Field experience has made it clear that if one is to be successful growing native plant communities, a functional microbial community is imperative. Use of native seeds in revegetation follows the current societal focus on multiple use of lands, a policy toward enhancing ecosystem diversity and function, and conservation of biological resources. Retention of and reapplication of predisturbance topsoil is fundamental to success with native seeding efforts. At drastically disturbed sites where topsoil is absent, the potential for success is generally reduced unless a carefully balanced restoration program is followed.
Baseline Information
Analysis of accurate baseline information helps guide decisions concerning constraints and opportunities for naturalizing a revegetation site. The end result of this data collection process is the delineation of naturalized landscapes to be created, restored, or enhanced. A site survey should be conducted that includes the biota and an analysis of the light, moisture, soils, slope, altitude, wind, and, if applicable, microclimate. This information is then used to select appropriate ecological plant communities as landscape models. The plant and animal data are especially useful to help identify the potential of existing communities for preservation, restoration, or enhancement.
Baseline information forms the framework of the naturalization plan. It is usually presented on a base map, which provides a visual record of the plan. Some types of baseline information to be included on the base map are such features as property lines, existing trees, shrub borders, meadows, water bodies, and other natural features; paved and unpaved roads; and permanent structures, with utility and service corridors noted. Symbols may be used to identify orientation, seasonal wind patterns, slope aspect, and solar aspect coverage. Information on existing local regulations concerning vegetation, setbacks, and other restrictions should be included. A series of GIS computer overlays can be used to easily record physical, biotic, and visual survey data.
Components for Creating a Site-Specific Revegetation Design
Each site is unique, and understanding the fundamentals of the subject area and the necessary requirements for successful revegetation is the first step. Until a satisfactory and realistic plan is prepared, the revegetation effort has only a random chance of success. Working closely with a botanist or knowledgeable plant ecologist is generally advisable to prepare a site-specific revegetation plan. The major components of such a design usually include the following.
Site-description predisturbance-Identify indigenous vegetation if possible. Include percentage tree, shrub, forb, perennial, and annual grasses. Pay special attention to early native seral stage species that naturally occupy disturbed sites. Identify concentrations of noxious weeds present in surrounding areas.
Characterize soil-Obtain preconstruction soil samples for analysis to establish topsoil baseline information. Be prepared to collect postdisturbance soil samples for analysis to identify the actual growth medium, especially if topsoil will not be returned to the site.
Catalog organic layer-Include depth, texture, color, consistency, and vegetative litter.
Microorganisms-Evaluate for presence of microphytes, mycorrhizae, and other soil organisms in soil predisturbance.
Exposure-Classify exposure: full sun, partial sun, full shade. Identify type of canopy cover: closed, broken or open, coniferous or deciduous, height.
Climate-Identify climate, temperature average, highs, lows, and corresponding dates. Include annual percentage of sunshine, estimated annual average precipitation, and rainfall periods.
Aspect-Distinguish aspect-i.e., north, south, east, west-and sub-combinations.
Slope-Categorize post construction slope, percent low, high, average, and aspect. Ascertain hydrologic potential of drainage areas above slopes that might provide the source for water erosion.
Primary objective-Select purpose of revegetation: aesthetic, habitat restoration, screening, erosion control, filter strip, soil stabilization, etc.
Secondary objective-Define subpurpose areas and categorize.
Attributes-Establish optimal plant characteristics and attributes: mixed rooting depth to control erosion and help dispense with water, non-palatability to wildlife, screening, filter sediment, color and texture as design/aesthetic elements, rhizomatous, tolerant of inundation, drought tolerance, etc.
Criteria-Establish general design considerations: e.g., priority areas, wetland areas, wildlife mitigation subareas, watercourse, and riparian plantings.
Hydrology-Identify function and type of hydrology to ascertain wetland characteristics.
Organic Matter
The organic-matter content of a soil is considered to be one of the most important factors relating to a productive soil. Organic matter performs several major functions in the soil. One is to improve soil structure for better soil aeration and moisture movement, to enable mineral-nutrient retention in the rhizosphere and the other is to facilitate plant nutrient uptake by plants through symbiotic processes involving soil microorganisms. The organic content of any soil is governed primarily by the quantity of stable soil humus, and secondarily by the amount of living plant material added and the rate that it decays. The distinction between these two is of great importance. Regardless of different climatic areas, other factors, such as drainage, soil texture, and kind of organic matter added influence the quantity and quality of soil organic matter.
Soil organic matter provides benefits far out of proportion to the percentages present in the soil. Soil organic matter accounts for 2% to 6% by weight of most soils. In semiarid regions the percentage is usually less than 2%. In a typical agricultural soil, organic matter contributes 50% of the cation exchange, water holding, and buffering capacity of soil. In sandy soils, organic matter may account for up to 90% of the absorptive capacity. Cation exchange capacity is important because it measures the ability of the soil to bind nutrient cations and prevent their loss by leaching. Increased water-holding capacity benefits plant growth by maintaining soil moisture for a longer period of time. This is particularly true for seeding on disturbed sites in semiarid regions. Relatively small amounts of organic matter provides the amount of moisture retention needed to germinate the seeds and sustain young seedlings until root systems are able to extract water from a larger volume of soil made possible by expanded root systems, and by fungal and related microbial associations that mature in symbiosis with the host plant.
There are an astonishing number of organisms in soil, as many as two billion per cubic inch. Humic rich soil contains complex networks of fungal threads called hyphae. Absent this humic condition fungal establishment and growth is seriously limited. Organic matter has a profound effect on soil structure. Organic matter is the “glue” that binds soil particles into functional aggregates. These soil aggregates are responsible for the crumbly, friable nature of productive soils. Soils with good structure have much less tendency for crusting, an event that is a frequent cause of poor native plant stands. Seedling roots elongate more rapidly through a soil with good structure, allowing establishment of young plants with strong root systems that will withstand the stresses of dry periods.
Soils at disturbed sites usually have below-average soil organic-matter levels, at times close to zero. Alternatives are to add high carbon-lignin material, such as livestock manure or composts. These compost products have diminutive levels of nutrients required for plant establishment, and the nutrient is in a chemically and biologically complex form beneficial to some microorganisms required for native species growth.
Stable Soil Humus
Stable soil humus, a small percentage of total soil organic matter, is the end product of organic matter decomposition when performed under anaerobic, or oxygen free conditions beneath the soil surface. The resulting organic structures can be hundreds of years old and are considered a slow renewable resource. Organic residues formed at shallow soil depths add little, if any, to the reserves of stable soil humus. Soil scientists tell us that stable soil humus is categorized into three distinct fractions. These fractions are typically found in nature in a balance consisting of 50% humins, 10% fulvic acids, and 40% humic acids. The thousands of individual organic structures within each of the classes are considered the “active ingredients” within soil organic matter. Soil productivity is enhanced when these components are found in abundance as they are in naturally organic soils.
The relative small quantity of stable humus in most soils ranges from 0.1% to 1%. It is a minute fraction of the total soil volume but plays a critical role in every function of productive soil. Even a slight reduction in actual humus can dramatically reduce soil productivity and ability to support plant life. Nutrient uptake in plant roots is virtually impossible in the absence of soil humus. Applied organic matter (compost, manure, etc.) is not stable soil humus. Conventional crop residues and manures incorporated into shallow depths are rapidly decayed by oxygen-loving aerobic bacteria. Applied organic matter is simply consumed (or oxidized) during the decomposition process, and has little, if any, potential to become stable soil humus. The addition of manure or compost contributes to the short-term health of the soil, but does not replenish stable soil humus.
Humus should be thought of as the “raw matter” or “building block” of life-the transition stage between one life form and another, mineral to plant. It is a part of a constant process of change and organic cycling, thus must be constantly replenished-for when we are removing vegetation and or crops we are depriving nature’s cycle of potential humus. This is why we need to substitute manures or other sources of organic matter to maintain the fertility of our productive land. The formation and decomposition of organic matter are fundamental to life-promoting processes that store and release energy derived from photosynthesis. Soil organic matter is the base product from which topsoil is made. Only 300 to 1,000 years are required to build an inch of topsoil. The average depth of topsoil is about 8 inches, indicating an earth covering living lens of soil about 8,000 years old.
Soil Balancing
Test the soil for macro and micro mineral balances. Soil lab testing is imperative at disturbed sites to balance the growth medium for native plant growth. In agriculture, a soil test is the analysis of a soil sample to determine nutrient content, composition, and other characteristics, including contaminants. Tests are usually performed to measure the expected growth potential of a soil. It measures fertility, indicates deficiencies that need to be remedied and determine potential toxicities from excessive fertility and inhibitions from the presence of nonessential trace minerals. The test is used to mimic the function of roots to assimilate minerals. Soil testing is often performed by commercial labs that offer an extensive array of specific tests. Choosing the test lab site is just as important as the test results. There are many soil testing labs in the United States, but finding the right one will take some research. Lab tests include, but aren’t limited to, major nutrients: nitrogen (N), phosphorus (P), and potassium (K); secondary nutrients: sulphur, calcium, and magnesium; and minor nutrients: iron, manganese, copper, zinc, boron, molybdenum, and aluminum.
The mixing soil samples from several locations to create an “average” (or “composite”) sample is a common procedure, but it must be used judiciously, as it can artificially dilute quantities/concentration to meet requirements for sampling. Make a reference map for the filing system to know where samples were collected, and how many samples were pulled in the field. All of these considerations affect the interpretation of test results. Careful soil sample collection often reveals a mosaic of mineral balance conditions on a given site, requiring a range of nutrient amendments to appropriately mitigate for imbalances as identified.
When imbalances are revealed by soil testing, mineral amendments may be added to the site to mitigate for excessive fertility and inhibitions and from the presence or deficiency of trace minerals. This is a critical step in revegetation at disturbed sites. Without testing and balancing to establish base fertility the likelihood of revegetation success is seriously limited.
Microbial Associations in Soils
Soil fungi and related microbial associations that allow plants to survive the stresses of extreme temperatures, drought, and soil infertility colonize soils in significant functional ways. Soils in regions with low rainfall tend to be low in organic matter, low in available phosphorous and nitrogen. Fungal colonies are present in the root systems of most indigenous plant species on arid and semiarid lands, as well as higher rainfall areas with coniferous forests. The role of mycorrhizal fungae in this habitat appears related to their capacity to acquire nutrient resources for plants. Beneficial mycorrhizae and associational microbes solubilize mineral elements such as phosphorous for uptake by plant roots. The plant roots utilize the fungus that surrounds them to acquire water and mineral nutrient. The root systems of these plants grow in a sheath of mycorrhizal fungai in symbiosis between plant and fungus. That is, energy in the form of organic carbon compounds moves primarily from fungus to plant, and inorganic resources such as phosphorous move from mineral to the fungus for uptake by the plant.
Microfauna
The unicellular eukaryotes, or protoctista include a wide range or organisms, which are more often called protozoans. These include the flagellates, naked amoebae, testacea, and ciliates. These organisms range in size from a few cubic micrometers in volume to lager ciliates, which may be up to 500 micrometers in length and 20 to 30 micrometers in width. Protozoa are quite numerous, reaching densities of from 100,000 to 200,000 per gram of soil. Bacteria, their principal prey, often exist in numbers up to one billion per gram of soil. All of these orgasms are true water-film dwellers and become dormant or inactive during episodes of drying in the soil. They can exist in inactive or resting stages for literally decades at a time in xeric environments.
It is generally not easy to classify genera of VAM fungi simply by their root colonization patterns, there are numerous species adapted to climate, aspect, soil chemistry, and climate and elevation changes throughout the American west. Morphological features that are important include variations in vesicles (size, shape, wall thickness, wall layers, position, and abundance), hyphal branching patterns, the diameter and structure of hyphae (especially near entry points), and the staining intensity of hyphae (dark or faint). These fungal species are specialist and have evolved to function in symbiosis with the host plant species in a given environment, adapting to the multiple variables of a given circumstance.
Soil Microorganism Population and Diversity
These diverse microorganisms constitute the “soil microbial community.”
- Soil microorganisms help do the following:
- Facilitate increased nutrient and water availability
- Conserve nutrients in the soils biological fraction
- Mineralize organic nitrogen and recycle nutrients
- Improve soil structure
- Essential to create productive and sustainable soils
Nutrient cycling is, in large part, controlled by bacteria and the relative growth rates of the active fractions of the bacterial biomass, including root algae. Loss of significant portions of bacterial biomass, or loss of certain nitrogen fixers or nitrifying bacteria severely limits the productivity of a site. The thin lens of topsoil common to the earth’s crust is in fact rich in bacterial, fungal and associated microbial biomass. Soil is a living system. Loss of this thin lens of topsoil during road construction, mining or other disturbance makes it difficult to reestablish vegetation on the exposed subsoil. For this reason, cut-and-fill slopes exposed during construction will generally not fully revegetate and thus remain sparsely populated with vegetation. Rebuilding the lens of biomass necessary for successful vegetation to establish can take many decades. Though the airborne spores of indigenous soil microbes may be present, absence of hospitable conditions for reestablishing biomass makes it difficult for conditions to improve.
Soil Ecology: Microbial Activity
In general, the organic content of a soil is an indicator of its fertility and the ability to support microbial populations, retention of mineral elements, and to retain water. This is an important issue, especially given the symbiotic relationship of native plant species symbiotically dependent on soil organisms.
Soil organisms perform many processes, often with varying degrees of redundancy. In healthy soil, there are usually several organisms that perform any particular process, and in highly disturbed soils these organisms might be lacking or so adversely impacted to dramatically limit the success of revegetation. Unless topsoil is returned to the disturbed site, optimum results are achieved by adding stable soil humus so that early seral-stage plant life can be established.
Disturbed soils, especially soils that have been mined, generally have low mineral and microbial diversity and, depending on the site, might be relatively sterile and poorly colonized with indigenous microbial and fungal populations. To mitigate for this, one must initiate the soil-building process by importing organic compounds containing stable soil humus with complete humic acid and fulvic acid. One of the questions that land managers face is what commercially available products function in this capacity that may be used on severely degraded sites. Products that act as catalysts to microorganisms and enzymes to activate soil microbial activity and improve soil fungal development are of necessary importance. Humic acid activates a broad range of soil microbes, improves soils physical properties, and aids in water retention. In addition, cytokinin and auxin are among a group of organic growth hormones that may be added serving to stimulate root growth and the development of essential microorganisms. Use of these naturally occurring catalyst products does much to aid the establishment of native plants on drastically disturbed sites.
Soil Microbes and Land Disturbance
One of the most successful land management methods for managing disturbed sites has been the retention of topsoil to reestablish the predisturbance microbial population. Concurrent reclamation techniques respread stockpiled topsoil, followed by reseeding the selected early seral stage species. This is consistently the preferred method of managing for revegetation efforts.
Native topsoil retention is one of the factors fundamental to being successful with native seeding projects. Given the close association of native plant species to VAM and the host of soil microbes and invertebrates in the soil, maintenance of intact topsoil must be a high priority for native vegetative habitat restoration operations. At drastically disturbed sites the potential for success is generally reduced in the relative absence of topsoil. A carefully monitored program is required to retain and redistribute topsoil to reclamation sites. The objective of this effort with topsoil is retention of native soil organic matter, including indigenous microbial populations.
It should be noted that use of inorganic fertilizers, especially those containing superphosphate should be discouraged since they can drastically inhibit microbial formation. This important point regarding fertilizer cannot be overemphasized. Uses of synthetic fertilizer alone in native seeding will generally not further establishment objectives. Often use of synthetic nutrient-especially nitrogen may result in weeds and non-native species occupying the site in question.
By contrast, even the addition of relatively small amounts of topsoil (more than 2 inches deep) to a site results in improved microbial population and subsequent establishment of indigenous grass, shrub, and forbs with superior erosion control and soil stabilization performance.
In the absence of topsoil reclamation on mine, road and bridge construction projects, the use of organically based soil amendments that facilitate initial plant establishment and microbial development is a functional alternative. Mycorrhizal inoculation products, roots dips, and similar applications have proven consistently unsuccessful for reasons having to do with the site-specific nature of indigenous soil fungi and associated biota. Soil microorganisms are not generic in nature; they are adapted to the specifics of given soils chemistry, plant-cover type, aspect, elevation, and climate.
Transplanting native and introduced species as container stock at mine, roadside construction and other construction projects is common and often met with failure when topsoil is absent. Frequently such transplants struggle and do not flourish for various reasons. A common reason is dysfunctional root systems missing necessary indigenous bacteria, fungi and associated microorganisms to obtain adequate moisture and nutrients. Nutrient cycling is in large part controlled by bacteria and the relative growth rates of the active fractions of the bacterial biomass, including root algae. Loss of significant portions of bacterial biomass or loss of certain nitrogen fixers or nitrifying bacteria, severely limit the productivity of a site. This limitation adversely impacts container transplants at revegetation sites.
Other soil limitation factors resulting from construction damage include reduced root volume area due to soil compaction, low organic matter and lack of fertility, adverse soil pH in subsoil, and undesirable moisture holding capabilities. In these cases the addition of humic substances aids in the colonization of plant roots by soil microbes, which significantly benefits plant growth potential. Generally adding synthetic fertilizer will not achieve these objectives. Use of appropriate humic acid, nutrient balancing minerals and biostimulants will definitively assist in the establishment of both native seed and container stock planted at disturbed sites. Avoid using wood-based mulch or wood-compost products that functionally tie up nitrogen and nitrogen-related decomposer microorganisms due to their high carbon-nitrogen ratio, which require substantial quantities of nitrogen to break down the cellulose fibers in the wood.
Seeding
Reestablishment of native vegetation is greatly expedited through direct seeding using a multibox range drill. When it is possible to minimize the size of disturbance, advantage can be had by taking the seed from vegetation at the edge of the disturbance. Locally collected seed almost always outperforms seed imported from distant locals. Topsoil should be salvaged and replaced as quickly as possible, thus utilizing a source of plant and seed materials contained in said topsoil. These techniques by themselves are generally insufficient to rapidly establish a vegetation cover but they are a highly constructive set of steps. Direct seeding is usually required using quality native seed.
Considerable progress has been made in direct seeding in the past decade. Much emphasis has previously been placed on introduced grasses and legumes, however. Seeds of these life forms generally germinate readily, have limited scarification or germination requirements, and are composed of relatively clean seed; thus they are easier to meter and to seed. Seed of native species has tremendous variability in size and shape and is often difficult to use with standard seeding equipment, which is why the range drill designed to handle the characteristics of native seed is an invaluable tool. Caution must be taken as some varieties of introduced grasses and legumes have been selected for aggressiveness, particularly in the seedling stage. These varieties successfully out-compete many native seeded species and even established native vegetation causing exclusion of some desirable plant species.
Legumes should be used primarily for immediate improvement of soil structure for soil aeration and water movement. Many legumes will naturally vanish from a planting site following completion of this function. Generally, nitrogen fixing and modification of soil structure can be achieved using legumes, and regionally native species are preferable to introduced varieties. Some introduced legumes, such as alfalfa, might persist in an ecosystem long after the goal for its introduction has been achieved. If at all practical, use of indigenous species is encouraged.
The seeding process is perhaps the single most important component of a revegetation plan to establish native plants at mines and on road side revegetation sites following disturbance. Care must be taken with the multiple steps involved with the seeding process.
Seeding Methods and Criteria
Seeding methods can be divided into three general categories: drill seeding, broadcast seeding, and hydroseeding. Selection of the appropriate seeding method depends on site accessibility and terrain, seedbed characteristics, time of seeding, species characteristics and variability within a mixture.
Drill seeding is usually limited to slopes of 3:1 or flatter and areas that are not extremely rocky. The “rangeland drill” is often the most effective machine for reclamation drill seeding, even when the soil is rocky or contains other large debris. The rangeland drill is a heavy-duty drill with a high-clearance, reinforced frame and disk furrow openers that are independently suspended. The furrows are covered with drag chains or packer wheels. The disks can be equipped with different-size depth bands to control furrow depth. Multiple seed boxes are used for metering different-size seeds, and planting different seed types at differing depths. A quality range drill should be capable of handling fluffy or trashy seed. This is usually accomplished by mechanical “pickers” (revolving teeth) that clean the delivery path within the bottom of the seed box. With this specialized capability the range drill can effectively plant valuable but trashy species such as sage and rabbitbrush.
Drill seeding improves seed coverage by soil at proper depths, allows reduced seeding rates for greater economies, provides more accurate seed metering with simplified calibration, and can be used to seed into difficult and uneven, rocky terrain. The rows created by the seeder might initially be aesthetically unappealing as the plants emerge, and may temporarily result in increased competition from the concentration of seeds in the row. Drill seeding, if not handled by an experienced operator, can also result in some extremely small seed being planted too deep. Optimum seeding depth sometimes varies widely among native plant species because larger-seeded species often require a deeper planting depth than small seeds. A properly adjusted rangeland drill can accommodate the seeding requirements of a variety of differing species types. The Bureau of Land Management (BLM) has done pioneering work with range drills in the American West, with laudable success.
Broadcast seeding is accomplished with a spinner plate or propeller device attached under a seed hopper or conical shaped container, scattering or throwing seeds in predetermined directions as a tractor or similar vehicle passes over the area to be seeded. Broadcast seeding is generally required on slopes steeper than 3:1, on extremely rocky sites, or on remote or inaccessible sites, and in areas where the appearance of drill rows are undesirable. Broadcast seeding usually requires raking, chaining, or harrowing to ensure seed coverage where possible. Broadcasting normally requires higher seeding rates one-and-a-half to two times greater than drill seeding, and consequently results in less efficient use of the seed. It is difficult to precisely calibrate seeding rates. Germination and seedling establishment may be somewhat slower, although apparent initial diversity is generally higher than in drill-seeded areas. Broadcasting into a rough seedbed, followed by harrowing, can result in a variable range of seed placement depths and may allow better establishment of small-seeded species. It is common to have a tractor with a mounted broadcast seeder tow a “furst” or “pasture” harrow to cover seed, provided debris is not present to clog harrow.
Hydroseeding is a modified form of broadcast seeding that involves using a pressurized spray of water containing seeds and other materials such as mulch and tackifier. Some seeds might become damaged from the mechanical action of the hydroseeding machine. Hydroseeding is dependent on local water supplies. It might be the only alternative for steep, inaccessible slopes. Generally, the freeze-thaw cycle helps incorporate surface applied seed into the soil. Hydroseeding is a suitable technique for areas with adequate and dependable moisture during the germination period, given the usually shallow seeding depth. When used with appropriate tackifier and soil bonding products, it can provide seeding capability at difficult steep sites.
Timing
Seeding of disturbed areas should begin during the first available seeding “window.” A seeding window is that period of time most suitable for native seeding and offers the best potential for seeding success. The factors that contribute to the determination of the seeding window can include the following:
• Seeding prior to a period of adequate moisture for seed germination
• Seeding prior to an extended period of adequate moisture for early seedling growth and establishment
• Seeding when soil temperatures are adequate for seed growth
• Seeding prior to a period that could meet the stratification requirements of the species (A common requirement of many native species for breaking of seed dormancy is a cold, wet stratification.)
The seeding window is greatly influenced by temperature and precipitation. Note that seeding is generally recommended just prior to periods when temperatures are low (above freezing) and precipitation is high. Fall and spring seeding windows exist in most areas of the West. Since many of these windows are brief because of precipitous weather, site preparation should be completed shortly before the window begins, thus allowing the maximum amount of time possible for seeding. Areas to be seeded during the spring should be prepared during the fall if possible. Contracts should be awarded and seed materials ordered well in advance in order to ensure prompt startup of seeding operations.
Generally, plan for seeding during the fall. Fall seeding allows utilization of soil moisture recharge during the winter. By the time conditions are dry enough in the spring to allow access for seeding, part of the soil moisture buildup has been lost. Some shrub and forb species require winter stratification for germination. Many native legumes have extremely hard seed coats and seem to establish more readily from fall seeding. The timing of seeding can be used to some degree to manipulate the species composition of the revegetation. Grass seeds generally germinate more quickly and seedlings establish more readily than shrubs and trees. The competition from grass seedlings might limit or prohibit establishment of shrub seedlings. On flatter areas where erosion control is not a major problem, slower growing forbs or woody shrub natives could be sown one growing season prior to more aggressive grasses to enable them to become better established prior to any competition from grasses.
It is important to revegetate disturbances as soon as possible during the first available seeding window following site preparation. The longer the time period between seedbed preparation and seeding, the more susceptible the area is to surface crusting, erosion, and weed infestation. For theses reasons, it is also important to revegetate correctly the first time.
Seed Requirements
All seeds furnished to or acquired by the seeding operator should be those specified in the project plan and should be measured by pure live seed (PLS) weight. The advantages of using seed on a PLS basis are that trash and empty or foreign species seeds do not confuse seeding-rate calculations. All seed should be tested by a certified seed analyst in an accredited seed testing laboratory within three months for grass seed; a maximum of six months for forb, shrubs and tree seed; and six months for all seed crossing state lines. Agency requirements vary. Consult the state seed laboratory in the respective area where the project is located to obtain current information regarding laws governing seed, and location-specific native seed. These regulations are rapidly changing and vary from state to state. Each species should be furnished with a bag-tag, and what is on the tag should be what is in the bag.
All legume seed should generally be treated with a commercial rhizobium inoculate at the time of planting to enhance the development of nitrogen-fixing root nodules.
The seed mix should not contain prohibited noxious weed seed, as listed by state or federal law. Note that no noxious weed seed may cross state boundaries by federal law. Wet, moldy, or otherwise damaged seed should not be accepted. If a specified seed variety is not available, the contractor should be required to consult with the specifying entity prior to any substitutions. All substation approvals must be documented in writing. It is best to use seed relatively soon after purchase and do not store it for any length of time unless using proper facilities, such as those that are cool and dry are readily available. Seed should be kept at room temperature or below but above freezing prior to use.
Seeding Rates
Direct seeding rates for individual species and species mixtures should always be derived on a PLS basis. Testing by a qualified seed laboratory to evaluate purity and germination should have been performed in accord with specified parameters of time to ensure an accurate determination of PLS values.
Seeding rates vary by revegetation zone and species composition. It is important to focus on the number of seeds per square foot or square meter more so than the number of pounds per acre or kilograms per hectare. The kg/ha rate should be derived from the seeds per square meter. A reliable rule of thumb for broadcast and hydroseeding is 480 PLS seeds per square meter for drill seeding, use 320 PLS seeds per square meter. The rate for broadcast seeded areas should be 150% (1.5 times) the normal drill-seeding rate. This allows greater numbers of seed per unit area to allow for poor seed germination and seedling mortality. Do not assume that more is better, the opposite is true. Having too many seeds per square meter is undesirable, creating undue competition among the germinating plants for moisture and nutrient. The cost of native seed is a considerable component of the revegetation program; it is not diminutive by any means.
Species Selection
Selection and use of native species for disturbed land revegetation depends on an integrated consideration of six basic factors: (1) adaptation capability—be certain to use early seral stage, not climax species; (2) ecotype limitations—elevation, moisture, nutrient requirements, shade tolerance, etc.; (3) germination and establishment requirements; (4) functional utility-above-ground portion of plant; (5) functional utility of the below-ground portion of the plant, rooting depth, association, and mineral fixing capability; and (6) seed availability and cost.
Given that most roadside revegetation projects involve use of federal or state funds, it is recommended that the project designer(s) work directly with botanists from the district of the agency near the subject property. It is important that regional botanical experts be consulted when developing plans for revegetation of disturbances. Revegetation specialists generally know many of the tolerances and requirements of native species and can provide valuable information about their proper use in seeding and planting of disturbed areas. Also, these agency personnel will have knowledge of compliance obligations and may be able to assist in this critical regulatory area.
Roadside revegetation projects have unique requirements different from other more generic seeding projects. Typically, they must avoid species that might be palatable to and thus attract wildlife, avoid especially flammable species, avoid species combinations not germane to the existing landscape, and use plants that blend into what is in the surrounding natural environment. Height and maintenance considerations are also relevant for roadside revegetation that may not be of consequence in other circumstances.
One factor that often limits the use of native species in revegetation is the general lack of knowledge concerning their proper use and adaptability. The literature on the use of natives is expanding rapidly and much more is known today than 10 years ago. As the use of native species increases, even more will be discovered about their basic growth requirements and adaptation.
Selection Criteria
Identify which native plants grow in the subject area are suitable for revegetation and base your species selection on data obtained from both the site and soil analysis, this is often a complex matrix as the constraints imposed by the disturbance and the growth medium may limit the options. Use the right plants for the job at hand. Start with early seral-stage colonizer species. Do not plan on putting higher seral-stage or climax-stage species into immature, sterile soils at disturbed sites; this approach will not succeed and is commonly the fundamental reason for failure in revegetation efforts. Only after the microbial community is established and the carbon cycling of vegetative cover process has begun will higher seral-stage plants be capable of establishment.
Early seral-stage plants are capable of germinating and establishing in primitive, undeveloped soil types such as subsoil, and they help build microbial associations required by higher seral-stage plants, which supplant early colonizers with time. Being cognizant of eco-specific adaptations in local plant species is important. Such adaptations may include the ability to accommodate higher levels of micronutrients such as boron and other minerals, or shorter, colder growing season at higher elevations. Pay attention to which native plants voluntarily colonize disturbed sites in the project area; these species might be among the most promising early seral-stage plants for inclusion in the revegetation species list for a given difficult site.
Local Collection
Given that eco-specific adaptations are common to native plants, local collection is highly advisable. This need not cost more if planned for well in advance of acquisition. Correlating conditions as closely as possible at the revegetation site with those of the seed-collection site will yield greater probability of success. Wetland species are often not as adaptive to different soil conditions and elevation, making local collection the optimum choice when success is imperative. Plan ahead. It is advisable to require that the contractor obtain seed for revegetation 60 days from start of contract work. This will greatly alleviate problems of availability and resultant attempts at substitution. Also this provides time for the project proponent to test the seed intended for use on a subject site—assuring that what is on the tag is in the bag. Having this language in the revegetation contract serves notice to the contractor that only quality seed will be accepted, and that quality assurance measures are in place. Most revegetation contractors are honorable and don’t take exception to this effort to insure quality seed.
Seed Availability
A major factor hindering the use of many native species is occasional lack of availability or high cost. With increasing interest in the use of natives since the early 1970s, a definite native plant and seed industry has developed. Numerous companies are now selling both seed and plant material of native species. A list compiled by the BLM and the Forest Service is available for review. These lists might be incomplete, and local directories should be consulted when purchasing seed.
The increase in the availability of native seed demonstrates that with increased demand for natives, many species have become commercially available. With adequate planning, most species could be available for revegetation along roadways and other native revegetation sites. Many of the more dominant native species are available commercially now, and as their use increases, more species will be made available and research and testing will continue to identify and improve selections of natives much the same as was done with many of the introduced species.
Revegetation of Road Cuts to Minimize Erosion
Reestablishing vegetative cover on the cut-and-fill slopes along roadsides mitigates for some of the soil particulate eroding from transportation corridors. Accomplishing this objective presents notable challenges given the conditions of the growth medium, the steep angle of hillside slopes, the absence of organic matter, and the lack of suitable quantities of soil microorganisms. Successful revegetation experiences at roadside slopes with disturbed soils offer instructive evidence of the potential revegetation success obtainable with a well-orchestrated plan. Seeding early seral-stage plants with a demonstrated propensity toward developing the soil microbial community aids in the establishment of deep rooting plants that are generally higher seral stage. Holding soil in place and having grasses act as a bio-filter. Most importantly, this group of early seral state plant species does grow and establish where other, climax species might not at first succeed.
Advances in soils research and technology have led to development of organic soil amendments that stimulate the development of micro flora in soil. Use of such humic and catalyst additives in revegetation efforts greatly speeds the establishment of indigenous vegetation at difficult sites and can literally make the difference between success and failure.
On areas subject to impacts of snowplowing and/or sheet-flow erosion potential, it is recommended that a semi-permeable polymer soil-stabilizing compound be applied to prevent the soil amendment and seed from eroding. This semi-permeable membrane should be flexible and water-insoluble and form a porous membrane in the topmost layer of soil that is permeable to rain and oxygen and won’t impair vegetative growth. This product line should provide surface stabilization in various soil classifications without inhibiting water infiltration.
