Managing Wetlands: Moist-Soil Management (Seasonally Flooded Impoundments)

Moist-soil management refers to management of land to provide moist soil conditions during the growing season to promote the natural production of beneficial plants. Seeds produced by these plants often attract and concentrate waterfowl and other wetland wildlife species. The decomposing vegetative parts of moist-soil plants also provide substrata for invertebrates, which are critical food for many wetland wildlife. Factors that determine the success of moist-soil management include: the timing and rate of the dewatering; soil disturbance and the stages of plant succession; and the timing and rate of reflooding. Best success is achieved when water levels can be controlled, although good results can be obtained under natural conditions when artificial draining and flooding are not possible. The following are techniques used in moist-soil management:

• Hold water on the land until late spring, generally early May through June. This will help prevent some undesirable plants that need dry conditions to germinate and will provide extended habitat for wetland wildlife. Drawdowns (dewatering) early in this time period tend to produce smartweeds, while mid period drawdowns often produce wild millets. Late drawdowns in this time period usually result in beggartick, panic grasses and crabgrass.
• Drop water levels at a slow rate - generally about one inch a day -until the marsh bottom is exposed. Slow drawdowns prevent soils from drying too quickly and produce desirable plants over an extended period. Slow drawdowns usually produce a greater diversity of plants, while a fast drawdown (dewatering in a few days) produces extensive stands of similar vegetation. Fast drawdowns late in the season tend to dry the soil too quickly and often result in less desirable vegetation. The stage of plant succession in a marsh unit will also affect the plant species composition following a drawdown Perennial plants like rice cutgrass and marsh smartweed tend to increase after three or more years without a soil disturbance (discing). Many of the high seed producing plants like wild millet and large seeded smartweed occur in units in the first few years following a soil manipulation.
• Start reflooding the moist-soil unit from early September to mid-October depending on your particular objectives. A slow, continuous flooding of the unit will provide optimum feeding conditions for many wetland species. Flooding depths are critical to successful moist-soil management. Many species of wildlife including most ducks prefer water depths of less than six inches and as little as two to three inches. Flooding marshes too deep is not only costly but often puts food too deep for wildlife to utilize.

The above drawdown procedure should produce a variety of moist-soil plants, including: smartweeds, wild millet, rice cutgrass, beggarticks, and many others. During long summer dry periods, shallow reflooding (irrigation) will stimulate moist-soil plants and can even kill or set back undesirable plant species, such as cocklebur and morning glory. Moist-soil plants are not adversely affected by summer flooding, as long as one-third of the growing plant is out of the water.

 

Common wetland plant species that serve as important waterfowl foods include: above left, beggartick; middle left, nodding smartweed; lower left, large seeded smartweed; above, wild millet

Some moist-soil plants can become too dense or reach heights which make them unavailable to many wildlife species. Midsummer mowing to a height of 18 inches will make these areas more attractive. Mowing native plants that have already produced viable seed is no longer considered baiting for waterfowl.

Most desirable moist-soil plants occur in early stages of plant succession in a marsh community. Seed production for these plants is also usually highest in the early stages. Although units vary, most marshes should be disced once every three to five years to set back plant succession. This discing will also help control woody plant invasion and can be used to control undesirable plants like cocklebur. When possible discing should be rotated over a unit so that all parts have been disced in a three-to five-year period. A late summer discing followed by a shallow flooding is especially attractive to shorebirds and early migrant waterfowl. Invading woody plants and other undesirables can also be temporarily controlled by mowing. Mowing is especially effective when desirable moist-soil plants occur under rank cocklebur. The mowing will remove shade and competition, and moist-soil plants will begin growing rapidly, especially if irrigated. Remember that mowing is usually only a temporary cure, as most woody plants will resprout after being mowed. Discing the ground is a better practice, because it will help remove woody plants and encourage moist-soil plant production.

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