Why is it that the perfect time to dig and divide your iris bed is in July at the height of the season’s heat? For bearded iris it’s because they go through a dormant period in the summer. This hardy perennial is a beautiful spring bloomer that is virtually immune to diseases. But to ensure health and vigor you should divide your bearded iris every three to five years. If they receive adequate sunlight (at least 6 hours a day) but do not bloom well then it is definitely time to divide.