Effect of artificial shading on growth and competitiveness of Alternanthera philoxeroides (alligator weed)

Terrestrial Alternanthera philoxeroides is difficult to control with current tools. Shading as a control tool depends on the relative shade tolerances of the target weed and co-occurring species. This study examined A. philoxeroides' shade tolerance with inter-specific competition from pasture species. In glasshouse conditions, shade and competition reduced A. philoxeroides growth but did not cause senescence. Grasses senesced in response to shade. In the field, competing species' biomass reduced by over 90% in shade. A. philoxeroides biomass was unaffected by shade, contrasting with glasshouse results and resulting in increased dominance in shade. Connection to unshaded ramets in the glasshouse reduced some, but not all, of the differences between shaded and unshaded plants. Thus, resource sharing likely facilitated A. philoxeroides growth in the field, but may have been less influential than release from interspecific competition. This research suggests shading may have limited use for controlling A. philoxeroides unless shade-tolerant inter-specific competition is provided.