Studies were undertaken on seeds of the summer annual mudflat species Ammannia coccinea and Rotala ramosior to determine the (1) effects of flooding during late autumn to late spring on dormancy break and (2) optimum temperature for dormancy break. At maturity in autumn, about 65–100% of the seeds of these species were dormant. Seeds of both species buried under flooded and under nonflooded conditions in a nonheated greenhouse germinated to 70–98% at 30(day)/15(night)°C and at 35/20°C the following June or July; seeds required light for germination. As dormancy break occurred, seeds of R. ramosior showed a decrease in the minimum temperature for germination, but those of A. coccinea did not. In another experiment, seeds buried under nonflooded conditions in the nonheated greenhouse were flooded in November, December, February, March, April, or May, and all flooded seeds and nonflooded controls were exhumed and tested in July. With few exceptions, seeds of both species flooded for short (May–July) or long (November–July) periods germinated to significantly higher percentages over a range of temperatures when exhumed in July than did seeds that had not been flooded. In a third experiment, seeds of both species were incubated on moist sand in darkness at 5, 15/6, 20/10, and 30/15°C for 0, 3, 6, 9, and 12 wk and then tested in light at 15/6, 20/10, 25/15, 30/15, and 35/20°C. The optimal temperature regime for dormancy break in seeds of R. ramosior and A. coccinea was 20/10 and 30/15°C, respectively. In the nonheated greenhouse, some dormancy break began in buried seeds of both species during late autumn and winter, and it continued as temperatures increased in spring and/or early summer. The ability of seeds of both species to come out of dormancy during flooding at field temperatures from late autumn to early summer means that seeds are nondormant when mudflats become dewatered in summer.