Darkness may induce a secondary dormancy in S. robustus achenes that may prevent their germination when buried or when exposed in heavy shade. Germination of achenes of both bulrushes increases with increasing daytime temperatures and when temperatures fluctuate. Regardless of storage conditions, S. robustus achenes subject to alternate freezing and thawing or constant freezing germinate better than those stored at room temperature. Under proper light and temperature cycles, S. robustus achenes show high germination rates on paper moistened with distilled water. Even small amounts of salts depress germination and seedling growth.
Salinity probably reduces germination by reducing water absorption by the achenes.
Salinity reduces the speed as well as the percentage germination of S. maritimus achenes. No chemicals found in other hydrophytes are known to significantly depress achene germination in either of the two bulrushes. Achenes of the two bulrushes may differ in moisture required for germination. As little as 2.5 cm of water over S. robustus achenes held in pure quartz sand stifles germination for at least 25 days, but the same achenes germinate normally when water levels are lowered to the sand surface. However, more S. maritimus seedlings emerge when core samples of bottom substrate are held under 4-5 cm of water than when they are merely kept moist, although the germination rate is highest when these conditions alternate.