The invention relates to 2-(3-alkenylbenzoyl)cyclohexane-1,3-diones of the formula I where the variables have the following meanings: R1, R2 are hydrogen, nitro, halogen, cyano, thiocyanato, alkyl, haloalkyl, alkoxyalkyl, alkenyl, alkynyl, —OR6, —OCOR7, —OSO2R7, —SH, —S(O)nR8, —SO2OR6, —SO2NR6R9, —NR9SO2R7 or —NR9COR7; R3 is hydrogen, halogen, alkyl, haloalkyl, alkoxy, alkenyl, alkynyl; R4, R5 are hydrogen, nitro, halogen, cyano, thiocyanato, alkyl, haloalkyl, cycloalkyl, alkenyl, cycloalkenyl, alkynyl, alkoxy, alkylthio, haloalkoxy, —COR10, —CO2R10, —COSR10, —CONR10R11, —C(R12)═NR13, —PO(OR10)(OR11); unsubstituted or substituted alkyl, heterocyclyl, heterocyclylalkyl, phenyl, phenylalkyl, hetaryl or hetarylalkyl; or R4 and R5 together form an alkanediyl chain which can be substituted and/or can be interrupted by a hetero atom; Q is an unsubstituted or substituted cyclohexane-1,3-dione ring which is linked in the 2-position; and agriculturally useful salts thereof; processes for the preparation of the compounds of the formula I; compositions comprising them; and the use of these derivatives or compositions comprising them for controlling undesired plants.