Abstract: The present invention relates to compositions and methods for the regulation of plant growth, especially for the inhibition of the growth of dicotyledonous plants by the use as active substance of quaternary ammonium salts of formula I ##EQU1## wherein R.sub.1 and R.sub.2 each independently represent an alkyl radical having 2 to 6 carbon atoms, an alkenyl radical having 3 to 5 carbon atoms,R.sub.3 represents an alkyl radical of 8 to 14 carbon atomsR.sub.4 represents an alkyl radical of 1 to 2 carbon atoms andX.sup.- is the conjugated ion of a strong acid.These quaternary ammonium salts are new compounds.