The Rhizome part of the plant – the swollen part – is actually a stem that has been modified by the plant to help it survive in certain conditions. It normally grow very close to the surface of the ground, or such as in the case of irises, on the surface of the soil.
The rhizome is different to a root, because it has leaf buds from which new plants can grow. A root does not have leaf buds in the same way.
The rhizomes of the iris grow on the surface of the soil, because they liked to be ‘baked’ by the sun. If they are buried too deep in the soil, they will rarely grow as well.
If you cut up the rhizome into sections in the autumn or winter, each section will re-grow as a plant the following year, providing that the sections are large enough to each have a bud. The buds are easy to see with lily of the valley and also - if you look carefully - the iris.
One of many beautiful Flag Iris varieties - It grows from a rhizome. -Hardy.
By David Hughes - firstname.lastname@example.org