Nearly every iris we grow needs digging and dividing every few years. There are a few exceptions, siberians only need dividing if they slow in flower production, spurias almost never need division. There is one iris that is so good at self sowing it seems pointless to divide, Iris pseudacorus. Grown in poor dry soil it can behave like a dwarf iris and is shy to flower or set seed. However, if it is growing in moist soil it can be extremely vigorous and produce copious quantities of seed. When it is in its prime habitat of shallow water it becomes a thug. It can seed over an immense area crowding out nearly every other plant. It is happy in anything from moist soil to nearly two feet deep in water.
I foolishly planted a few select Iris pseudacorus in a pond about twenty years ago. A couple white flowered ones, a yellow with extra large flowers, and a giant form. I had previously planted a pseudacorus with variegated leaves that was eaten by muskrats. I assumed the green leaved forms might be just a tasty and did not actually expect them to survive predation by the pond's resident muskrat family.
Not only did they survive they thrived. I suspect the muskrats are farming the Iris pseudacorus. The pond is a glorious sight in Spring covered in golden flowers with an occasional white flower still making an appearance.
The sad part is, I was warned. Nearly everything I had read cautioned against planting Iris pseudacorus near water. So, even if you have the knowledge it is worthless unless applied.
I do not want to discourage anyone from growing the cultivars of Iris pseudacorus in the garden as there are many wonderful selections available. There are also some really cool hybrids that do not set seed and would be perfect at the edge of a pond or lake.
