Pyrimidine compounds useful as plant growth regulators have the formula (I): ##STR1## wherein Y is an optionally substituted secondary or tertiary alkyl group containing from 3 to 7 carbon atoms or an optionally substituted cycloalkyl or alkylcycloalkyl group containing from 3 to 7 carbon atoms; R.sup.1 and R.sup.2, which may be the same or different, are hydrogen or a lower alkyl group; A is an optionally substituted heterocyclic group, for example thienyl or pyridyl; n is an integer which may be from 0 to 2 and is preferably 0; and R.sup.3 is hydrogen, an alkyl group containing from 1 to 4 carbon atoms, or an alkenyl or alkynyl group containing from 2 to 4 carbon atoms.