That depends entirely on how developed your hedge is: In the early stages of hedge formation, we recommend trimming the plants twice — once in May and then again at the end of June after the lammas shoots have formed. In this way, you are effectively exploiting the potential of two formation periods within one growing season. If the hedge is already fully formed, however, cutting it twice a year is rather cumbersome. In that case, it is better to prune the hedge only once, namely after the appearance of the lammas shoots and at a time when a few overcast days mean that the fresh cuts are not immediately subjected to blazing sun. Finally, if you plant narrow thuja cultivars like "Columnaris" or especially "Smaragd", you will only need to trim the hedge rarely ("Columnaris") or not at all ("Smaragd").