Why are bare root irises only dispatched in September, October and March?

All of the bare root plants we dispatch grow in our fields here in Shropshire.

Irises need to develop new roots after flowering and the rhizome needs time to swell (grow and store nutrients). By September the rhizomes have filled out sufficiently for us to be sure we are sending the plant at its best. The ground is still generally warm and moist, due to rain, allowing the plant to get off to a good start.