Density‐Dependent Suppression of Invasive <i>Hydrilla verticillata</i> by Two Native Submerged Macrophytes Under Greenhouse Conditions

ABSTRACT Hydrilla ( Hydrilla verticillata ) is an invasive submerged macrophyte that can dominate freshwater plant communities and complicate restoration efforts. Management options may be limited in systems containing sensitive native species, making competitive suppression by native macrophytes a potentially useful complementary strategy. We evaluated interactions between H. verticillata and two widespread native species, Heteranthera dubia and Potamogeton illinoensis , in greenhouse mesocosms using additive‐density and equal‐ratio experiments. Apical tips of the three species were planted into sediment‐filled 19 L mesocosms and grown under winter, summer, or spring greenhouse conditions. In additive‐density experiments, hydrilla root and shoot biomass and relative growth rate generally declined as the initial density of hydrilla decreased relative to the native species. Suppression of hydrilla growth was more evident in the winter experiment, although hydrilla tuber production was also greater during winter. In equal‐ratio experiments, H . dubia showed growth rates comparable to hydrilla, whereas P . illinoensis allocated proportionally more biomass to roots and had lower relative growth rates. Overall, the results suggest that dense plantings of native submerged macrophytes may reduce hydrilla growth under controlled conditions, particularly when combined with prior hydrilla removal. Field studies are needed to determine whether winter restoration plantings can suppress hydrilla regrowth from established tuber banks.