Three cohorts of seedlings of Phragmites australis (Cav.) Trin ex Steud., germinated in May, June and July, were allowed to grow in shallow water (depth 5 cm or less) in southern Sweden. In the autumn, size parameters were measured on the plants. In the second year, the water level was raised to 0.8 m and emergence of shoots, plant survival and size parameters were recorded. The mean plant weight by the end of the first year differed markedly between cohorts. Rhizome biomass showed a relationship of 700:70:1 between the May, June and July cohorts. In the second year, rate of emergence above the water surface, and maximum height of plants that did not reach the water surface, was positively related to the size (mass) the plants had achieved after the first year. Only plants that emerged above the water surface survived the second summer, resulting in survival rates for the May, June and July cohorts of 90%, 68% and 0%, respectively. The rhizome weight of the smallest survivors had decreased after the second summer compared with values after the first summer. Hence, they were not capable of ‘reloading’ their rhizomes during the second year. In a temperate climate, the size of juvenile plants after the first year, which is strongly dependent on early germination on exposed bottoms (i.e. bottoms without standing water), determines their water depth tolerance during the second year. The timing and duration of exposure, as well as the subsequent depth of re-flooding, are all of fundamental importance for successful ‘lakeward’ seedling expansion of P. australis.