To produce a large amount of moist-soil plants a high level of soil moisture should be maintained throughout the growing season. Amount and timing of the drawdown can me manipulated to produce the desired amount of moisture.

A slow drawdown is an effective way to conserve soil moisture early in the growing season. In most cases, frequent, complete to partial re-flooding or flushing the impoundment throughout the growing season is desirable, followed by fall and winter shallow flooding to ensure food availability.

The importance of complete water control or the ability to flood and drain impoundments as needed cannot be overstated when managing moist-soil. This is not to say that moist-soil impoundments cannot be successfully managed without complete water control, but management options are certainly increased with the ability to flood and drain when necessary, especially if each impoundment can be flooded and drained independent of all other impoundments. Stoplog water control structures that permit water level manipulations as small as 2 inches provide a level of fine tuning that facilitates control of problem vegetation or enhancement of desirable vegetation. If 6-inch and 4-inch boards are used to hold water behind stoplog structures, 2-inch boards need to be available to facilitate water level management during drawdowns.
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Fast drawdown:
Impoundment is drained all at once. Moist-soil plants start to grow. Then the area is re-flooded for use by waterfowl.

Slow drawdown:
Impoundment is drained a little at a time. Different types of moist-soil plants grow depending on the amount of moisture in the soil. The area is re-flooded for use by waterfowl.

Without the ability to re-flood or irrigate an impoundment during the growing season as needed, it has been our experience that a better plant response is achieved by keeping water control structures closed to hold winter water and additional rainfall, allowing water to slowly evaporate through the growing season. The practice of opening structures to dewater the impoundment during the spring and leaving it dry all summer generally results in poor moist-soil seed production.

Another option for impoundments with partial water control is to conduct an early drawdown and then replace boards to catch additional rainfall that may or may not occur at a rate fast enough to compensate for evaporation and transpiration later in the summer. If adequate rainfall is received, this option can result in a plant community important to waterfowl (e.g., barnyard grass and smartweed). However, if inadequate rainfall results in moist-soil seed production well below desired levels, other options (e.g., disk, plant a crop, etc.) should be considered. As a general rule, desirable moist-soil plants can tolerate more flooding than nuisance plants such as coffeebean and cocklebur, two plant species that can dominate a site and virtually eliminate more preferred species in an impoundment.

The following table shows soil temperatures as they relate to drawdown timing and moisture conditions.