The carbon (C) and phosphorus (P) dynamics of a stream macrophyte, Justicia americana, were assessed within watersheds of the Piedmont region of Maryland. Little is known about this emergent plant species that is rapidly proliferating throughout many rivers and streams in the mid-Atlantic region of the U.S. Aboveground biomass was collected at three stream locations in Frederick County, Maryland for determination of total aboveground C. To evaluate biomass densities of J. americana across stream sites, patches were measured as percent of total reach area. Leaf decomposition rates were measured for J. americana and two tree species, Ailanthus altissima and Acer saccharum, by placing leaf litter bags in a stream. Aboveground biomass indicated variable macrophyte densities across three streams and that J. americana is a major contributor of autochthonous C in the sampled reaches. Decomposition of J. americana leaves and stems was faster than tree leaves indicating temporal differences between autochthonous and allochthonous C and P contributions to stream nutrient budgets. In both J. americana stems and leaves, the highest rate of TP release occurred in the first 14 days of decomposition. The hypothesis that macrophyte and tree biomass would decompose at different rates was supported. The results also indicated that there are differences in the quantity and timing of C and P release between J. americana leaves and stems. J. americana is an important source of autochthonous C and P to stream ecosystems due to rapid decomposition.