Depending on the variety of purpletop vervain, it can grow between 10 to 140 cm in height and 2 m wide in its spread.
The purpletop vervain will bloom from the beginning of summer until the middle of winter, when the first frost arrives. Because of the plant's large amount of bloomage, it can quickly start to look a little disheveled if not pruned regularly. To avoid this, prune every month in the summer. Throughout the rest of the year, consider pruning your purpletop vervain twice each season if it requires it.
During summer, when the plant is in bloom, trim back any spent flowers or withered foliage. If regularly pruning, trimming back by 2.5 to 5 cm each time will help to maintain your plant's shape. Also, pinch tips to encourage more abundant blooms in other areas of the plant. Avoid removing over a third of the plant overall, as this may affect flowering in the future.