The distribution of plants on flood plains depends on the hydrological regime on the flood plain and the hydrological requirements of the plants. The aims of the present study were to: (1) determine the relationships between germination of tangled lignum (Duma florulenta) and nitre goosefoot (Chenopodium nitrariaceum) and hydrological regime; (2) determine the buoyancy of the seeds of the two species, and hence the ability of the seeds to disperse by water; and (3) inform environmental flow requirements for the two species. Seeds of tangled lignum germinated best on soaked soil and on soil inundated for 20 days. Seeds of nitre goosefoot germinated best on soil inundated for 5 days and on soaked soil. The majority of tangled lignum seeds floated for at least 7 days. The majority of nitre goosefoot seeds sank within 7 days. The results of the present study are consistent with the observed distributions of the species on flood plains. Tangled lignum requires ~20 days of flooding and wet soils following flood recession for optimal germination. Nitre goosefoot requires a few days of flooding and wet soils following flood recession for optimal germination. The inundation requirements for germination of tangled lignum and nitre goosefoot should be considered in the management of environmental flows.