Nowadays substances stimulating growth and development of different agricultural plant species are used in intensive farming. Some of such plant growth regulators are Asahi SL and Moddus 250-EC. So far the literature dealing with growth regulators has concentrated mainly on their effects on deciduous trees. Because no data are available on how those substances affect conifer trees, this paper presents description of experiments on the effects of Asahi SL and Moddus 250-EC on the increase of tree height and branch length of Picea pungens and Thuja occidentalis. The research has proved that morphological features of the plants tested in the experiment varied according to which growth regulator was used and in what month the measurements were taken. Conifer trees sprayed with Asahi SL had a bigger yearly height growth rate and bigger growth of branches than those sprayed with the stimulator Moddus 250-EC.