The perennial submersed noxious aquatic plantHydrilla verticillata(Royle) was treated with well water solutions of methylene blue, hematoporphyrin, and Eosin Y, and compared with control systems. Statistically significant reductions in growth were observed for treatments with all three dyes at initial dye concentrations of 10‐4M. Typically, reductions in growth of 30 percent on a relative basis for the test systems were observed. Methylene blue was adsorbed onto the plant, raising the possibility that shading could be affecting plant growth. Similar adsorption was not noted for the other two dyes. Though two dyes are used as redox indicators, the third — hematoporphyrin — is not but a common mechanism of action may be proposed: that the dyes are involved in photodynamic action. It is possible that natural products may act in a similar manner to manage hydrilla through photodynamic action, and the implications of this hypothesis are considered.