The present invention relates to piperazine compounds of the general formula I defined below and to their use as herbicides. Moreover, the invention relates to compositions for crop protection and to a method for controlling unwanted vegetation. R1 is selected from the group consisting of halogen, cyano, nitro, Z—C(═O)—R12, phenyl and a 5- or 6-membered heterocyclic radical which has 1, 2, 3 or 4 heteroatoms selected from the group consisting of O, N and S as ring atoms, where phenyl and the heterocyclic radical are unsubstituted or may have 1, 2, 3 or 4 substituents R1a; Z is a covalent bond or a CH2 group; R12 is hydrogen, C1-C6-alkyl, C3-C6-cycloalkyl, C2-C6-alkenyl, C5-C6-cycloalkenyl, C2-C6-alkynyl and the like; R2 is hydrogen, halogen, nitro, cyano, C1-C4-alkyl, C1-C4-haloalkyl, C2-C4-alkenyl, C1-C4-alkoxy, C1-C4-haloalkoxy, benzyl or a group S(O)nR21 in which R21 is C1-C4-alkyl or C1-C4-haloalkyl and n is 0, 1 or 2; R3 is hydrogen or halogen; R4 is C1-C4-alkyl, C3-C4-alkenyl or C3-C4-alkynyl; R5 is hydrogen, C1-C4-alkyl, C3-C4-alkenyl, C3-C4-alkynyl or a group C(═O)R51 in which R51 is hydrogen, C1-C4-alkyl, C1-C4-haloalkyl, C1-C4-alkoxy or C1-C4-haloalkoxy; R6 is C1-C4-alkyl, C1-C4-hydroxyalkyl or C1-C4-haloalkyl; R7, R8 independently of one another are hydrogen, OH, C1-C4-alkoxy, C1-C4-haloalkyloxy, C1-C4-alkyl or C1-C4-haloalkyl; R9, R10 independently of one another are selected from the group consisting of hydrogen, halogen, CN, NO2, C1-C4-alkyl, C1-C4-haloalkyl, C2-C4-alkenyl, C1-C4-alkoxy and C1-C4-haloalkoxy; and R11 is hydrogen or C1-C4-alkyl; and the agriculturally useful salts of these compounds.