One of the easiest plants to learn how to propagate is the iris. They’re so easy because the plant does most of its own propagation.
Irises grow from rhizomes, which are fleshy structures that store nutrients for the plant. Rhizomes resemble roots but actually are modified stems; roots grow downward from the rhizome, and leaves grow upward. As the plant grows, the rhizome, which lies horizontally near the surface of the soil, expands and sends more roots down and more leaves up. This is called vegetative propagation. When a clump of iris gets too big, however, it will run out of nutrients if it is not divided. This is when human intervention is necessary.
Irises should be divided every two or three years. To do so, dig up the rhizome; this should be easy because the rhizome is very near the surface. Cut it into two or three pieces, leaving about six inches of leaf on each piece; this is important because the rhizomes need to be oriented correctly when they’re transplanted. Discard the oldest part of the rhizome; allow the remaining pieces to dry for one to two days, then dust them with fungicide. Prepare the new flowerbed by adding organic material such as manure, peat moss or compost. When the new bed is ready and the rhizomes are dry, simply lay them on the surface of the soil. Partially cover them with soil if you live in a very hot climate. Make sure that the leaf side of the rhizome is upward; they won’t grow if they’re upside-down. That’s all you need to do for another two or three years. The transplanted, divided rhizomes will propagate themselves until then by the vegetative method.