Have I left it too late to divide my bearded irises?

I’ve held off from dividing the large clumps of bearded iris in my garden this summer because of the drought, but now I’m worried I’ve left it too late. Have I run out of time to do it this year or could I still risk it?

E Ryan, Galway

Prized for its intricately beautiful, sculptural flowers which appear in early summer, this sun-loving rhizomatous perennial slowly forms a dense, spreading clump which should be divided every three to five years to encourage the plant to continue to put on a good flowering display. Typically, this is done roughly six weeks after flowering – so in July or August – to allow the newly divided rhizomes enough time to re-establish properly before growth slows down in late autumn.

[ My bearded irises did not flower: what am I doing wrong?Opens in new window ]

While you’ve left it a little late, I agree with your reasons for postponing. You now have the choice to divide the plants or wait until next year. On balance, my choice would be the latter. Given how dry and parched most gardens are, the freshly divided root systems would almost inevitably struggle to establish themselves well, even allowing for the fact that bearded irises like a warm sunny spot and a very free-draining soil. You’d need to water them carefully and regularly, yet not too much given their dislike of wet soil – a delicate balance that would be both time-consuming and a little nerve-racking.

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If you do decide to go ahead, then you could always hedge your bets by using a sharp spade to slice some viable root sections away from the edges of each parent plant, leaving the original but now smaller clumps in situ. Cut back any leaves to a height of 15cm to minimise water loss, and quickly replant these root sections into pre-dug, well-watered holes, bearing in mind that the rhizomes of bearded irises should be planted at or just below the soil surface. Speed is also of the essence to minimise stressing the plants’ vulnerable root systems. Finish off by labelling these divisions clearly and then generously watering them to get rid of any air pockets and ensure their roots make good contact with the soil.