Growth of Typha latifolia L. and its effects on sediment methanogenesis were examined in a natural organic sediment and a sediment enriched with acetate to a concentration of 25 mM in the interstitial water. The lower redox potential and higher oxygen demand of the acetate-enriched sediment did not significantly impede growth of T. latifolia despite some differences in growth pattern and root morphology. Plants grown in acetate-enriched sediment were ca. 15% shorter than plants grown in natural sediment, but the former produced more secondary shoots at earlier stages, which resulted in similar total biomasses after 7 weeks of growth in the two sediment types. Plants grown in acetate-enriched sediment had thicker and much shorter roots than plants grown in natural sediment. This difference did not significantly affect the release of oxygen from the roots when measured under laboratory conditions, which was 0.12–0.20 mmol O2 g−1 DW h−1. Enrichment with acetate resulted in much higher sediment methanogenesis rates (643 vs. 90 nmol CH4 g−1 sediment DW h−1). Growth of T. latifolia significantly reduced methanogenesis in both types of sediment, but the effect was twice as marked in the natural sediment (34%) as in the acetate-enriched sediment (18%), although in absolute terms the reduction was higher in the enriched sediment. The data suggest that this effect of plant growth was via root oxygen release and its effect on redox conditions. In the natural sediment, oxygen release resulted in a significantly higher redox potential and lower sediment oxygen demand, whereas there were no significant changes in the acetate-enriched sediment. The very high oxygen demand of this sediment probably masked the effect of root oxygen release so that a significant reduction in methanogenesis occurred without any significant increase in the redox potential. This demonstrates how root oxygen release from plants like T. latifolia can significantly alter rates of biogeochemical processes such as methanogenesis, even in sediments with high oxygen demands where this is not evident from instantaneous parameters such as redox potential.