This research evaluated the use of dried distiller grains with solubles (DDGS) as a soil amendment to suppress weeds in container-grown ornamentals. DDGS is a byproduct of ethanol produced from corn, and developing new uses for DDGS could increase the profitability of ethanol production. Adding DDGS to a commercial pine bark potting mix reduced emergence and growth of common chickweed (Stellaria media) at concentrations of 5% (by weight) or greater and annual bluegrass (Poa annua) at concentrations of 10% (by weight) or more. Herbicidal activity of DDGS was maintained in methanol-extracted DDGS. Rosa hybrid ‘Red Sunblaze’, Phlox paniculata ‘Franz Schubert’, and Coreopsis auriculata ‘Nana’ transplanted into potting soil amended with 20% by weight DDGS were severely stunted and nearly all plants died. Plants survived when transplanted into potting soil containing 10% DDGS by weight, but growth was greatly stunted and flowering of rose and coreopsis was reduced. Addition of 20% DDGS decreased the C:N ratio from 90:1 to 24:1 for the potting mix and from 23:1 to 10:1 for a soil. The decrease in C:N ratio resulted in a twofold increase in microbial respiration at 3 d and 14 d of incubation for both the potting mix and soil. As a result of the phytotoxicity observed on ornamentals transplanted into DDGS-amended potting soil, subsequent studies evaluated surface-applied DDGS to suppress weeds. DDGS applied at 400 g·m−2 or less to the soil surface at transplanting did not reduce emergence or growth of common chickweed or annual bluegrass. DDGS applied at 800 and 1600 g·m−2 to the surface of transplanted ornamentals reduced number of annual bluegrass by 40% and 57% and common chickweed by 33% and 58%, respectively, without injury to transplanted ornamentals. DDGS may be useful for reducing weed emergence and growth in container-grown ornamentals applied to the soil surface at transplanting.