The largest Maryland population of the federally threatened tidal freshwater wetlands legume, Aeschynomene virginica (Sensitive Joint-vetch), is comprised of two subpopulations in the Manokin River watershed. To provide baseline data fundamental to understanding annual fluctuations and long-term trends, annual censuses began in 1991 and 1994 for Taylor Branch and Manokin River subpopulations, respectively, and continued through 2009. In addition, distributional limits, phenology, average plant height, associated plant species, and muskrat activity were recorded to better understand plant and habitat characteristics. Large annual fluctuations in size were common to both subpopulations. The Manokin River subpopulation (2 km in extent) averaged 1,867 ± 268 plants (n  =  16), ranged from 715–4,394 plants, and inhabited marshes (56%) and banks (44%). Subpopulation size increased significantly during the survey period. June rainfall may have contributed to a tripling in subpopulation size from 2005 to 2009, but a significant increase from 1994–2006 was related to some undetermined environmental factor. The Taylor Branch subpopulation (0.6 km in extent) averaged 400 ± 122 plants (n  =  18), ranged from 42–1,797 plants, occurred almost solely in marshes, and did not change significantly in size over the study period. Seed germination occurred in late April, shoot growth through July, and flowering and fruiting peaked by mid-August and early October, respectively. Muskrat effects were negligible for both subpopulations. Although continued subpopulation monitoring is important, sediment accretion and wetland hydrology studies are needed to predict long-term viability of A. virginica in the Manokin River watershed.