Managing Brackish Impoundments for Widgeon Grass

By R.K. Williams


brackish-mainRuppia Maritima, commonly known as widgeongrass, has long been recognized as a valuable waterfowl food plant. When managed for in coastal brackish impoundments, widgeongrass has proved to be very attractive to waterfowl wintering in North and South Carolina. The vegetative portions of the plant are readily consumed by northern pintails, American widgeon, gadwalls and coots, while the widgeongrass seeds are heavily utilized by northern pintails and blue-and green winged teals.

 Brackish tidal marshes along the coasts of North and South Carolina occur in the zones where tidewater salinities range from 5 parts per thousand (ppt) to 20 ppt. Dominant vegetation in those zones are normally big cordgrass, Spartina cynosuroides. This plant is also well established in many coastal brackish impoundments that have not been properly managed and should be eliminated to as great an extent as possible before optimum management for widgeongrass can occur.

​There are many methods practiced today aimed at eliminating or reducing large stands of big cordgrass in brackish impoundments. One very effective method involves thoroughly draining the impoundment and allowing the area to dry during late winter (February-March). Once dry and with favorable winds, the impoundments should be burned with a very hot burn. Immediately after burning, the area should be flooded as deeply as dikes and water control structures will allow. The area should be kept deeply flooded for an extended period. If the cordgrass problem is extensive, the deep flooding should be maintained throughout the growing season (March-October). This necessitates the sacrifice of the growing season as far as more desirable plant species (such as widgeongrass) is concerned. If the cordgrass problem is not extensive, partial eradication of existing stands can be accomplished by deep flooding for a period of 90-120 days (March-May or June) following a hot burn.

Deep flooding to control cordgrass should result in an interspersion of open ponds and emergent vegetation inside the impoundment. Higher elevations should be colonized by the emergent salt marsh bulrush (Scrirpus robustus) and open ponds should produce stands of dwarf spikerush (Eleocharis parvula) and widgeongrass. The bulrush produces seeds readily consumed by mallards, black ducks and pintails. The spikerush plant is utilized extensively by teals, pintails, American wigeon, gadwalls and coots.

​Once the big cordgrass has been eradicated or severely reduced, the impoundment can be intensively managed for widgeongrass in conjunction with the salt marsh bulrush and dwarf spikerush previously mentioned. Saltwater bulrush will thrive and produce more seed in brackish impoundments in the 3 ppt to 7 ppt range. Dwarf spikerush and widgeongrass will thrive in brackish areas ranging from 5ppt to 20ppt.

​Assuming that the big cordgrass problem is under control, start the widgeongrass cycle with a partial drawdown beginning in late February and March. Continue a partial drawdown while circulating tidewater into and through the impoundment using the properly adjusted water control structures throughout the month of April and part of May. Care should be taken to monitor salinities of tidewaters during this phase of the management cycle. Keep in mind that ranges between 5 ppt and 20 ppt are acceptable for widgeongrass and dwarf spikerush, while saltmarsh bulrush, if established in higher elevations, will do better at the lower end of the salinity scale. This flushing action during the partial drawdown period (late February-late May) will result in shallowly-flooded marsh and saturated marsh soils. A complete drawdown for up to two weeks in late May to early June will allow marsh soils to stabilize for optimum widgeongrass production later in the growing season. The impoundment should be allowed to completely dry out during the complete drawdown due to the potential for acid formation in the upper few inches of marsh soils.

​After the brief drawdown to stabilize marsh soils (late May-early June), water control structures should be set to flood the impoundment with 6-8 inches of water in the range of 5ppt-15ppt. Care should be taken not to flood brackish impoundments with waters above 15ppt initially since salinities tend to increase (in years of normal rainfall) in the impoundment during hot summer months due to evaporation.

​During the remainder of the growing season (until mid-October), water levels should be gradually raised by 4-6 inch increments twice per month, usually on the new moon and full moon tides, to enhance the growth and maturation of the submerged aquatics, as well as the emergent salt marsh bulrush. Water levels can be effectively increased by addition of flashboards to the riser built into the water control structure. Water control structures must be carefully manipulated during the growing season to maximize the growth of desirable food plants. Salinities should be carefully monitored during times of flooding so that water over 15ppt will not adversely effect the growth maturation of salt marsh bulrush.

​Waters flooded into the impoundment exceeding those twice per month additions can be used to effect the widgeongrass circulation. There will be times during excessive rains when rainwater should be allowed to drain from the impoundment over the flashboard risers so as not to dilute the salinity base. Conversely, during periods of drought and higher than optimum salinities in the tidewater, care should be taken not to add water to the area above a maximum of 15pt.

After the growing season (mid-October) it is desirable to hold water levels high as early migrating waterfowl pass through the area. These waterfowl will utilize floating seeds of the salt marsh bulrush and the vegetative portion of widgeongrass plants, which are readily available on the surface of open ponds within the impoundment.

​Around the middle of November, water levels should be gradually lowered to a depth of 12 to 15 inches on the impoundment to allow for maximum utilization of the area by waterfowl through the wintering season.

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