Divide at the right time of year, after flowering, when irises become dormant during late summer, reducing the chance of bacterial soft rot. Avoid dividing during winter when irises are trying to survive on stored energy in their rhizomes.

When can you dig up and separate iris bulbs?

Late July through early August is the best time to divide iris, as they are dormant during the summer. They then put on a flush of growth in preparation for winter. This dormant period makes it the ideal time to renovate a clump. Iris, as a general rule, should be divided about every three to five years.

What happens if you don't separate iris?

Over time, clumps of iris become overgrown and they don't bloom well anymore or they stop blooming all together. Old undivided iris are also more prone to disease, insect damage, and root rot. This is one plant that really benefits from being divided on a regular basis - maybe every 3 or 4 years.