Answer: Irises can stop blooming if they are not getting all day sunshine, or if they are given too much fertilizer or are overcrowded (they have to compete for water and nutrients). Digging and dividing them should promote bloom. You may need to dig and replant them to get them to bloom, and you can take advantage of the opportunity to amend the planting bed with organic matter to help with drainage and a steady source of nutrients. Irises grow from rhizomes which elongate on both ends. It is this new growth at the ends of the rhizomes that produce foliage and flowers. When you dig the irises (August is a good time), first cut the foliage down by two-thirds (for ease in handling) and then use a garden fork to lift them out of the soil. Break off the ends of the rhizomes where the new growth is and discard the old center portion. After amending the soil with compost or aged manure, I dig shallow trenches (4" deep and wide) and lay the rhizomes in the trenches, then cover with soil. The rhizomes should be just barely below the soil surface (too deep and they won't bloom for a couple of years). Water them in well and then just forget them. Next spring they will grow and flower for you, provided they're getting adequate sunshine and water.
Best wishes with your garden!
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