Alligatorweed is subject to an eradication program in Victoria, Australia. In aquatic situations, the herbicides glyphosate and metsulfuron are used. Alligatorweed has been shown to break up soon after the application of these herbicides, resulting in the production of many stem fragments that are viable and capable of downstream colonization, compromising the effectiveness of the eradication program. This paper reports on an experiment to investigate the usefulness of commercially available plant growth regulators (PGRs) in reducing the number of viable propagules produced post-herbicide application. Three herbicide treatments (no herbicide, glyphosate, and metsulfuron) and four PGR treatments (no PGR, aviglycine [AVG], naphthalene acetic acid [NAA], and 2,4-D) were investigated in a factorial experiment. Chemicals were applied to alligatorweed growing in separate aquaria, the resulting stem fragments were collected and counted, and a subset was tested for viability. There was no evidence of PGRs having any effect on the total number of viable stem fragments produced. However, AVG reduced the total number of fragments produced. PGRs in combination with herbicide treatment had an antagonistic effect on the efficacy of the herbicides. PGRs increased belowground biomass of alligatorweed, as well as the number of apical growing tips present. Results indicate that although PGRs, particularly AVG, may be of benefit in reducing the number of alligatorweed propagules produced post-herbicide application, at the application rates tested here there would be no benefit from incorporating them into herbicide control programs for alligatorweed.

Nomenclature: Glyphosate; metsulfuron; aviglycine; naphthalene acetic acid; 2,4-D; alligatorweed, Alternanthera philoxeroides (Mart.) Griseb. ALRPH.