Landfill Revegetation: The Hidden Assets

Deborah Marton

Nearly one-half of the nation's 6,000 municipal landfills closed in the early 1990s and thousands more will close by the end of the decade. What are we going to do with all this land?

Since construction on these sites involves high costs and serious structural problems, the most common use will be parks and recreational areas. So far, problems with capping, leachate and methane have limited the number of successful revegetation projects. Technological advancements, however, have significantly addressed these problems, clearing the way for restoration.

On The Technological Front In the past, restoration has focused on selecting plant species believed to tolerate landfill gas (LFG). But successful growth is actually due to a plant's development of a shallow root system - not to any unique gas tolerance.

The most successful species completely avoids the highest concentration of gases by keeping roots less than 20 centimeters below the soil surface. To avoid unstable root systems, landfill gas should be controlled during the capping stage.

Since most landfill settling occurs during the first few years after closure, postponing final planting - especially of trees - will minimize problems. Initial plantings should be designed to rapidly form a root mat to prevent soil erosion, which exposes the cap and leaves it vulnerable to the freeze/thaw cycle and to the shrinking and swelling of clay cap material. Flaws resulting from weather exposure could be made worse by root intrusion or burrowing animals and may lead to leachate breakout.

Vegetative deaths caused by leachate generally are due to waterlogging and consequent soil anaerobism - not leachate toxicity. Similarly, LFG's harmful effects to vegetation stem from its displacement of soil oxygen, which produces anaerobic soil conditions on the surface.

Ongoing improvements in capping and venting are helping to eliminate upward diffusion and 'hot spots' of anaerobic conditions. Eventually, these technological improvements should dramatically minimize or even prevent potential damage to plants and trees from landfill gases.

Soil Characteristics Ideally, soil should have drainage, substrate and planting layers, all of which drain easily with good aeration, resisting compaction and settlement. Organic matter could be high (10 percent or more) to compensate for coarse texture and to enhance water and nutrient retention capacity.

The need for adequate drainage must be balanced against a more compact soil's capacity to help stabilize woody plant roots. For example, if a major concern is adequate drainage after storms, soil specifications might include a drainage layer of gravelly sand, a substrate of loamy coarse sand (USDA) and a top soil layer of sandy loam (USDA).

In areas not subject to extreme storms, the top soil layer could be replaced with silt loam (USDA). Silt loam retains nutrients and upper soil moisture while providing increased stability for tree roots.

Proper soil placement and sufficient gradients will encourage drainage by rapid lateral water movement, and determine the success of any plantings. Due to density and low oxygen levels, compacted soils inhibit root growth both physically and chemically.

Avoid compacted soils by using appropriate earth-moving machines and techniques. Do not perform final grading until the soil is dry and easily crumbled.

To achieve the optimum drainage and soil stabilization, use a post-settlement gradient of at least 3 percent (30:1). Slopes steeper than 30 percent (3:1) will inhibit soil stabilization. In addition to aesthetics, the final landform design should reflect climate, predominant wind direction and soil quality.

Realistically, the minimum soil depth appropriate for sealing cap protection and plant growth should be at least 60 centimeters. In areas where trees and shrubs will be planted, the depth should be at least 90 centimeters.

Soil depth is a tricky issue; depending on the capping material, plant species and geographical area, depths greater than 60 to 90 centimeters may be appropriate as a safeguard against root penetration.

If freeze/thaw is a concern, the soil layer should extend below the frost line for the greatest cap protection. If the site will be revegetated by natural succession, the soil depth should accommodate native species' rooting depths. In some regions this could be two to three meters or more. Mowing or regular pruning of large woody plants will decrease root depths and allow for thinner soil.

Grasses And Wildflowers Even under the best circumstances, growing conditions on closed landfills will be stressful to plants due to limited moisture retention capability, erosion, possible gas seepage, shallow and/or poor quality soil and a host of other likely problems. Nevertheless, plants are integral to closure, since their root systems ensure that soil remains in place to protect the cap. Plants also increase evapotranspiration, moving moisture away from the cap to the soil surface.

Grasses and wildflowers are the most common cover because their root systems are believed to pose less of a risk to cap integrity than the woody plants' roots. However, some perennial grass species may extend roots several meters deep, especially when the grass is unmown. If grassland or wildflower meadow is the desired cover, choose species for their shallow root systems, as well as drought tolerance, low fertility demand and regional suitability (see plant list on page 70).

Every planting plan should include plants from the family Leguminosae, which will increase the health of other species. Legumes can fix atmospheric nitrogen, aiding the build-up of available organic nitrogen in newly formed soil and minimizing the need for chemical fertilizers (see plant list on page 70).

Wildflowers may be combined with legumes and grasses, but they require intensive maintenance in the first few years to prevent other species from overtaking the slower establishing wildflowers. Wildflowers are also less effective at controlling erosion than grasses.

Trees And Shrubs While it may be appropriate to restrict vegetative types where the budget is low and soil cover thin, the aesthetic, ecological and recreational benefits of a broad plant palette may outweigh the risk of cap penetration.

In general, selected trees and shrubs should have shallow root systems, pollution tolerance and high adaptability. The first phase of revegetation should include some nitrogen fixers (see list). With adequate soil cover, sites should be able to sustain complex natural systems or even woodlands.

Landfill restoration is relatively new, and no long-term studies document woody plants' effects on sealing caps. A study conducted at Fresh Kills Landfill in Staten Island, N.Y., found no significant cap penetration after seven years.

Trees' rooting habits in natural habitats will be similar to those on landfills. Most trees growing on undisturbed forest soils have relatively shallow root systems. Although popular conceptions depict a tree's root system as a mirror image of its trunk and branches, in fact, lateral roots often extend up to three times the tree's height, while vertical roots rarely grow deeper than three feet.

Roots will only grow in soil with favorable moisture, aeration and mechanical properties. The bulk density of clay capping materials prevents root penetration, where the cap is not flawed. Roots cannot decrease in size to enter the minuscule pore space in this material.

Using woody plants on landfills in Europe is widespread. In Finland, for example, a short-rotation mixture of willow, poplar and birch species have been planted on a former landfill to increase evapotranspiration and to harvest as a biomass crop. Although irrigated with leachate, crop productivity reportedly was among the highest in Finland. At Vinkeveen, in the Netherlands, a former landfill site has been planted as a formal park with a range of viable trees and shrubs.

Timber crops and recreational woodlands have been successfully established on many sites in Great Britain, including Befordshire, East Sussex and Atlas Mill in West Yorkshire. The Atlas Mill site, which includes a wetland, is showing signs of success as wildlife habitat.

In the United States, however, many cultural, legal and economic constraints have limited the diversity of plantings at many sites - with some outstanding exceptions (see the Trend on Danehy Park, page 16.)

At Dyer Landfill in Florida's Palm Beach County, George Gentile & Associates pursued a naturalistic approach. By literally picking up and transplanting large tracts of vegetation from an area slated to become a new landfill, Dyer Landfill was developed into a wetland surrounded by cypress forest. Although weekly monitoring of the wetland's water quality will always be necessary, maintenance steadily decreases as the ecosystem begins to function naturally, with plants and animals regenerating the system.

Finally, plans for a park at Fresh Kills Landfill also include a range of vegetation, from native grasses to small trees. Experiments are under way to determine which species will thrive at the site.

One significant risk that cannot be controlled through barrier quality is 'windthrow,' which occurs when trees are blown over by strong winds. The uprooted tree may have soil bound in the roots, leaving the cap exposed. Luckily, cap exposure is not inevitable. Adequate cover material depth (including both top soil and sub-grade materials) for the expected rooting depth will limit the amount of soil dislodged. As a further precaution during the final cover planning process, planners should take into account the region's climate, windiness, elevation, topography, soil and rooting depth.

To minimize the possibility of wind-throw, consider harvesting and selling larger trees. Funds could be used for replanting and upkeep. Another option is to plant trees that will remain relatively small or to plant only shrubs.

Windbreak techniques also will increase protection. For example, protect tall species by surrounding them with smaller, more dense plantings to break up the wind. Even a single row of trees beyond the waste deposit area can be an effective windbreak.

Irrigation Soil aridity may be a problem at many sites, because of shallow and/or poor quality soil, high elevations and wind exposure. Depending upon waste content, one cost-effective solution may be to irrigate with leachate, which typically contains nutrients plants need and may reduce the need for chemical fertilizers. Irrigation systems should be installed after the majority of subsidence has occurred to avoid damage.

Wetlands can be created as retention basins for surface runoff and stormwater. These natural systems slow down evapotranspiration by allowing moisture to percolate into soil instead of evaporating from the surface. An added benefit would be more diverse plant and animal life. In arid areas, retention basins still are recommended for moisture retention, but annual rainfall may be insufficient to sustain a wetland.

Design Strategy For the most part, the restorations described here require extensive maintenance. If a large maintenance budget isn't available, consider developing a landscape whose maturation depends on natural succession and not human intervention. (Succession refers to the gradual establishment of a self-sustaining ecosystem through colonization by native species.) Although specifics will vary depending upon regional climate and ecology, certain methods will advance this process.

Landscape should be designed to connect with the region's natural habitat. However, highways, parking lots, buildings and even the ecologically impoverished landscape of some suburban areas may be significant barriers. Consequently, colonization by plants and animals must be encouraged.

Typically, landfills are sited at the edge of development; by adding new habitat that could help link remnants of natural forests and wetlands, urban greenbelts could be enhanced or buffered. Plant diversity is crucial to jumpstart this process.

For example, throughout much of the United States, many plants depend on birds to disperse their seeds. But many birds will not land in areas without perches at least 1.5 meters high. As a result, areas seeded with commercial grasses may remain barren indefinitely because they lack the minimum ecological complexity necessary.

Obviously, one solution would be to include some trees to attract seed-dispersing birds. However, tall trees and shrubs could be costly and would not adapt to landfill conditions as easily as saplings. In the interim, artificial perches could be constructed. Pay close attention to the region's seed dispersal patterns, including prevailing wind direction, landform effects and colonization patterns of invasive species.

Finally, whether the final landscape will be high or low maintenance, consider 'cluster planting.' With this strategy, woody plants are concentrated in specific areas provided with deeper soil cover. Clusters also could be reserved for colonization by native species, with the remaining area receiving more maintenance, such as an annual mowing.

This strategy would be useful where the budget for soil cover won't pay for covering the entire site. However, without maintenance, species in these pockets may spread to areas with inadequate soil cover to sustain their rooting depth.

Although design options for landfill restoration may be limited by budget constraints and regulations that proscribe barrier requirements, soil depth/quality and plant palette, many opportunities exist to create rich landscapes on these sites.