Solutions (5 × 10‐5M) of humic‐like model compounds, chiefly, hydroxybenzoic and cinnamic acids, were tested for their ability to inhibit the growth of hydrilla (Hydrilla verticillataRoyle). This is a perennial, adventitious plant that has the ability to occupy the top meter of water in sub‐tropical and temperate zones and eliminate benthic or rooted plants through shading. Humic acid and a naturally occurring soils extract have the ability to inhibit the growth of this plant (total organic carbon 0.4 mg/L), and the humic‐like models were tested for comparison. None appeared to be as effective as the natural product. Chelation does not appear to be the explanation for inhibition, though there was a good negative correlation between the degree of growth inhibition and the pKAof the model compound.