Wild celery (Vallisneria americana L.) has coexisted with the dominant species hydrilla (Hydrilla verticillata (L.f.) Royle) since the resurgence of submersed aquatic vegetation in the tidal Potomac River in 1983. In 1989, particularly turbid, cool, and cloudy spring conditions were associated with a substantial decrease in hydrilla coverage. We measured growth and elongation potential of wild celery and hydrilla propagules under various temperature and irradiance conditions to compare these two species and in part explain the stable persistence of wild celery and the variability in hydrilla coverage. A plant growth experiment was conducted to simulate actual temperatures in the Potomac River during spring of 1986 (plant coverage increased) and 1989 (plant coverage decreased). In the 1989 temperature treatment, final heights of hydrilla and wild celery were unaffected by a 6-C decrease in temperature 2 weeks following tuber germination. Heights of wild celery, however, were more than twice that of hydrilla, and elongation rates of wild celery were greater than those of hydrilla when temperatures reached 17 to 22C. Laboratory studies conducted in complete darkness showed that wild celery tubers germinate at 13C, whereas hydrilla tubers germinate at 15C, and that wild celery elongated to heights twice those of hydrilla. Heights were positively correlated to tuber length. If irradiance is diminished at incipience, differences in tuber reserves and elongation potential may be sufficient to ensure that wild celery can survive when hydrilla is not successful.