This work constitutes a part of a wider study examining the degradation and release of plant DNA into the environment. Microcosm studies investigated the kinetics of leaf and DNA content degradation in a specific variety of tomato (Admiro) after incubation in sediments over 30 days at 20, 10, and 4 °C. Temperature and microorganisms have been found to play a key role in the decomposition of plant material in freshwater sediment. A two-compartment first-order function fitted well both tomato leaf matter degradation and DNA content mass loss. Genomic analysis indicated that before having been released, an important part of DNA may be degraded inside plant tissues during decomposition in sediments. PCR amplification demonstrated that, after having been released, DNA can both be rapidly adsorbed onto sediment particles and persist as dissolved extracellular DNA in the water column.