Late summer is the perfect time to lift and divide overcrowded rhizomatous irises - those with fleshy stems at soil level. if you're looking at a crowded mass, often interspersed with grass and maybe weeds and starting to look buried beneath the soil, then this is a job that needs doing. I find it easiest to lift the entire mass and work on dividing whilst it's out of the ground. Pull the grass/weeds away from the rhizomes and roots and separate the plants. You're looking to keep the rhizomes with leaves, the rest are spent and won't flower again. Snap or slice them at the junction between the old plant and new growth - ideally sections of 12-15cm. Then trim the foliage back to a fan of around 15cm to reduce plant stress and encourage strong roots. Replant with spacing of about 20cm with the rhizome sat proud on the soil, and tuck the roots down below into the soil as deep as you can. Give them a watering to settle in.
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