“Why don’t my iris flower anymore?”
This is most likely one of three things, either the plant has got overcrowded, the rhizomes are planted too deep or they aren’t getting enough sunlight.
If you haven’t divided the plant for 4-5 years you’d better put it on the ‘To Do’ list for the coming Autumn. Anytime from July to late September, or even early October if you forget. The important thing is the plant must re establish its roots before the frosts start.
The rhizome MUST be on/above the surface of the soil, you mustn’t bury it.
The rhizome is where the plant stores all its energy & nutrients. It needs sunlight to convert that stored energy into sugars for growth and flowering. So make sure the rhizome is in as much sun as possible, ideally pointing south where it can get baked in the sun all summer.
Shading from weeds or overcrowding in the border will result in lots of leaves but no flowers!
Of course, it may be pest or disease attacking the plant, in which case apply the appropriate insecticide or fungicide. I prefer to use Rose Clear, as one spay will see off most pests and deal with disease such as black spot on the leaves