The present invention relates to the use of piperazine compounds of the formula I or of the agriculturally useful salts of piperazine compounds of the formula I as herbicides, where in formula I the variables are as defined in the claims and the description.Moreover, the invention relates to compositions comprising, in addition to auxiliaries customary for formulating crop protection agents, piperazine compounds of the formula I or agriculturally useful salts of I, and to a process for preparing these compositions. Furthermore, the invention relates to certain piperazine compounds of the formula I, a process for preparing piperazine compounds of the formula I and a method for controlling unwanted vegetation which comprises treating plants, their seeds and/or their habitat with at least one piperazine compound of the formula I.