The hydrilla variety found in Washington will also make tubers in the spring and will produce nondormant turions throughout the growing season. Tubers and turions can withstand ice cover, drying, herbicides, and ingestion and regurgitation by waterfowl. There are two varieties of hydrilla in the United States. In New Zealand, where hydrilla has also been introduced, the hydrilla plants are all male. Generally the northern-most populations of hydrilla in the United States are monoecious.
Monoecious hydrilla looks and grows somewhat differently than the southern female populations.
It tends to have a delicate appearance and sprawl along the lake bottom. The tubers from these monoecious plants are smaller than tubers produced by their southern female relatives. Management Hydrilla is not being sold today, but it was recently introduced to California as a contaminant of water lily rhizomes. The hydrilla discovered in Washington was growing in two interconnected privately-owned lakes near Seattle. Because introduced water lilies are common in these two lakes, we suspect that that water lilies may have been the method of hydrilla introduction to these lakes.