Glasshouse competition experiments with Hydrilla verticillata (L.f.) Royle indicate that plants grown from turions are weaker competitors than those grown from tubers, when compared to the widely distributed macrophyte, Potamogeton pectinatus L. These results support an earlier hypothesis about the importance of propagule size for predicting the outcome of plant competition (Grace 1985; Schaffer and Gadgil 1980). Results of outdoor growth experiments indicate that even though Hydrilla plants from turions are relatively weaker competitors, they are able to grow succesfully in an existing macrophyte bed composed of either, P. pectinatus or P. gramineus. During the early stages of Hydrilla invasion into an area of existing macrophytes, native plants may coexist with Hydrilla. However, once the abundance of Hydrilla tubers in the sediment increases, Hydrilla may displace existing plants.
Keywords: Aquatic macrophyte; Competition; Tuber; Turion.