Mustard seed meal is a byproduct of mustard (Sinapis alba L.) grown for oil production. Developing new uses for mustard seed meal could increase the profitability of growing mustard. Seed meal of mustard, var. ‘IdaGold’, was applied to the soil surface to evaluate its effect on several common weeds in container-grown ornamentals. Mustard seed meal applied to the soil surface of containers at 113, 225, and 450 g·m−2 reduced the number of annual bluegrass (Poa annua L.) seedlings by 60%, 86%, and 98%, respectively, and the number of common chickweed (Stellaria media L.) seedlings by 61%, 74%, and 73%, respectively, at 8 weeks after treatment (WAT). Mustard seed meal applied to the soil surface after transplanting Rosa L. hybrid, var. ‘Red Sunblaze’, Phlox paniculata L., var. ‘Franz Schubert’, and Coreopsis auriculata L., var. ‘Nana’ did not injure or affect the flowering or growth of ornamentals. In separate experiments, mustard seed meal applied at 225 g·m−2 to the soil surface reduced the number of emerged seedlings and fresh weight of creeping woodsorrel (Oxalis corniculata) 90% and 95%, respectively, at 8 WAT. Mustard seed meal applied at 450 g·m−2 completely prevented woodsorrel emergence at 8 WAT. Mustard seed meal applied postemergence to established liverwort (Marchantia polymorpha L.) at 113, 225, and 450 g·m−2 did not injure container-grown Pulsatilla vulgaris Mill., var. ‘Heiler Hybrids Mixed’ up to 6 WAT and controlled liverwort from 83% to 97% at 6 WAT. Weed suppression with mustard seed meal generally increased as rate increased from 113 to 450 g·m−2. Mustard seed meal may be useful for selective suppression of annual weeds when applied to the soil surface of container-grown transplanted ornamentals.