Physical and chemical methods of managing invasive plants (weeds) create disturbances that paradoxically often promote these species because weeds tend to have traits that confer competitive advantages over desired species in disturbed habitats. A more holistic and sustainable method of managing invasive plants is to design disturbance regimes to favor desired species over weeds. This study investigated how the biomass of a herbicide-tolerant plant, alligator weed, and its competitors respond to different chemical disturbances over a 2-yr period. We compared the response of alligator weed and its monocotyledon competitors to 16 different herbicide treatments in a blocked 4 by 2 by 2 factorial design. Treatments included broad spectrum (nonselective) and dicotyledon specific (selective) herbicides applied at two concentrations (variable depending on herbicide) and two frequency regimes (three or four applications). Belowground biomass of alligator weed in unmanipulated control plots was 10 times greater than aboveground biomass, highlighting the need to reduce belowground material if control is to be achieved. All herbicide treatments reduced belowground alligator weed biomass when compared with controls; however in the short term (8 d after the final treatment), even four applications at the highest listed concentration were not sufficient to eliminate alligator weed from study plots. Over the long term (15 mo after the final treatment), selective herbicide application resulted in a sustained reduction in alligator weed biomass and an increase in monocot biomass.

Nomenclature: Alligator weed, Alternanthera philoxeroides (Mart.) Griseb