Disclosed is a method of improving the growth of plants wherein at least one compound of formula (I) wherein A is 2-chloropyrid-5-yl, 2-methylpyrid-5-yl, 1-oxido-3-pyridinio, 2-chloro-1-oxido-5-pyridinio, 2,3-dichloro-1-oxido-5-pyridinio, tetrahydrofuran-3-yl, 5-methyl-tetrahydrofuran-3-yl or 2-chlorothiazol-5-yl group, R is hydrogen, C1-C6alkyl, phenyl-C1-C4alkyl, C3-C6cycloalkyl, C2-C6alkenyl or C2-C6alkynyl; Y is —N(R)(R2) or SR2; R1 and R2 are independently of each other C1-C4-alkyl, C1-C4-alkenyl, —C(═O)—CH3 or benzyl; or together form a group —CH2—CH2—, —CH2—CH2—CH2—, —CH2—O—CH2—, —CH2—S—CH2—, —CH2—NH—CH2— or —CH2—N(CH3)—CH2—; and X is N—NO2 or N—CN or CH—NO2, or, where appropriate, a tautomer thereof, in each case in free from or in salt form; is applied to the plant or the locus thereof.