The presence of the vegetation and litter in the wetland system is absolutely critical for successful performance, but establishing this vegetation is probably the least familiar aspect of wetland construction for most contractors. In recent years, a number of specialty firms have emerged with the necessary expertise for selecting and planting the vegetation on these systems. The use of such a firm is recommended for large-scale systems if the construction contractor does not have prior wetland experience.
Wetland plants can be established from seeds, root and rhizome material (tubers), seedlings (sprigs), and locally obtained clumps. The use of seeds is a low-cost but high-risk endeavor. Hydroseeding has been attempted for FWS wetland systems with marginal success.
Clumps of existing wetland species can sometimes be harvested from local drainage ditches or other acceptable sources. In these cases, most of the stem (to about 1 ft) and leaves are stripped off and the material is planted in clumps of at least a few shoots. Root and rhizome material can be obtained in the same way. If the lead time is sufficient, it is also possible to establish an on-site nursery so seedlings or clumps are available on schedule for planting. A number of commercial nurseries have been established in recent years that can furnish and plant a variety of species in a variety of forms (e.g., seeds, root/rhizomes, seedlings). When selecting such a nursery, it is desirable to utilize a source with a climatic zone similar to the intended site. Commercial seedlings have been used successfully on a number of projects. On larger projects, the use of seedlings allows the use of existing mechanical agricultural equipment for planting. A mechanical tomato planter, for example can easily be adapted for planting wetland seedlings.
Planting in the spring will provide the most successful results for seedlings, root/rhizome stock, or clumps. The planting density can be as close as 1.5-ft (0.45-m) centers or as much as 3 ft (1 m). The higher the density the more rapid will be the development of a mature and completely functional wetland system; however, high-density plantings can significantly increase construction costs. If planted on 3-ft (1-m) centers, a wetland system in a cold climate will typically take two full growing seasons to achieve expected performance objectives.
Planting seedlings or clumps is the simplest approach, as the green part goes up. Some experience with rhizomes is necessary to identify the node that will be the future shoot. The soil should be maintained in a moist condition after planting seeds or any of these other materials. The water level can be increased slowly as new shoots develop and grow. The water level must never be higher than the tips of the green shoots; otherwise, the plants will die.
Providing the necessary water for initial growth can be more complicated for large FWS wetland cells, as it may not be possible to plant the entire surface in a cell at one time. In this case, with a flat-bottomed cell, planting should occur in bands perpendicular to the flow path and a temporary shallow ridge of earth should be created on the upstream side of the band (if the bottom is sloped toward the effluent end, planting should begin at that end and proceed toward the inlets). Sprinklers or shallow flooding can then be used to keep the previously planted areas wet. If mechanical equipment is used for planting, it is important to keep the unplanted areas relatively dry until planting is complete. If hand planting is used, the entire area can be flooded with a few inches of water. The water depth can be increased gradually as the plant shoots grow until the design level is reached. If the FWS wetland is designed to treat primary effluent, the use of a cleaner water source is recommended for this initial planting and growth period. If the intended influent is close to secondary quality, it can be used immediately. If acceptable agronomic soil has been selected and used as the rooting media for the wetland, it should contain sufficient nutrients, and a preliminary application of commercial fertilizers should not be necessary.
It may not be economical to plant very large FWS wetlands on 3-ft (1-m) centers if the total system area comprises at least 1000 ac (400 ha). In this case, the amount of planting that is cost effective should be done in separate bands extending the full width of the wetland cells, with at least some plantings in each cell near the discharge end. These bands can then serve as source material for the future spread of the vegetation; it may require many years for such a wetland to reach complete plant coverage. In some cases, it may be necessary to protect the newly emergent vegetation from the birds and animals that are drawn to the wetland. These new plant shoots are a succulent and an attractive food source for both birds and animals during the early growth stages. The natural emergence of native plant species can also eventually vegetate large wetland areas if a suitable seed bank is available in the local soils.
If the FWS wetland system has been designed only for treatment functions, the plant species to be used are likely to be either bulrush (Scirpus) or cattails (Typha). Both are hardy plants that can survive some abuse during construction and still be successful. If a portion of the wetland has been designed to provide habitat values it is likely that a larger variety of plants will be specified. Some of these may require special conditions, and a knowledgeable person should be retained for their planting.
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