The definition of growing season in the 1987 U.S. Army Corps of Engineers Wetland Delineation Manual is derived from the soil biological-zero temperature concept. Lacking direct information on soil temperatures, minimum air temperature thresholds are used as indicators of the beginning and ending dates for the growing season. The 1987 Manual regional supplements allow for field observations of above-ground plant growth to estimate the growing season period. Since acceptance of the 1987 Manual, the growing season concept has been controversial. Soil biological zero does not apply to large areas of the continental United States, minimum air temperature thresholds appear inconsistent with observations of above- and below-ground biological activity, and photoperiodism and thermoperiodism result in local, regional, and annual variations for determining the growing season period based on plant activity. Additionally, the belief that wetlands perform ecological functions year round supports the argument that defining the growing season is irrelevant. A literature review of the environmental factors that influence above- and below-ground biological activity is presented. Recommendations are made on the use of the growing season concept to support jurisdictional wetland delineation determinations.
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