poor maintenance or neglect. This mistaken perception has been addressed by clearly defining the boundaries between areas of regular maintenance and natural areas. An easy and effective technique is to maintain a closely mowed 15-ft zone behind the curb or edge of the travel lane.
Water quality is the most pressing environmental issue that TxDOT and most other DOTs must deal with today. Heavy metals and automobile fluids deposited on the pavement eventually enter the roadside and then on to a stream, river or other body of water. Large interchanges can be significant watersheds in themselves (over a hundred acres in some cases) but they often have water flow from off-site properties entering and exiting them. Interchanges could function as a sink for these pollutants, trapping and holding them while they degrade over time. Vegetation has been shown to be very effective at trapping chemical pollutants as well as silt. Organic compounds found in decaying plant material act as magnets, chemically bonding with and trapping pollutants such as phosphates and nitrates as well as heavy metals.
Putting plants in the ground
The plant community approach to large-scale landscape development was first tried at the large interchange of Loop 1 (north-south) and U.S. Highway 183 (east-west) in Austin, Texas. This TxDOT-designed project was competed in 1990 and covers almost 100 acres (including pavement). The great majority of the site was cleared of what little vegetation existed, so the finished interchange was largely barren turf with the exception of a tree group in the southwest quadrant.
A major drainage enters the northwest quadrant of the interchange, travels under Loop 1 and exits the northeast quadrant under U.S. Highway 183. The area surrounding the interchange was developed on three sides including commercial and office buildings with a shopping center planned for the northwest corner.
Landscape architects from the TTI were asked to assist the TxDOT Austin District landscape architect in developing a landscape planting program for the site.
A traditional approach to this type of planting would have been a park-like tree planting. Other factors suggested looking for a different approach. Because of the size of the interchange, maintaining a park-like planting would be quite expensive for TxDOT. Maintenance engineers were concerned about trying to keep the highly visible and highly traveled site in an acceptable visual condition within an urban context. Edging around bridge columns and trees in addition to frequent mowing to keep the site in a neat condition would significantly increase maintenance costs. The design team felt this problem could be lessened if large areas could be removed from regular maintenance (or received less than normal mowing).
The site is at the eastern edge of what is known as the Texas Hill Country. Here the vegetation changes from landscapes of taller, thicker woodlands to the east to shorter escarpment live oaks, ashes and cedar elms. The rugged beauty of the regional landscape has helped make Austin a very environment-conscious city. Home to the Wildflower Research Center established by Lady Bird Johnson, the area contains many caves, clear-water springs, a tourist-attracting bat colony and some endangered bird species.
The runoff from and through the site forms the headwaters of a creek that empties into Town Lake, a key part of the visual character in the heart of the state capital. The runoff issue alone was felt to be sufficient justification for a landscape that looked more like prairie than park land.
The design focused on establishing large groups of native plants situated throughout the site to frame large areas of native grasses and to create the look of savanna-type grassland. The design was not intended to faithfully re-create a historically precise native ecosystem. Nor did it assume that these communities would reach a point where they stopped changing. It was expected that some species might not fare well and would disappear. Other plants would do very well, become prolific and thrive. Birds and wind would bring seed from other sites, and new species would establish themselves if the conditions met their needs. In other words, the plant community will “act natural.”
Let nature finish the work
The design team did not consider it necessary to understand the minute details of the complex natural interactions in order to put them to use. If the system could be started, it was felt, natural processes could finish the design over time. Some fundamental characteristics of a successful planting were considered critical:
- A diversity of plant types, species and sizes: Plants employed should be native to the regional conditions of soils and climate but adapted, non-native species that are not invasive or noxious may be used;
- Sufficient plant mass to create an interior zone that is shaded and protected from drying winds and extreme exposure: 40 ft was established as a desired minimum width of planted community; the wider the planting, the better; and
- A ground surface rich in organic matter and micro-organisms that effectively conserves moisture and recycles plant nutrients: Thick, organic mulches would be used until the natural cycle has begun.
The Austin area receives about 30-35 in. of rain per year, but long droughts with high temperatures are common. A supplemental irrigation system was installed in the naturalized planting at the Loop 1 interchange. After establishment of the plant community, the irrigation system was intended to be used only if needed during periods of severe stress.
A large number of species were used to increase the likelihood of a large number of plants establishing on the site and to more closely mimic the natural diversity. The plant list included canopy trees (trees that would attain a height greater than 20-25 ft with a crown as large); understory trees; large, woody shrubs; small, woody and herbaceous shrubs; and perennial native grasses and seasonal plants (e.g., wildflowers).
The “ponds” on the site were actually flow-through areas that were not designed to impound any water. The bottom of the ponds was an irregular bed excavated in solid limestone with a thin cover of eroded silt. A porous rock dam was installed at the outflow of the lower pond to slow water flow and allow more time for sediment to settle in the pond. Native grasses were seeded to trap more particulate matter as well as slow the rate of flow.
Hands-off management
Management during the first year after installation included manual removal of noxious weed and tree species. Plants that died during the establishment period were not replaced and were removed only if highly noticeable. It was preferred to let them decay where they were. No steps were taken (such as mowing, edging or herbicides) to control any grassy or weedy vegetation within the plant groups, except as mentioned above. These invaders would disappear once a canopy cover was established. Mowing within the interchange would be limited to a 15-ft strip behind the curbs and in areas requiring clear sight for safety.
Because this design approach was very different from past practices, it was expected to meet with skepticism from engineers within TxDOT. To make sure that all possible reservations were addressed, bridge, design and maintenance engineers within the Austin district were included in the design team.
Special measures were taken to ensure public acceptance. This involved providing information to educate the public on the goals of the project. TxDOT’s public information officer was provided with details of the project to use for an article in the local newspaper describing the project during construction and to answer calls from citizens later on.
To ensure that the design team did not miss any important issue and to assure engineers and the public that the project was carefully considered, key public and private organizations were consulted for their review and recommendations. These included the City of Austin Parks Department, the Texas Parks and Wildlife Department, the Wildflower Research Center and the local representatives of the Texas Society for Ecological Restoration. The project was let for competitive bid in the fall of 1993 and installed in the spring of 1994. The total bid price was $250,000, which equated to $1.10/sq ft or $48,000/acre of planted area at the time. This cost included the irrigation system, grass and wildflower seeding and 15 months of supplemental maintenance by the contractor after the 90-day warranty period.
Some problems were encountered in the maintenance period. After installation, the contractor’s crew was found to be removing plants that were not on the original planting plan, a common practice in commercial landscape contracts. Also, the crew could not differentiate between weeds and desirable native grasses, plant species that might have invaded or desirables that come up from seed or suckers from planted species. The contractor’s concern for preventing losses due to inadequate water led to some crown-root rot in one important plant species. The 5 in. of composted tree trimmings preserved moisture so well that frequent watering was unnecessary.
Jim Shepperd is the district vegetation manager for the Austin District and has had direct responsibility for overseeing the site since its inception. Shepperd’s program includes regularly mowing within 15 ft of the curb as part of the normal six to eight cycles per year. Larger grassy areas are mowed only once or twice a year depending on rainfall. The flow-through ponds are not mowed. The upper pond has developed a large population of invading trees and cattail reed. Although not seen from any travel lane, the reeds catch large amounts of trash and litter, preventing it from reaching the creek system. The planted areas have received no maintenance or plant replacement, and yet have thrived and become densely vegetated. The irrigation system has not been used for the last six years. Shepperd reports that TxDOT views the project as a definite success and considers the concept valid for future development of other interchanges.
The effect of the site design and management on water quality is unknown, and there are no plans to attempt to assess this issue. It was felt that doing so would prove very costly and time consuming. Also, an abundance of research confirms the beneficial effects of thick vegetation and particulate entrapment to the point that the practice has become an established best-management practice. Since this project was installed, similar projects have been created in other TxDOT districts including Houston, Dallas, Waco and Fort Worth. Current work involves incorporating water harvesting techniques to make the approach viable for more arid regions of Texas as well.
TxDOT and many other DOTs have felt pressure to improve the aesthetic appearance of their roadways, and landscape is typically involved but maintaining ornamental planting projects can become prohibitively expensive. Plant communities allow TxDOT to avoid accumulating maintenance costs by abandoning the plantings once they are established. The natural system processes of a plant community enable the planting to conserve its own moisture, recycle its nutrients and improve water quality. This is the original sustainable landscape.
