Answer: There are several possible reasons I can think of why this would happen. One is that the soil fertility is low and so a little all purpose fertilizer (eg a granualr 10-10-10 at the label rate) and/or compost applied in early spring and again right after bloom time might be helpful. Another is that the rhizomes may be old and tough and possibly too crowded. Iris usually need to be divided about every three to five years. During that process the oldest rhizomes are discarded because they bloom less than younger more vigorous rhizomes. Perhaps in the beginning you started out with larger older rhizomes and now they are simply in need of rejuvenating.
Q&A Library Searching Tips