Rarely do landfill owners specify native plants for vegetative cover, but with patience and care the results can be rewarding.

By Penelope Grenoble O'Malley

A successful native-plant revegetation program achieves 100% cover in the first year (based on a stratified biological assessment), but the learning curve is steep. Managing the revegetated acres to ensure continued operation of the landfill's environmental control systems is a challenge in southern California, where wildfire is also a threat.

Nick Morell, environmental planner in the planning and permitting section of the Los Angeles County Sanitation Districts' Solid Waste Management Department, sums up the challenge of revegetating with native plants in a landfill environment. "This is not your typical revegetation program. What we have here is native plants growing on an engineered landfill surface adjacent to a natural environment that is prone to wildfires. In designing the revegetation layout, we had to allow for inspection of our landfill gas systems as well as monitoring of the landfill surface for gas emissions. In a typical fish and game habitat restoration project, you would pick an area that was already disturbed or might have less-than-ideal conditions and bring it to premium conditions, then plant trees and shrubs based on those better soil conditions. On the landfill surface, the cover soils have already been put in place and compacted, and those are the conditions you've got to work with."

The Calabasas Landfill

In 1984 Congress passed legislation designed to limit the creation of new solid waste disposal sites in units of the National Park System (NPS) and to reduce the potential for adverse effects from existing operations. The Calabasas Landfill, a 505-ac. canyon cover operation in Agoura Hills, CA, just west of the San Fernando Valley, is owned by the County of Los Angeles and operated by the Los Angeles County Sanitation Districts under a joint-powers agreement. Under the new federal regulations, the Sanitation Districts was required to apply for an NPS Special Use Permit (SUP) to continue operation. The park service permit amounted to another layer of oversight for the landfill, which operates under a Conditional Use Permit from the Los Angeles County Regional Planning Commission, specifying long-term contour levels for the site, as well as other numerous technical permits. The landfill is currently operating at about one-third capacity, receiving 1,100-1,200 tpd of solid waste. At current levels, closure is estimated to be approximately 30 years.

The landfill is located in a northern segment of the Santa Monica Mountains National Recreation Area (SMMNRA), across the Ventura Freeway from the main body of the 150,000-ac. federally protected preserve but immediately adjacent to a 4,000-ac. unit of the SMMNRA heavily used by hikers, mountain bikers, and equestrians that also provides a significant wild-habitat link between the Santa Monica Mountains north into the Santa Susana Mountains. Existing plant communities in the vicinity of the landfill include valley oak savannah, coast live oak woodland, coastal sage scrub, California black walnut woodland, chaparral, riparian woodland, and native and non-native grassland (the two canyons that comprise this bulk of the park were once used as a cattle ranch). Existing visitor-serving facilities include a trailhead and a picnic area with direct views of the west-facing slope of the landfill and a 30-mi. trail system that connects the two canyons.

During the landfill permitting process, the immediate goal of the NPS, as the manager and administrator of the SMMNRA, was to ensure that visitor use and enjoyment would not be negatively affected by landfill operations. At the same time, it was continuing to allow the solid waste facility to operate as a component of Los Angeles County's solid waste management system. After a lengthy process that included an environmental review and public input, the NPS issued the Calabasas facility an SUP requiring the Sanitation Districts to clear the west-facing slope of existing ornamental vegetation and replant it with natives to blend in with surrounding plant communities. The conditions under which the permit was issued also required the Sanitation Districts to submit its plans for a native-plant restoration program to the NPS within three months of the effective date of the permit, including a detailed listing of plant species, a planting layout, methods of weed eradication, and irrigation.

The first in a series of annual reports required as a condition of the permit is now in the hands of the park service, and a year later, Morell characterizes the effort. "One of our goals is not only to visually and aesthetically blend the landfill into the park but also to allow visitors to be educated and understand that this facility is an integral part of the local environment. We want the visitors' experience and perception of the landfill to be a positive one, where they can say, 'Look, they've revegetated the landfill with a native-plant community.'

For its part, the NPS wants the site to function as both a restoration and landscaping project. "We don't want it to look like a manicured environment," says Ray Sauvajot, local NPS chief of planning, science, and resource management. NPS concerns about long-term irrigation include the consideration that too much water could force unnatural growth in the species used to revegetate and could attract exotic plant and animal species that require extra water, which could compete with the natives and upset the local ecological mix. Sauvajot says the park service is pleased with the project so far and notes that final evaluation will include the ecological functioning of the site, the appropriateness of the final species mix, and whether what was planned achieved "good cover."

At the time the project was initiated, vegetation at the site included the typical ornamental vegetation the Sanitation Districts uses to screen environmental control facilities (e.g., erosion control, gas system piping, roads), including acacia (Acacia spp.), California pepper (Schinus molle), Mexican fan palm (Washingtonia robusta), oleander (Nerium oleander), myroporum (Myroporum laetum), bottlebrush (Callistrnon citrinus), and eucalyptus (Eucalyptus spp.). A variety of invasive weed species (castor bean [Ricinus communis], mustard [Brassica spp.], and alyssum [Alyssum spp.]) was established on-site along with minor quantities of native plants, including encelia (Encelia californica), mule fat (Baccharis salicifolia), and California buckwheat (Eriogonum fasciculatum). Non-native grasses provided 100% ground cover.

The cover material on the 20-ac. project site was evaluated after the removal of the ornamental species and was determined to have anywhere from 7 to 12 ft. of soils, derived primarily from onsite shale, siltstone, and other bedrock materials. Soil samples from nine test sites indicated sandy loan, sandy clay loam, and loam soils. Naturally occurring organic material was found to be generally higher in quality than in the native soils outside the landfill boundaries, which was due to the fact that the project site had been vegetated and irrigated for an extended time (the landfill has been in operation since 1961). Except for one test site, levels of soil pH, salinity, boron, sodium, calcium, magnesium, nitrogen, phosphorous, and potassium were found to be suitable for native species. Surficial soil in the northern half of the site was fairly compacted but in the southern half only loosely compacted. The contours of the finished slopes were accomplished through a series of visual berms and benches (for drainage), resulting in a terraced appearance, and an aboveground piping system that connects to a series of vacuum-aided vertical wells and horizontal trenches that collect landfill gas.

Site preparation required modification of the existing overhead irrigation system with a localized deep-water drip system to encourage deep root growth in the areas to be planted with mosaics of trees and shrubs. It also included herbicide eradication of the existing weeds and removal of any remaining ornamental species throughout the site. All removed vegetation was shredded and recycled at the landfill. Because the goal of the final planting program was to provide visual screening as well as habitat continuity with the natural slopes immediately adjacent to the landfill, container plants were established in groups of 150 mosaics to mimic naturally occurring plant communities and as an aid in achieving density in the landscape. Prior to container installation, the entire site was seeded using three different seed mixes (predominate plants: bush sunflower, California sagebrush, caterpillar phacelia, black sage, and native bunch grasses). Seed collection was done locally, and the container plants were grown from collected seeds or from seed stock germinated in the nursery, which allowed for the preservation of genetic diversity while ensuring that local natural resources were not depleted. "I like to think that the Sanitation Districts and the Calabasas Landfill will be resources to provide seed for future projects," says Morell.

Performance standards for the project were established in the NPS SUP as a minimum of 50% survival of the container plants installed after the first year and each year thereafter and 30% minimum native-species cover throughout the 20-ac. site in the first year. The goal of 75% total relative cover with native species was established for the project after five years. To that end, a five-year maintenance and monitoring program utilizing an NPS-approved biologist was also stipulated in the permit, with the monitoring to begin one month after installation. The goal of the monthly inspections is to evaluate the overall status of the site, including the condition of the container mosaics and the drip-irrigation system and to make recommendations on weed maintenance. Following the first year, monitoring will be quarterly and results will be included in an annual performance monitoring survey. Although an original condition of the permit specified that the irrigation eventually be phased out, the Sanitation Districts committed only to minimizing irrigation in an effort to consider the competing needs of meeting NPS regulatory requirements and protecting its operation against the potential of wildfire.

Implementation of the program began in January 2001, and seed placement and plant installation was completed in less than four months. Overhead irrigation was provided three to four times a week and supplemented as needed with drip irrigation for the containers. The schedule was modified based on review by the monitoring biologist and adjusted for rainfall during the 2001-02 winter season. The site is currently irrigated once every seven to 10 days, a regime that allows the uppermost soil layer to dry out. Weed maintenance is accomplished using an herbicide when application won't affect native vegetation; otherwise removal is accomplished with hoeing and hand-pulling. All green material from weeding and from clearing trails for gas monitoring is used at the landfill.

"The time of year of installation was very important," says Morell. "We were able to install the containers and the seed mix in the winter of 2001 to take advantage of natural rainfall. Because we applied the seed mix late in the year, the biologist recommended that we continue to irrigate through the summer to ensure the established seed would flourish. That, plus the biological monitoring and continuous weed control, gave us very good results, but it took this kind of concerted effort to ensure the natives had that extra boost."

The "concerted effort" paid off, with both cover and survival of native species exceeding the NPS's first-year performance standards. The mean cover of native species on a stratified basis was approximately 100.3%; in fact, cover of greater than 100% was found on several transects, reflecting the multilayered nature of native-plant communities. Overall cover of native shrubs was 83.1% and native herbaceous material 17.1%. Non-native cover was measured at 9.1%, while approximately 26.6% of the site remained open ground. California bush sunflower was established as the most abundant species, with more than 3,000 individuals per acre, while a variety of other native shrub species had established themselves, including California sagebrush (Artemisia californica) with 12.1% cover, as well as ashy-leaf buckwheat (Eriogonum cinerum), purple sage (Salvia leucophylla), and black sage (Salvia mellifera), which together contributed 12%. Native grasses were abundant at 1,389 individuals per acre. Non-native cover included mostly non-native grasses, including Bermuda grass, which was controlled by an aggressive initial eradication effort, and onion weed (Asphodelus fistulosus), mustards (Brassica spp.), and an exotic species of statice (Limonium sinatum). Total relative cover of native species is estimated at approximately 75%.

Overall container-plant survival was 81.3%, and survival of container plants in mosaics was approximately 85%. Of the 40 trees planted on-site, 16 were found alive at the end of the first year for an overall survival rate of 40%. The Mexican elderberry (Sambucus mexicana) was established at approximately 14 individuals per acre, but the California walnut (Juglans californica) experienced high mortality with a survival rate of only 30%. Walnut mortality occurred in both moist and dry conditions (i.e., with and without overhead irrigation); additionally, walnuts have been noted to be sensitive to transplantation and do not typically establish naturally on western-facing slopes. Five each of coast live oaks (Quercus agrifolia) and valley oak (Quercus lobata) were planted; three of the coast oaks were found alive (60% survival) and four of the valley oaks (80% survival). Oak trees planted during this project were not planted on the landfill surface but were limited to adjacent areas and only on native soil. The Sanitation Districts has not planted the coast live oak, which is the dominant native tree found in this area, on the landfill surface primarily because the Los Angeles County Oak Tree Ordinance (Title 22 of the Los Angeles County Code - Planning and Zoning) protects all oak trees of a certain permit size from damage, encroachment, and removal and does not exempt voluntarily planted trees. Morell reports that the Sanitation Districts is currently seeking a modification to the ordinance to permanently exempt all voluntarily planted trees - especially trees planted on areas subject to differential settlement - from the requirements of the protected-tree permit process.

Native cover in fully irrigated areas was approximately 100%, compared to 13% in areas not irrigated from overhead but receiving water only from the drip system. Although the success of the native-vegetation cover is attributed partially to overhead irrigation provided to the seeded areas immediately after installation - both cover and density were greatest in areas irrigated by both the overhead and drip irrigation installed for the container plant mosaics - Morell says the Sanitation Districts has no immediate plans to add more overhead systems. "We're all in a learning curve, and at least for the immediate future, we've decided to leave things the way they are, in part to study the long-term effects."

In fact, establishing the correct level of irrigation might become one of the crucial factors in site management, requiring a balance of three priorities: appropriate plant growth, integrity of the landfill surface, and fire control. "The container plants and the seed mix have to tolerate a certain amount of supplemental irrigation," says Morell. "Otherwise you create a hazard condition on the landfill surface instead of an environmentally beneficial program. We are attempting to manage the system for evapotranspiration. These soils have high clay content, and if the surface form cracks, we may be forced to use heavy equipment to repair the slope, and this will mean removing the vegetation.

"Fire control is another issue you don't have to deal with when you use ornamental vegetation. If a fire in the adjacent coastal sage community burned up to our site and our vegetation wasn't moist, it could burn right onto the facility and impact all the environmental control systems." One solution Morell is considering proposing to the park service as the vegetation on the site matures is using dolomite or shale excavated on-site to create natural-looking fire breaks. This solution would replace cutting or mowing perimeter vegetation and include monitoring to ensure that tall vegetation establishes itself in front of, but not immediately adjacent to, gas system components and irrigation lines.

In March 2002, not quite a year after project installation was complete, temporary access trails were established through heavily vegetated areas to ensure that technicians could make quarterly gas monitoring sweeps. South Coast Air Quality Management District regulations require that a landfill be divided into grids of 50,000 ft.² and the technician walk a continuous course of 2,600 lin. ft. in each grid while taking an integrated air sample within 3 in. of the ground. Additional revisions of the original rule require slope and deck monitoring. In the revegetated site, the trails needing to accomplish this were cleared by hand in areas where the vegetation had formed a closed canopy, the goal to trim rather than remove individual plants. "With ornamental vegetation, we'd just mow," says Morell. A particular target has been the well-established bush sunflower (59.4% cover in the shrub layer), and preliminary results have indicated that trimming the abundant shrub has had a beneficial effect by allowing light to reach smaller shrubs, such as the ashy-leaf buckwheat, needle grasses, and purple sage. One goal of the monitoring paths and walkways needed to maintain the gas system - which the park service preferred be no wider than 3 ft. - is to mimic existing natural game trails. Deer are already using the site, and Morell suggests that the monitoring trails already tie into the natural deer trails in the adjacent recreation area.

Lessons Learned So Far?

"One recommendation I would make to anyone who revegetates using natives under less-than-ideal conditions is to establish performance standards based on total cover rather than species composition. You want to work with the species that are available and be prepared to make substitutions if some species does not establish well, either in the nursery or the field. Figure out what exists in the natural environment, then what works in your superimposed man-made environment, and work with that. What this amounts to is acknowledging the unknown variables that might exist in the environment so you end up with a result you can measure, plus a result that provides a lot of wildlife habitat value.

"We found the native plants prefer to be installed in the winter and some seeds do better in the summer than others, so in terms of your seed mix, you want to keep that in mind when you set the timing of your contract. Also, if you have enough lead time that you can have seed germinated in the nursery, your project won't have as much impact on the local native environment. Native plants don't establish as quickly as ornamental vegetation, and they don't give you the instant gratification of a forested appearance. But I think that many people who enjoy the natural environment remaining in southern California see the importance of using native vegetation. It takes more patience, but what we're seeing here in Calabasas is that within a year, with proper management and a very successful seed mix, we have a facility that's starting to blend in with the natural areas."

Journalist Penelope Grenoble O'Malley is a frequent contributor to environmental publications.

MSW - March/April 2003