Disclosed herein are methods for modulating (e.g., inhibiting) emergence and growth of monocotyledonous or dicotyledonous plants (e.g., weeds and grasses) using sarmentine and/or a sarmentine analog and a second herbicide. The application of sarmentine and/or a sarmentine analog with a second herbicide to a plant and/or its growth substrate results in a synergistic pre- and/or post-emergence herbicidal activity against the plant. The present method also provides compositions comprising sarmentine and/or a sarmentine analog, in combination with a second herbicide.