Boxwoods are constantly pinched to shape and form hedges, and Promalin® application is used to boost their shoot yield. Having good root development is crucial for the healthy development of plants; however, the effects of such applications on plant root development have generally been ignored in previous studies. Hence, this study evaluated the effects of applications on the root growth of two boxwood species (Buxus sempervirens L. and Buxus balearica Lam.) during the summer dormancy period. Pinching and Promalin® (0, 1 000, 2 000, and 4 000 ppm) were applied to one-year-old seedlings at three different times. Results showed that pinching had a negative effect on root development in both species and both years. Root development was lower in pinched plants than in control plants, which were not pinched. Promalin® at 2 000 ppm was found to be the most effective application on the root parameter. Root development was also higher in the second year than in the first year, depending on temperature, humidity, and the duration of sunshine. It is recommended to apply 2 000 ppm of Promalin® and not prune B. sempervirens. However, if pinching is to be performed, application of Promalin® will be useful to improve root development. For B. balearica, applying pinching and Promalin® together will help increase root development depending on the application time (in periods when the temperature is high).