Silver nanoparticles (AgNPs) were biosynthesized from the leaf extracts of Lantana camara L. (Lc), Parthenium hysterophorus L. (Ph) and Coleus amboinicus Lour. (Ca), which possess allelopathic effects on the aquatic weed ‘water hyacinth’ (Pontederia crassipes Mart.). Their herbicidal efficacy was physically characterized and assessed. The absorption maxima of the synthesized nanoparticles (NPs) were typical to those of AgNPs according to spectroscopy, and they possessed a face-centred cubic structure according to X-ray diffraction. Among the three biosynthesized NPs, Ph-AgNPs exhibited better stability, with a zeta potential of -32.7 mV, and dynamic light scattering at a size range of 213 nm. There was a significant difference in necrosis on water hyacinth leaves after spraying with a 1 ppm Ph-AgNP suspension as compared to that on leaves sprayed with Lc-AgNP and Ca-AgNP suspensions with the same concentration. The water hyacinth plants sprayed with 10 ppm Ph-AgNPs had the lowest number of leaves, leaf length, leaf width, bud diameter, root length and plant height at 15 days after treatment (DAT). Ph-AgNPs at a concentration of 10 ppm significantly decreased the total chlorophyll content, carbohydrate content, protein content, photosynthetic rate and stomatal conductance in the leaf tissue of water hyacinth at 5 DAT. The phenol content and superoxide dismutase (SOD) activity in water hyacinth plants significantly increased in response to the application of the biosynthesized NPs. This study revealed the feasibility of utilizing a noxious weed, by exploiting its natural chemical properties, to manage by suppressing growth of another difficult-to-control noxious weed. The use of AgNPs biosynthesized from P. hysterophorus leaf extract effectively inhibited the growth of water hyacinth. However, accurate evaluation of AgNPs toxicity requires comprehensive, long-term studies under realistic environmental conditions to understand ecological factors interactions.