A series of trials to increase understanding of the summer dormancy trait in Phalaris aquatica was conducted. Autumn-sown and younger, spring-sown plants of two cultivars (cvv), known to contrast in expression of summer dormancy, were established and then tested over the following summer under three moisture regimes: long drought; drought + mid-summer storm; or full irrigation. The autumn-sown plants of cv. Atlas PG expressed substantial but incomplete summer dormancy under all moisture regimes and exhibited the characteristic responses including significant growth reduction and herbage senescence. The summer-dormant cv. Atlas PG used 31 mm less soil water over the summer and also began to rehydrate its leaf bases from conserved soil water before the drought broke. The non-dormant cv. Australian grew whenever moisture was applied and also responded to the mid-summer storm with a decline in dehydrin expression in leaf bases, whereas no decline occurred in Atlas PG, presumably because it remained dormant. The irrigated, younger, spring-sown swards of cv. Atlas PG had restrained growth and produced only about 37 % of the herbage of cv. Australian. Drought reduced activity and growth of young plants of both cultivars but while Australian regrew in response to the storm, cv. Atlas PG, grew much less (59 % of Australian) indicating that dormancy, although only partially expressed after spring sowing, was reinforced by summer drought. Australian used more soil water over summer than Atlas PG, and it is suggested that this is why cv Australian is limited to the higher rainfall zones of south-eastern Australia, particularly where soil water holding capacity is limited.