I'm a great believer in “The right plant in the right place”.

Irises need as much sun as you can give them.  

They hate cold wet feet in winter. So don’t plant irises under a tree, or in that damp spot where nothing seems to grow.

They need a South facing spot, such as at the bottom of a wall in a dry soil.

Tip: Planting a single specimen iris? Orient the rhizome to the South so that it is not shaded by its own leaves.

Tip: Planting a cluster of the same iris? Keep all the rhizomes in the centre of the planting, with the leaves in an outer circle. As the cluster grows, the leaves will move outwards, keeping the rhizomes in the centre. When you come to divide the cluster, all the rhizomes in the centre will be the same variety, allowing you to maintain a mass-planting of the same variety.