Root growth increased during the early growing season in Spartina alterniflora salt marsh plots. While fertilization with nitrogenous fertilizer did not affect initial growth, a marked decrease in root biomass followed the spring peak particularly where nutrient doses were highest. A sharp reduction in roots occurred in enriched areas covered by Spartina patens, although, as with S. alterniflora, aboveground biomass increased. Roots disappeared during autumn leaving rhizomes as the only part of the plants to overwinter. The maximum standing crop for roots was 0–2 cm deep, for rhizomes 2–5 cm. Net annual underground production was calculated from annual increments in dead matter belowground. Total production, underground and aboveground, exceeds that of any marine vegetation, ranging from 3,900 to 6,600 g m‒2 yr‒1 in S. alterniflora areas and 3,200 to 6,200 g m‒2 yr‒1 in S. patens areas. Fertilization increased production particularly aboveground where dead plant parts are subject to export.