We compared growth and survival performance of the detritivorous amphipod Hyalella azteca fed lab-conditioned leaves of either Typha angustifolia (narrow-leaf cattail), Phragmites australis (common reed) sprayed with the herbicide Glypro®, and unsprayed Phragmites in microcosms. Leaves were from plants varying in time of senescence. Herbicide application advanced plant senescence and death by ~3 months, whereas the other two leaf types senesced naturally. Amphipods grew, on average, ~100 µg d⁻¹, and amphipod growth was positively related to fungal biomass on leaves, suggesting that fungi were a key nutritive source. Average fungal biomass was significantly greater on herbicide-treated, early sensecent Phragmites (317 ± 43 µg ergosterol g⁻¹ dry wt.; mean ± 1 SE) and Typha (226 ± 39 µg ergosterol g⁻¹ dry wt.) than on naturally senescent Phragmites (114 ± 21 µg ergosterol g⁻¹ dry wt.). Leaf toughness, an indicator of unpalatability, was greatest on naturally senescent Phragmites leaves. Even so, amphipod growth, survival, and offspring numbers did not differ among litter types, probably because individuals were mainly eating surface biofilms rather than the actual leaf matter, and biofilm abundance was relatively high in all leaf treatments. We also quantified decay rates of each leaf type, and amphipod (Hyalella and Gammarus pseudolimnaeus) abundance in leaves in a Lake Erie coastal marsh (i.e., drowned river mouth) to relate amphipod performance and leaf properties in microcosms to natural patterns. Average rates of leaf breakdown (~10.3 mg d⁻¹ over 126 d) in coarse-mesh litterbags and amphipod numbers in litterbags were similar among leaf types. Leaf mass decreased by ~20% during October but slowed from November to January when amphipod abundance and water temperature sharply declined. An increase in leaf loss from February to March, when Hyalella numbers and water temperatures (~0.1°C) were low, coincided with river channel thawing and opening, suggesting that ice and sediment movement affected litter breakdown. The ability of amphipods to perform, and fungi to grow, equally well on reed and cattail implies that leaf detritus of these macrophytes plays a similar role in marsh detrital food webs.