VEGETATION MANAGEMENT AND PROPAGATION IN CONSTRUCTED WETLANDS
including WATER TREATMENT WETLANDS
by George Waller
For those involved in developing wetland systems intended to improve water quality, the choice for plant species is not difficult. Wetland projects with other goals require more detailed analysis to make correct species choices. The following article describes the plants and animals (biota) used in constructed wetlands, describes the ecological pressures to which these plants will be exposed, identifies two fundamentally different constructed wetland environments, and provides some suggestions for specifications regarding plant choices, spacing, and first year propagation. The information in this article is based on the author's personal experience of 15 years working with biota for constructed wetlands.
Specified and Ancillary Species -=- Seed Bank Pressure -=- Succession Pressure -=- Pressure from Exposure to Natural Forces
Installers Response to Natural Forces -=- Type of Wetland to be Vegetated -=- Hydrocharacter of Wetland types
Vegetation for Treatment Marshes -=- Vegetation for Enhancement Marshes -=- Plant Acquisition -=- Installing Strategies
Transplant Size and Biological Condition-=- For More Information-=- About the Author
SPECIFIED AND ANCILLARY SPECIES
Biota used for constructed wetlands includes the plants which have been chosen and written in plans and specifications for the project, as well as additional other plant and animal species (ancillary species) which may be installed along with the specified plants to bring about a timely and successful wetland environment. The ancillary species include native varieties of mosquito-eating fish as well as other plants and animals residing in the water column and other compartments of a natural marsh. Among these are open water species of small plants and animals, and species residing in the mud and substrate of the marsh. The ancillary species act as seeds which (hopefully) quickly spread throughout the system.
When an existing marshy area is used as rootstock source for the vegetation specified in the constructed project, the ancillary species are easily transferred along with the waters and mud clinging to the transplanted rootstock. If nursery stock or washed rootstock is used, these ancillary species may be introduced separately.
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SEED BANK PRESSURE
Once the ground is cleared, water is added to the wetland, and specified plants are installed, other species quickly (within hours) invade the marsh on their own accord.. This seedbank invasion (includes plants and animals) is often out of the control of the wetland builder and may cause problems of predation, shading, or competition from non-specified plants.
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SUCCESSION PRESSURE
A related ecological challenge arises from the fact that the installation and invasion of species begins a parade of different environments which will occupy the constructed wetland site until it outgrows its banks or its water supply. Most rapidly immediately after installation, the marsh will show constantly changing species composition and distribution as it occupies available open water and land space. The environmental character of the site will change as plants fill the biological space available in the constructed wetlands, and as the wetland soils and environment build. This changing environment will likely require changing response from the person growing and maintaining the wetland.
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PRESSURE FROM EXPOSURE TO NATURAL FORCES
It is the (often achieved) goal of most users of constructed wetland technology to operate a system which utilizes the natural processes occurring at the project site. The ideal wetland system would be self functioning with minimal human maintenance or manipulation. This goal requires that the system elements of the project be exposed to, and indeed captive to, the natural forces and processes characteristics of the site, including wind, rain, sunlight, gravity, seedbank pressure, biochemical reactions, predation, temperature fluctuations.
During at least the initial one year of active propagation in a newly constructed marsh, there is a fluctuating and sometimes chaotic distribution and abundance of wetland species resulting in an unbalanced ecological environment and increased exposure to these natural forces. This may increase vulnerability of plants installed in a constructed wetland.
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INSTALLER'S RESPONSE TO NATURAL PRESSURES
A successful response to seed bank pressure and changing conditions related to biological succession, is to actively propagate the project site during at least the first growing season after installation of the specified rootstock. During this time mortalities are replaced (sometimes with different species), predation by animals is identified and dealt with, invasion by undesirable species is minimized, water level is manipulated, general good health of the system is monitored and maintained, and adaptive management strategies are implemented. Successful installation of vegetation can be assured only by including in installation contracts, a one year period of maintenance, propagation, and adaptive management.
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TYPE OF WETLAND TO BE VEGETATED:
TREATMENT WETLANDS OR ENHANCEMENT WETLANDS
To help identify suitable species, we can divide constructed wetland environments into treatment wetlands and enhancement wetlands. Some projects are strictly one type of wetland or the other, but most projects have combinations of these two basic types of environments. In the latter case, the different environments can be separate cells in a system, or they can be distinct zones in one cell of the system. These two wetland environments have very different (often incompatible) vegetation requirements.
Treatment wetlands are free water surface wetlands designed solely or primarily to provide the environment which removes pollutants from water flowing through them. These are ponds and emergent marshes following each other in a series. The emergent marshes must provide the detrital build-up or plant structure (thatch) which produces the substrate and/or energy source for the micro-organisms which actually do the biological filtration treatment. The ponds must allow for open water process and the presence of oxygenating subsurface vegetation.
The marsh vegetation must evenly spread and slow the water flowing through the treatment marsh. All of these are strict requirements, any compromise of which will adversely affect the water treatment function of the project. Consequently, the choice of species for treatment wetlands is straightforward and easy to accomplish, based on what has (and has not) worked in a 20 year history of treatment wetlands.
Enhancement wetlands are designed to provide benefits other than water treatment, such as bank erosion control, temperature moderation, habitat benefits, park and recreation benefits, educational benefits, aesthetic benefits, food and fiber production, etc. Mitigation wetlands and other wetlands designed for non-treatment purposes are enhancement wetlands, for vegetation classification purposes. Enhancement wetlands provide a much greater challenge for the engineer or designer who wishes to specify these plants for a constructed project. Stormwater management wetlands can have both elements, but they must by hydraulically separate or in series. Stormwater management wetlands also often require primary settling before water enters the biofiltration treatment area.
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HYDRO-CHARACTER OF WETLAND TYPES
The hydro-character of the site is the key to specifying wetland plants correctly for your project. A look at the hydro characters of the two types of constructed wetlands will help clarify the differences between them.
Treatment wetlands have strict hydrological requirements (slow and spread the flow) which must be present in 100% of effective marsh vegetation area. Failure of vegetation to spread and slow the flow will result in a short circuit and treatment will be compromised. This hydro character, which is critically designed, constructed and maintained, is almost always different from the hydro character present with naturally occurring forms and water, where seasonal flows and short circuiting through a chaotic, patchy, series of wetland forms is more typical.
Constructed enhancement wetlands are most often designed to mimic or utilize the chaotic hydrology resulting from the natural forms and climate of the site. A non short-circuiting homogeneous thatch is only rarely desirable. Water flow is still affected by vegetation patterns, but the design is for specific enhancement goals (including mimicking nature in a mitigation) rather than water quality improvement.
Determining vegetation specification for treatment wetlands is fairly easy. There are really no more than a hand-full of species and only four or five genera that are recommended, and these are very common genera, listed below. Specifying plants for enhancement wetlands is much more difficult, because there are many dozens of species from which to choose, and hundreds of reasons one might want one particular species over another in terms of the goals of the enhancement project or the hydro character of the site. In all cases, the system hydrology must fit the plant species, or the species will not predominate on the site.
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VEGETATION FOR TREATMENT MARSHES
Constructed wetlands for water quality are typically designed with emergent marsh zones and open water zones to accommodate design characteristics such as flow rates and pollution removal targets. These zones have strict and critical functions by design, and only certain plants provide these functions.
For emergent plant zones (less than 3 feet depth), in a treatment marsh, plants recommended include Scirpus acutus (hardstem bulrush) and Typha latifolia (cattails). A mixture of species is desired, which can range from 90% cattails (to maximize carbon source) to 90% hardstem bulrush (for durability and maximum buildup of recalcitrant detrital thatch material). A mixture is recommended because diversity is less vulnerable to failure from environmental pressures.
Typha species are cattails. They are ubiquitous (grow everywhere) in North America, and there are only a few different species in the US. There are most likely intermediate types, varieties, and sporting individuals, but they look very much the same, grow in the same environment, and create a semi-recalcitrant thatch rich in available carbon. Typha propagates in nature by windblown seeds, among other ways. It will normally quickly invade any wetland environment, growing well in mud or shallow water. Typha are quite susceptible to predation, and all parts are edible by humans.
Scirpus are rushes and bulrushes, they are also ubiquitous in North America, but they enjoy a much greater diversity in form and function than the Typha genus. Hardstem bulrush (Scirpus acutus, Scirpus californicus) is recommended for treatment wetlands not only because it creates the recalcitrant thatch necessary as substrate for bio-chemical pollution treatment by bacteria. Hardstem bulrush at maturity is practically indestructible, it tolerates a wide variety of water quality conditions, and it also tolerates a wide variety of water levels, being able to survive intermittent depths of zero (including total dry-out) to over 6 feet, and steady state operating levels of anything less than 3 feet. Hardstem bulrush makes the best thatch, has positive habitat function, and effectively shades out direct sunlight, (helping in algae control and temperature moderation). Scirpus do not propagate by use of windblown seeds, and will not quickly invade wetland areas unless they are installed within the system.
For aerobic open water zones, the use of Sago Pond Weed, Potamogeton pectinatis, is typically used, although any submerged oxygenating plant will likely function in the same manner. Sago Pond Weed creates oxygenated substrate throughout the water column and does not shade out the sun with floating leaves. Sago is some-what difficult to install, but algae will do similar treatment while pond weed is developing. Floating leave pondweed and lemna communities do not afford the same functioning environment as submergent oxygenating plants and cannot be used as alternatives to provide the same design functions, although they are very valuable environments for other treatment functions, such as algae removal, settling.
Ancillary species for all projects include mosquito eating fish, which are the local native fish that exist in wetlands in the area of your project site. It is fairly easy, but of critical importance in wetland construction, to utilize mosquito eating fish, and to encourage bat and swallow habitat, to provide long term balance to mosquito populations. Bats and swallows and Dragon Flies are utilized by protecting or creating habitat. Mosquito fish and frogs are utilized by introducing them during the installation phase of the project, or whenever water is introduced to the system, or ponding occurs at the site for any reason. Mosquito problems have shut down wetland projects.
Other ancillary species can be used to accelerate propagation and to create some interim treatment during the first few years when the newly planted marsh is growing. These include Lemna and Azolla, which will shade out algae in the new marsh. These small floating plants propagate quickly into floating mats, when given space, nutrient, and protection from the wind. Another ancillary species is Hydrocotyl, which can also shade out algae and offer some interim treatment while Typha and Scirpus are maturing. Hydrocotyl also has been observed to protect young Scirpus plants from predation by coots. Coots (Fulvica americana) are duck species which are mostly herbivorous and will eat young Scirpus shoots. Hydrocotyl (and all similar plants) should be used with care because it can dominate into the areas designed for open water, and in some cases it will provide mosquito larval refuge.
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VEGETATION FOR ENHANCEMENT MARSHES
Species for enhancement marshes marshes (and stormwater marshes) cannot be stated with such degree of certainty without close analysis of the intended hydrology of the system and the intended function of the plants and the project itself, but some general principles can be stated. First it is necessary to realize that what we plant is simply the first in a series of successional communities which will exist at the site once we plant it. We may wish to hold back sensitive species until suitable environment has developed or plant pioneering species to produce a better environment.
Details of the hydrologic regime of the site are always the most important clues to wetland species choices, especially in enhancement marshes, where we cannot always use the most forgiving species and where we are often trying to mimic the nature's chaotic hydrocharacter. Details of the hydrologic regime must include average water depth, low water depth, high water depth, duration of time for the low water condition and the high water condition, frequency of the low water- high water oscillations. A design based solely on an estimate of average water depth will likely fail.
To adequately understand this hydrologic regime of your site, it is required to know the rainfall rates and expected storms, evaporation rates, groundwater interactions, soil conditions, the flow patterns of the point and non point sources which will be supplying water to the system, and the characteristics of the inlets/outlets of the system. There are uncountable permutations of these elements and therefore an unlimited number of different hydrologic regimes which might be encountered for an enhancement wetland.
Very few enhancement wetland designers have sufficient biological and hydrological information at their fingertips when they specify plants, and a matter of a few inches of water can determine which species will predominate an area. Because of this, plants are often (always, according to some installers) mis-specified in construction documents. In these cases, installers often must adjust the specified plant choices to fit the actual site conditions or face likely failure.
Wetlands for treating domestic sewage, on the other hand, often have lined systems with controllable point source inputs, and functioning water level controls. In these cases, describing the hydrologic regime is often less complex, the vegetation choices are straightforward, and designers who mis-specify plants in these projects have fewer excuses.
In all cases, the installer must deal with the pressure from existing local biota which will immediately invade the marsh. This pressure can be used to provide a diversity of beneficial species in the marsh; indeed we require certain local species (swallows, insects, bacteria) to establish in the marsh to provide all of the pieces necessary for a balanced, functioning wetland community. On the other hand, this pressure may exert itself and replace plants specified for the project if the specified, installed plants are in water quality regime or hydrologic regime which allows competitive advantage to the local biotic pressure. If Typha is in the area of the project (almost always is) one can expect its invasion into the system.
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PLANT ACQUISITION
Plant acquisition is an issue because of the continuing destruction of critical natural wetlands. The best source for rootstock is from a previously constructed wetland project. Agricultural ponds and regularly dredged flood control ditches can be a source of hardy, locally acclimated rootstock which would otherwise be wasted. The advantages of using this rootstock are twofold: the varieties are guaranteed to grow in your area and any biotic pressure from outside your system will produce the same species which have been designed to be in your system. Established custom growers and nursery folks are almost always responsible, but damage to natural systems by harvesting for profit has happened.
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INSTALLATION STRATEGIES FOR TREATMENT WETLANDS
SPACING AND ROOTSTOCK QUALITY
Specifications usually read 1 to 5 foot centers for vegetation spacing depending on economic constraints, acceptable risks, time constraints, rootstock size, and nutrient levels. If it is desired to have thatch built in minimum time, closer centers are required. If smaller rootstock is to be used (such as seedlings), closer centers are required. To minimize risk of failure, closer centers are required. The cooler and more nutrient poor the water, the slower the rootstock will colonize, and closer centers are required.
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TRANSPLANT SIZE AND BIOLOGICAL CONDITION
Transplants of any size and condition can be used, but are treated differently. Seedlings can be planted in the spring in low water. Clumps of tubers, soil, and died-back tops can be planted in winter. Rooted units of one to five gallon size can be planted year round, but must be carefully secured if planted in late fall or winter. Bare tubers can be planted in mud, but require special care because they can be drowned out or decompose before emergence in the spring. Small transplant forms are usually more susceptible to failure. (Vegetation has been known to float out of position, to be washed out of position, to be eaten by animals, to be killed by overtopping with water for too long a period, to be killed by drought, to rot in place, and to be out-competed by seed bank plants).
FOR MORE INFORMATION
I hope this article has given readers some insights into the issues and details of installing vegetation into constructed wetlands. The information is part of a series presented at an annual short course focusing on using constructed wetlands for water quality improvement. This course is given by the Environmental Resources Engineering Department at Humboldt State University, each year in mid May. The campus is located in Arcata California, where constructed wetlands have treated the domestic sewage for over 14 years, while providing world famous wildlife refuge. This course is directed by Dr. Robert Gearheart, who pioneered the technology and has not stopped working on constructed wetlands for over 20 years. Experienced guest instructors from the industry bring the latest information on constructed wetland developments, and trips to six local full scale treatment wetlands are scheduled. The course is intended for wetland engineers, scientists, and others involved in the planning, designing, and management of wetlands for wastewater treatment and pollution abatement. For more information on this short course, contact: Barbara Smith at (707) 826-3619 or the author at George Waller Wetland Systems, (707) 839-8167.
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ABOUTTHE AUTHOR
About the author: George Waller is a part time lecturer in the Environmental Resources Engineering Department at Humboldt State University, Arcata, CA., and a full time owner/operator of George Waller Wetland Systems, which for over 14 years has been helping public agencies, corporations, individuals, and non-profit organizations develop wetlands and improve water quality. He can be visited at www.northcoast.com/~wallergm or e-mailed at wallergm@northcoast.com