Compositions for plant growth regulation, comprising a synergistic mixture of at least one quaternary ammonium salt which acts as a growth regulator and one imidazolinyl derivative of the general formula II in which the substituents have the following meaning: R is alkyl; R is alkyl or cycloalkyl; R is hydrogen; hydroxyl; alkyl; alkoxy; alkylamino; dialkylamino; optionally substituted phenyl, phenoxy or phenylamino; R is hydrogen; halogen; alkyl; haloalkyl; alkoxy; haloalkoxy; alkylthio or haloalkylthio; R and R independently of one another are one of the groups mentioned under R or together are an optionally substituted 1,3-butadiene-1,4-diyl group; and inert additives.i