A Guide to the Revegetation and Environmental Restoration of Closed Landfills

Chapter 8: Considerations in Vegetation Selection

Prior to initiating a vegetation or restoration project, several criteria must be considered in selecting the plants that will be utilized in the final vegetation population.

Site-Specific Considerations

The plant assemblage to be selected must be compatible with the soils that will be placed as the vegetative layer. Most soils that are utilized for vegetative cover are generally not the original native topsoils and may not be quality topsoils, lacking the primary nutrients found in natural topsoils.

Borrow soils may require amendments. This can be accomplished by adding mulch or wood waste material that has been properly processed and sorted, to the soil. Soil chemistry may be considered to anticipate potential problems with trace chemicals or salts. Relatively sterile soils may require augmentation with fertilizers, and compost to satisfy plant growth needs. Stockpiling the original topsoil layer until the project is done is a sound plan; replacing it on the cap upon completion.

Soil compaction may be an additional consideration in providing the optimum soil conditions for vegetative cover. If soil is compacted excessively, this will inhibit root development of the vegetation. Some grasses can serve as soil softeners, which can allow succeeding vegetation to establish itself.

When a landfill is closed, the final layers of cover material may include a clay or geomembrane layer in the moisture barrier. Often, when this clay layer is utilized, irrigation or soil moistening must be designed to maintain required moisture levels in the soil to safeguard against desiccation of the clay layer. This moistening activity will be conditioned upon the annual precipitation and climate of each individual site. The moistening function can be taken advantage of as a means to support vegetative cover that may be planted on the final cover. The moisture level must be designed to account for the climate of the site location including annual rainfall volumes and periods of occurrences, the type of clay material to be utilized in the barrier, and the planned plant community to be installed on the landfill site. Once these conditions are established, the volume of water required for proper moisture control can be determined. Water Use Classification of Landscape Species (WUCOLS) is a reference published by the University of California Cooperative Extension that discusses all of the parameters involved in designing a vegetation cover.

If artificial irrigation is planned for the site, the source water must be tested for contaminants and the chemical constituents in the planned water must be considered. The chemical constituents in some water may include salts and other trace materials that may accumulate in the irrigated soil which can cause salt or chemical buildup leading to plant damage or mortality. A project planned to employ irrigation water from a cooling plant was designed for a Southern California landfill.⁷ In time, the vegetation on the irrigated landfill site began dying. A study of the source water revealed high concentrations of boron and other dissolved substances. These salts were injuring the vegetation being irrigated on the landfill. To correct the problem, plans are being developed to process the effluent water from the power plant to remove these salts prior to irrigation.

In addition to moisture being introduced, control of drainage off site must also be properly designed. Some sites may require less artificial irrigation with more surface runoff facilities provided at the site than other locations.

Maintenance of the final cover and its accompanying vegetation can be influenced by the proper design steps taken in constructing the vegetative layer. Selection of slow-growing plants will reduce the numbers of times maintenance crews may be needed to thin or control growth of grasses or other foliage. Selecting clean plants will help in cutting cleanup costs.

Some trees such as eucalyptus are extreme generators of litter. They produce an abundance of debris from fallen branches, topping from high winds during storms, bark debris, a hard nut-like seed pod and the thick lignin rich leaves that do not decompose readily. Vegetation that produces such volumes of litter can require more frequent cleanup maintenance than most native plants require. In addition to the cleanup requirements, unmaintained litter buildup of this kind can pose a potential fire hazard that could present added emergency costs for fighting a fire. The high oil content in eucalyptus, in addition, creates hot fires.

Native plants especially adapted to the environmental conditions of the candidate project site can reduce irrigation requirements as well as maintenance and pest control. Native plants are adapted to defend themselves from indigenous pests.

These plants can possess defense mechanisms effective against native plant pests. Using native plants in final cover planting should only increase the chances of plant survival. This can help in reducing costs of cleanup and replacement of plants lost to pest infestations.

Plants in their natural assemblages have formed compatible community associations. Some plants have special defenses, usually chemical, to promote their survival. Attempting to place competitive vegetation too close to such defensive varieties will reduce the chances of survival of the "invasive" plant. Usually the protective zone of a plant will extend to the edge of its drip zone; the area directly below the crown of the tree or bush. Eucalyptus (non-native), oaks, and California Bay (native) are some potent defenders of their root zones.

Climbing plants should be avoided because they can overpower the supporting tree they may establish in, by blocking the sunlight to the leaves. Certain California varieties of wild grapes can overrun a large tree in a matter of a few years.

An awareness of the chemical by-products plants produce (such as terpines) can help in determining the data that may be generated during water quality and soil quality analyses. Decomposing leaf detritus or bark debris can accumulate in the soil and percolate out as complex compounds that can mimic certain manmade hydrocarbons or leachate compounds. This can confuse data sampling and source determination. Eucalyptus, California Bay, and most conifers produce high concentrations of these chemicals. An understanding of this can reduce confusion when data sampling shows unusually high readings of hydrocarbons. The use of chips from these trees as wood wastes or soil amendments can create the same effects, leaving dissolved terpines in the soil.

Native versus introduced (non-native) plants.

The role of the vegetative cover is to stabilize landfill slopes, mitigate or control moisture levels in the soil, control surface runoff and comply with regulatory requirements. The landfill designer must also realize that this cover will continue as a long-term impact on the environment (hopefully beneficial). The landfill cover also should ideally serve as a repair to the environment. When possible, the vegetation should provide a near reconstruction of the ecosystem that the landfill had displaced, providing the native plant makeup to maintain the plant and wildlife diversity remaining in California. A vegetative assemblage that most completely integrates itself with the surrounding environment under given circumstances is what an operator who is attempting restoration is trying to achieve. A natural restoration would be the ideal result for the vegetative cover project; providing renewed natural environments for future generations. A landscape planner must consider several things in the overall look of the plant cover.

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Planting Considerations

When designing the final plant distribution, will the vegetation compliment or blend in with the surrounding vegetation and environment? Can it at least present an acceptable appearance? A landfill in open grassland with 40-foot tall trees would present an obvious visual impact inconsistent with the natural surroundings, focusing attention on the landfill rather than blending it into the surroundings.

Consideration must be made for power lines in the site area. The designer must make sure mature plants do not interfere with power lines or other overhead structures.

Will the selected plant species work compatibly with the designed cover layers without causing moisture intrusion or damage to barrier layers? The topic of root depth is an issue of concern for landfill revegetation planning. The erosion layer is generally built up to only the minimum California required depth of 12 inches over the moisture barrier. This affords little volume for deeper root zones to develop without possibly damaging the underlying moisture barrier. This thin layer in turn provides little soil depth for plant growth beyond grasses. The clay moisture layer, blocking root development or gas concentrations in the soil, affecting the roots could compromise larger plants. In turn, potential root penetration of the barrier layer could compromise the barrier’s functions of controlling moisture and landfill gas control. This root penetration would expose a plant to the damaging effects of landfill gas.

Some techniques available to resolve this problem are to build up berms or hillocks or to utilize benches as areas for large vegetation. Simply providing a thicker erosion layer, even in small special areas on the landfill, will improve the options for vegetation selections and location of plants on the final cover. This can provide more opportunities for "naturalizing" final cover vegetation.

Planter containers are considered as a means of providing locations for planting larger trees or shrubs. This technique has questionable merits. For planters to work, the container should be placed partially or fully below grade. This may provide a more secure foundation for the tree/planter to remain upright.

The conflict with this concept is that the planter will be set into the minimal soil cover layer (12 inches) and it negates the planter’s advantage of deeper soil for the tree. This method also puts the tree roots in the hydraulic barrier zone. Higher water levels in this zone at the roots will drown the plant. This practice can also put the roots closer to landfill gas concentrations in the soil that could be harmful to the plant. A planter will restrict root development, negating lateral root growth, thus rendering the tree vulnerable to wind throw (toppling). Unless the planter container itself is anchored to the substrate, both tree and container may blow over. The problem with this technique is the anchors could penetrate the moisture barrier, jeopardizing its primary function. Thicker soil cover at the tree site would not require a planter, so a planter is unnecessary. Most landfill surfaces are sloped, making use of the planter technique impractical or difficult.

The popular conception is that plants develop deep (vertical) root systems as they grow. In reality, they actually develop more lateral and shallow root systems. This follows along the fact that moisture and nutrients are found in their greatest concentrations in the first 12 to 24 inches of natural soil. The 3- to 4-foot taproots in larger plants are primarily for the survival of the developing sapling. The removal of this taproot upon planting smaller nursery plants should alleviate the problem of a deep taproot penetrating into the clay barrier layer. The lateral roots will develop to compensate for the removed taproot. Plants adapted to arid conditions develop deeper rooting systems to reach deeper groundwater sources. These plants should be avoided in situations with thin vegetative cover.

What must be done to maintain the developing vegetation and the mature planting for the next 30 years?

• Plant litter. Considerations must be made that the species selected for the site produce easily maintained quantities of leaf, bark, and plant litter.
  
• Mortality or vulnerability. Wind-throw (blow-down), disease, drought and frost can kill or weaken plants making them vulnerable to other diseases or pests. The eventual removal of plants adds extra maintenance costs.
  
• Regenerative ability. Capacity for the plants to re-seed and reproduce at self sustaining levels through the life of the landfill, especially grasses and smaller annual or perennial types while the cover is just establishing itself. Local native plants, able to cope with regional conditions, will be the best choice in this aspect.

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Vegetation Types and Considerations in Program Planning

Revegetation plans will have to satisfy regulatory requirements and program needs. Considerations must be made with regard to plant, irrigation and drainage requirements, landfill cover design limits on plant selection, costs to acquire materials and plants, costs to install the project and long-term maintenance requirements. Use of local native plants on a site-by-site consideration will eliminate some of the following considerations such as seeding, reseeding and growth cycles.

The final role of the vegetative cover may determine the types of plants to be utilized. Application of plant diversity in the proposed vegetation selection is very important. Avoid "mono-cropping" or planting of only one type of tree or shrub. Species diversity helps reduce disease dispersal or blights and it encourages wider environmental diversity in the restored habitat; more like a natural ecosystem.

Vegetation should be planted in random or natural looking distributions; not in rows like an orchard. If a revegetation project using non-native plants is located close to natural native plant environments, the invasive nature of the proposed project’s plant inventory should be checked before final selection. A "mitigation" with incorrect, yet aggressive, substitutes could actually cause displacement of adjacent native plants. This has been common in "mitigation" projects as in the Monterey dunes where similar plants were introduced years ago and are displacing rare natives today. A landfill revegetation project incorrectly stocked could provide a "jump point" for non-native invasive plants offsite. Rather than employing non-native species, one restoration project on an abandoned landfill in Spanish Bay, Monterey County, took cuttings of adjacent native plants to create the population supply "propogules" (cuttings) for the project.⁸

Grasses are generally small and herbaceous, lacking woody tissues and generally limited to prairie-like habits or appearances. Grasses are part of most open environments of bright sunlight but can be in all environments including understory conditions in woodlands environments. Grasses grow in even carpets of mixed species.

They may grow up to 24 inches or more in height. Grasses can grow as annuals or perennials depending on species.

• Annual Grasses
  Annual grasses grow for a single season with seeding and germination in winter. Grasses will mature and produce seed by the following spring. Maturation of grasses occurs by late spring or early summer with seed dropping.

By early summer, the mature plant is dead and dried, turning gold or brown. Annuals depend upon their next generation to continue growth and soil coverage. For successful establishment by annual grasses, the project manager must consider:

• Seeding Cycle. Will the growth and reseeding cycle of the selected grass coincide and be compatible with the local rain cycle?
  
• Reseeding Dependability. Will the succeeding generations provide sufficient numbers to sustain coverage, or will supplemental seeding be required?
  
• Native Grasses vs. Compatible Introduced Strains. The option of using introduced European grasses instead of native annuals in revegetation projects will not radically alter vegetative community appearance, but it does reduce the opportunity for native California grass populations to reestablish their numbers. Large areas would lend themselves well to native propagation. Native grasses, once established, do not require fertilizing and as much irrigation as non-natives.
• Perennial Grasses
  These are commonly small and herbaceous, lacking woody tissues and limited to prairie or grassland habit. Perennial grasses can grow carpet-like or in clumps as "bunch grasses" because of their long-term, multi-generational life cycles. Perennial California native species (bunch grasses) were dominant in California until the westward migration of European settlers who inadvertently brought the European strains of annual grasses (Bromus, Festuca, etc.) as seed on livestock and in the livestock's manure and overgrazing. Though not a grass, an excellent example of an imported invasive and its impact has been the yellow star thistle.

• Seeding Cycle. Will the growth and reseeding cycle of the selected grass coincide and be compatible with the local rain cycle?
  
• Reseeding Dependability. Will the succeeding generations provide sufficient numbers to sustain coverage or will supplemental seeding be required?
  
• Life Span. A perennial grass will last for many years (some species may live hundreds of years), dying back in the summer months, but regrowing from the always-viable root. It will produce seed each spring/summer yearly.
  
• Native Grasses Versus Compatible Introduced Strains. The option of using introduced European grasses instead of native annuals will not radically alter vegetative community appearance, but it does reduce the opportunity for native grass populations to reestablish their numbers.
  
• Resistance. Will the native perennial grass population hold up against the aggressive competition of European annual and perennial grasses? Native perennial grasses require attention in the beginning year or two until they are established but eventually displace annuals and require much less maintenance later.
  
• Root Depths. This must be considered. Some species of perennial grasses, especially long-lived species, will develop very deep root systems that could penetrate the barrier layer of a landfill, compromising the moisture barrier's integrity. A deep vegetative soil layer will prevent root penetration by these grasses.

Wildflowers are generally herbaceous and some species are woody-stemmed. Wildflowers provide a broad selection of plant heights, root depths, and aesthetic choices to provide ample selection in planning vegetative cover. These plants provide a wider choice of appearance for cover.

• Annuals
  The growth cycle is one year. Different species of the same genus can have different growth cycles through a given year. A dormant period after seeding occurs with annuals; the next generation will grow by winter.

• Seeding Cycle. Will the growth and reseeding cycle coincide and be compatible with the local rain cycle?
  
• Reseeding Dependability. Will the next generation require supplemental reseeding?

• Perennials
  The growth pattern of perennials can be for several years with woodier species living for extended periods. Perennials can have different growth, and germination cycles at slightly different times of the year, both within species and between different species. A dieback period may be involved with some species of perennials.

• Seeding Cycle. Will the growth and reseeding cycle coincide and be compatible with the local rain cycle?
  
• Reseeding Dependability. Will the next generation require supplemental reseeding?
  
• Life Span. Some perennials may live two to four years, some have longer life spans. Allow for maturing stock in planning cover communities.

These are woody perennials with growth heights from several inches to several feet.

• Plant Habit
  The "habit" is what the plant looks like as it grows. Some species can grow very large. Habit or foliage can be a consideration in planting on maintenance roads or benches. Design needs to allow space for maintenance vehicles to pass by on bench access roads. Planning may be required for multilevel "forest" to avoid conflicts with other neighboring plants in a planned plant community. Trimming and pruning of larger shrubs can modify them to fit special locations.
  
• Root Depths
  Root depths will vary from species to species, and with consideration for irrigation or moisture control purposes. Root depths are a concern when available final cover depths are limited or geosynthetic cover layer (GCL) or other low permeability clay layers are installed in the barrier layer.
  
• Irrigation Requirements
  Will the plant variety to be selected for an area be compatible with that area (climate, soil, moisture)? Low maintenance or minimal irrigation plans will require hardier plants than other more aggressively maintained postclosure programs.
  
• Layering
  If a more complex planned plant community design is envisioned, how will the candidate shrub work in a multi-story final plant community? Will it grow in low light, or bright sun, or transition over time from one to the other as the cover matures?
  
• Growth Rate
  If shrubs are planned (as well as other perennial plants) how will the selected plants interact with other selected plants in growth rate compatibility? Will a shrub actually outgrow a young sapling tree and displace it? The same applies to smaller shrubs. Will larger shrubs, causing loss of vegetative cover, displace them?

These are the longest lived plant group and can be the largest element in a plant community with the greatest influence on overall vegetation design. Trees can assume an understory as well as an overstory niche in a final revegetation plan. Healthy trees of many species will live hundreds of years.

• Root Depths
  The root depths of trees can vary from species to species and even within a species, due to vigor or planting conditions. This can be of concern when final cover depths are limited or GCL or other low permeability layers are in the final cover design. A minimum 3 feet vegetative layer will permit smaller trees to be utilized.

Trimming off of the taproot, a common practice in nursery supply, will allow planting saplings of large trees in relatively shallow soil layers. The sapling will adapt by developing its lateral roots, compensating for the absent taproot. The lateral roots, up to three times the tree’s canopy width, will provide ample anchorage and nutrient absorption for the tree. An alternative is to select indigenous tree species that lack the taproot. Smaller trees in high wind environments or at the periphery of planned tree groves may reduce the chances of wind damage or wind throw.

• Irrigation Requirements
  Will the tree variety(ies) to be selected for a project be compatible for that area? Low maintenance programs with minimal watering may require selecting hardier plants than projects with more generous irrigation programs, at least in the early growing stages as the plants establish themselves.
  
• Growth Competition
  In their early growth, can the candidate plants survive possible radical changes in their position in a forest hierarchy as the forest matures? Some plants may grow more quickly than selected trees as they compete for sunlight and soil nutrients. This may be monitored as part of the maintenance program to give the less competitive plants a chance to survive.
  
• Competition
  If a complex plant community design is envisioned, consideration must be made for the assembled plant selections in a multi-story final plant community. The defensive nature of some trees as they mature could block out competitive understory plants.
  
• Maintenance
  If a long-term vegetative cover plan is envisioned, how will the plants reduce maintenance costs? Messy plants will demand more attention to reduce litter and fire hazards than cleaner varieties.
  
• Irrigation System Design
  Will the selection of vegetation create a low or high demand on the planned irrigation system? This may affect water source options, types of irrigation fittings to be installed and moisture control requirements for the landfill cover (moisture control).
  
• Debris
  What steps will be taken to keep the site clean of plant litter and debris for fire safety and site access for other maintenance requirements (gas monitoring, slope inspections, etc.)? Selecting "clean" trees and shrubs will reduce this maintenance task. Eucalyptus, because of its debris, is one of the most fire hazard prone trees. Excessive debris can also provide protective shelter for rodent pests. Some shelter is good to encourage natural animal activity, but too much protective material can shield prey from their controlling predators. Excessive rodent burrowing can damage the cover layer.
  
• Understory Trees
  Will they cope with less sunlight until they mature in the final forest setting? Caution must be exercised to avoid over crowding of vegetation. This could interfere with plant growth and hinder maintenance and inspection activities.

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Last updated: April 18, 2008