Back: Table of Contents | Planning A Native Plant Project
PREPARING THE SITE
Before reintroducing native plants into the restoration site, it is important to ensure that the site conditions are what the plants need to grow and survive. If the site's soil is too compacted, has the wrong moisture levels or there are thriving invasive species, it will be difficult to establish a successful, self-maintaining native plant community. These are problems that should have become apparent during the site evaluation phase of the project. This section addresses some of the more common problems that restoration projects should address before planting or seeding begins.
One important factor to consider when making any kind of site modification is the importance of site complexity. Ensuring that the site is complex means that there will be a greater diversity of habitats available for species to use. For example, when preparing the site for planting, instead of smoothing out the surface of the soil, it may be desirable to purposely create a rough site. This can help to create little shallow depressions, which can act as microsites that capture moisture and protect small seedlings that are germinating. On a larger scale, complex sites may have a few different kinds of habitats such as a wetland, meadow, and a forest adjacent to each other. This diversity of habitats may be important to some types of wildlife that like to forage in the meadow, require the wetland for drinking water, but use the nearby forest to hide from predators.
A complex restoration site has a much greater chance of supporting a higher level of biodiversity and is less likely to suffer from catastrophic losses due to unexpected disturbances. Nature itself is complex and unpredictable. Diversity will help to create a site that functions as much as possible like a natural site so that it can be resilient to environmental stress. Every possible contingency cannot be anticipated when designing a restoration project. However, creating a complex restoration site will give the project a greater chance of success in becoming a self-sustaining diverse community of native species.
Noxious Weeds and Invasive Plants
The greater the amount of weeds that can be removed from the site prior to planting, the greater the chance that the restoration project will succeed. The most effective weed removal methods will differ for various types of weeds. Sometimes a combination of two or more of the removal methods is most effective. For example, one commonly used combination of weed control for herbaceous weeds is to shallowly till the site, allow the weeds to begin growing, spray with glyphosate, wait ten days, then spray again if weeds resprout. Also note that if an herbicide is used, a non-residual should be chosen to prevent detrimental effects on the site later.
Invasive plants are always a problem and the site will need continual monitoring. In one of the most monitored restorations - Curtis Prairie in Madison, Wisconsin - the invasive weeds that were a problem 55 years ago continue to be a concern today.
Below is a list of most of the typical methods used for invasive plant and noxious weed removal:
- artificial mulch: plastic, landscape fabric, cardboard, newspaper
- biological mulch: hay, wood chips, compost
- cover crop: growing a sterile (non-reproducing) annual that will outcompete weeds then die off
- hot water: pouring hot water over noxious weeds or invasives
- pre-emergent herbicide (prevents seeds from germinating)
- post-emergent herbicides (kills plants after they begin to grow)
- contact: kills the surface part of the plant that it comes in contact with
- systemic: affects the whole plants' internal biochemical pathways
- selective: only harms specific plant types (e.g. only broadleaf grasses)
(changing an ecological function to discourage weed growth):
- change disturbance patterns
- change available nutrient levels
- change soil pH
Reduced Soil Function
Often in disturbed soils there is a loss of nutrients, soil structure or ability to retain moisture. In these cases soil amendments can be used to repair these damaged soil functions. Typical soil amendments include topsoil, organic fertilizers, peat and lime.
A common concern in restoration sites is soil nutrient availability. If there has been excessive removal or disturbance of topsoil, there may not be sufficient levels of nutrients to support a native plant community. Soil amendments can add nutrients to the soil. However, before doing so, a good understanding of the typical nutrient levels in an undisturbed soil of that area is necessary. Some native plant communities are adapted to low nutrient levels and the addition of nutrients may encourage the invasion of weeds. In that case it may even be desirable to reduce the amount of nutrients available in the soil in order to discourage weed growth. For example, accumulated organic matter as a result of management may need to be raked from the site.
If the topsoil has been removed from the site or damaged, one option may be to replace topsoil on the site. This topsoil can be salvaged from the site prior to disturbance or removed from another site prior to its disturbance. When acquiring topsoil, keep in mind that a major value of topsoil is in its living component - the microbes and invertebrate animals that cycle nutrients, maintain the soil structure, and aerate the soil. Topsoil should be stored in small, shallow piles that allow air to circulate through the soil. It should also be used as quickly as possible to prevent organism mortality or soil loss due to erosion. In addition, in order to preserve soil structure, topsoil should not be salvaged when it is very wet or very dry. Moving excessively wet or dry soil will cause it to lose its soil structure, reducing soil aeration and killing off soil organisms. If the topsoil is from a different site, it should also be checked for weeds or weed seeds before it is transported to the restoration site. Topsoil with a lot of weeds should not be used (Colorado Natural Areas Program 1998).
If topsoil is not available or does not seem like the most appropriate option, but there is still a need to improve soil function, a commonly recommended method is to add some type of organic matter. Organic matter such as biosolids (sewage sludge) or compost can provide a slow release of nutrients while helping the soil retain moisture and develop soil structure. The slow nutrient release helps to prevent a sudden flush of nutrients that could encourage weed invasion. Slow nutrient release also means that the nutrient levels will be maintained for a longer period of time, giving the new ecological community time to establish its own nutrient cycle.
Using commercial fast-release fertilizers is not usually recommended to amend the soil. Typically these fertilizers will add too many nutrients at once, leading to an increased invasion of weedy species and possible fertilizer burn of young native species.
If the soil pH is not appropriate for the desired plant community, there are soil amendments that can be used to alter the pH. Copper sulfate, elemental sulfur, peat, shredded pine bark or pine needles can be used to reduce the pH (increase acidity). Addition of organic amendments, such as biosolids, will also decrease soil pH as they decompose. If the soil is too acidic, lime can be added to raise the pH.
Restoration of a native plant community on a site that had previously been contaminated can pose additional complications. If the removal of contaminants has changed the soil structure or soil chemical properties, soil amendments may be required before a vegetative cover can be established. Several of the variables to check for have been previously outlined. They can include soil pH, electrical conductivity (EC), and soil structure.
In some cases, compounds can be added to remediate a soil that will alter soil pH. Bringing the soil pH back to normal levels, either through the addition of lime (to increase pH, or increase alkalinity) or elemental sulfur (to decrease soil pH, or increase acidity), may be necessary. If high rates of material are required to correct soil pH, the soil solution may be too salty to permit plant growth and a waiting period that includes rain events or irrigation may be required. Salt concentration of the soil is measured using a conductivity meter. These are similar to pH meters and are easily used. Generally, an EC measurement of greater than 2 deciSiemans per meter is an indication of a potential salt problem. If remediation has disturbed the soil structure, addition of organic matter (in the form of manure, biosolids or composts) will rapidly improve soil physical properties.
In many cases, soil remediation will involve addition of large amounts of organic material or lime. Under these circumstances, plants that would normally be appropriate to use on a site may not readily establish. If surrounding naturally occurring soils are acidic, low in organic matter or nitrogen, plants adapted to these conditions are not appropriate for restoration. To provide a temporary cover until soil conditions return to what would be considered normal for the site, an annual cereal such as rye or wheat grass may be appropriate. If the soil needs to remain calcareous as part of the remediation, it will be necessary to identify plants that are adapted to highly alkaline soils as opposed to acidic soils.
If the soil had been contaminated with an organic material, it is important to determine if the compound is toxic to plants (phytotoxic), and if so, if it is toxic to all plants. If it is not phytotoxic, then there should be no additional obstacles to revegetation. However, if particular plant species have been used for remediation, then removal of these species and replacement with tolerant natives will be required.
If a phytotoxic compound such as an herbicide is the source of contamination, it may be possible to establish a plant cover on the site if the herbicide was specific to a particular family of plants. In this case, selection of appropriate species will be limited to those that are not affected by the herbicide. As the contaminant is broken down in the soil, volunteer species of the targeted families may start growing.
While some native prairie seed needs a firm seed bed in order to establish well, most soils that have been compacted to the point at which it is difficult for plant roots to penetrate the ground need to be loosened. This is usually accomplished through the use of a sharp metal implement such as a ripper or a chisel that can be attached to the rear of a tractor and pulled through the compacted soil. However, this method will only provide temporary relief from soil compaction. Repeated plowing to reduce compaction on agricultural fields will generally increase soil compaction over time. This method should be used with caution. If it is used in a site with a lot of rhizomatous invasive weeds, its use can spread the rhizomes, and thus the weeds, making weed control more difficult. Also, some mechanical methods of reducing soil compaction can have a detrimental effect on soil structure, making the soil more susceptible to erosion or less able to provide the necessary nutrients and moisture. Addition of organic matter with its accompanying soil fauna and plants to the soil will reduce compaction, as well as improve soil structure. This is the preferred alternative to simply plowing the soil.
If soil erosion is an issue at the site, there are some structural adjustments that can be made. If previous disturbance at the site has resulted in unnaturally steep slopes, the slopes should be regraded to decrease the chance for soil erosion. Ideally, to minimize soil erosion, slopes should be complex (rough and uneven, instead of smooth) or concave on the surface, low gradient, and short in length. Complexity is not only valuable in minimizing soil erosion, but increases the site's chance of supporting a diverse ecological community.
If steep slopes cannot be regraded or there is still some soil erosion taking place, then physical barriers to slow or stop the loss of soil can also be used. There are many types of erosion control mats available that can be placed on top of the bare soil to keep soil in place until vegetation can reestablish itself. It is best to use ones that are made with some type of natural biodegradable material such as coconut fiber, straw or wood shavings that will decompose after a few years. In addition, using physical barriers such as hay bales or silt fences can help prevent soil from being washed off the site. Straw of hay should be certified free of weed seed to avoid introducing weeds to the site. Mulching with weed-free straw or local native weed-free hay which contains up to 2 pounds of local native seed per bale can also help minimize soil erosion. Installation of such control measures may be required due to local ordinances for on site work to proceed, please check for this when planning the project.
If a long period of time is required for the native vegetation to reestablish itself, another method to minimize soil loss would be the use of a temporary cover crop. Usually some type of sterile non-persistent member of the grass family specifically developed for this purpose (REGREEN, Barley, cultivated oats) can be quickly seeded and established on the site but will die off in a year or two when the native vegetation begins to take hold.
While the hydrology of the site is important in any restoration project, it is especially critical if the restoration involves some type of aquatic ecosystem. Many wetland plants are adapted to specific degrees of soil saturation, water depth, and duration and frequency of inundation. If the current hydrology of the site does not provide the conditions necessary for the desired plant species, it will need to be altered.
Altering the hydrology of the site is a complex undertaking and should be done with the assistance of a professional hydrologist. It should also be kept in mind that a successful long-term restoration project is one that can function as a natural system with a minimum of continuous human intervention. Consequently, if the hydrologic system can be altered by taking advantage of natural patterns of water flow, this would be preferable to alterations which require construction of artificial flow control structures.
One simple way to alter hydrology is to reshape the contours of a site. Gently sloping shorelines surrounding lakes or wetlands are preferable to steep slopes. A gentler gradient means larger areas can be created that have similar levels of soil saturation or water depth, allowing a larger area for certain plant species to establish themselves. Steep slopes make it more difficult to identify the particular microelevation where planted plant species will be able to grow and survive.
Often at a wetland restoration site, the hydrology had been previously altered in some way to make the land available for other uses. Common examples of this are wetlands that were drained or river flow that was diverted so that the resulting drier land could be used for agriculture. In order to restore the original hydrology of these types of sites, these flow alterations must be reversed. This could include rediverting flow of a river or stream back through the wetland, removing flood control structures or removing artificial drainage systems.
For more management intensive projects, water levels in the wetland can also be controlled through the construction of a water level control structure. However, it should then be kept in mind that there should be some long-term plan for maintenance of that control structure.
Finally, when altering hydrologic patterns at the site, it is important to take into consideration periodic flooding events. Flooding is a natural part of any aquatic system. Its occurrence should be expected and perhaps may even be necessary for the success of the restoration project.
Detrimental Disturbance Patterns
As was explained in the section concerning site evaluation, disturbances can be either detrimental or beneficial to a native plant community, depending on the requirements of that particular community. If a native plant community is to be reestablished on the site, disturbance patterns that will harm these plants will need to be eliminated as much as possible and disturbance patterns that the community requires will need to be reinstated. Try to minimize the amount of ground disturbance for each project, especially in areas such as alpine tundra where microbiotic soil crusts and alpine plant communities take a long time to recover.
Next: Planting The Site