Iris rhizomes should be divided every 3-4 years.
What is a rhizome?
If you’ve dug up a bearded iris, a canna, or a lily of the valley, you’ve dug up a rhizome. What’s a rhizome? It’s a thick, elongated horizontal stem that stores food.
How does a rhizome grow?
Not all rhizomes grow alike. Canna rhizomes, for example, grow underground, while the rhizomes of bearded iris must be at least partly above ground to avoid rot. All rhizomes, however, store food and create new plants by creeping outward from the original plant.
Nodes occur at points along the rhizome. At each node, leaves grow upward, while roots grow downward, anchoring the plant in the soil and drawing up food and moisture.
Some gardeners might confuse “rhizome” with “stolon” (runners). Rhizomes are thick (like bearded iris). Their main purpose is food storage, and they lie partially or entirely underground. Stolons are wiry horizontal stems that lie mostly or entirely above ground. Examples of stolons are strawberries, creeping charley, a number of grasses. As with rhizomes, nodes occur all along the length of the runner. When the node encounters soil, it creates leaves and roots—then continues “running.” The purpose of a stolon is to create new plants.
It’s easy to propagate rhizomes. Dig up all or part of the original plant and cut the rhizome into sections that have at least two or three nodes. Let the cut edges dry then replant at the original depth. Avoid bruising rhizomes, as they may rot if bruised.
If, on the other hand, if you want to remove a rhizomatous plant, be sure to get all of it, or new plants will spring up from whatever is left in the ground. The same is true of stoloniferous plants like creeping Charley!
Common and Less Common Rhizomes.
Common rhizomatous hardy perennials include brunnera, iris (bearded and Siberian), lily of the valley, Solomon’s seal, native switch grass (Panicum virgatum), and some varieties of coreopsis, mint, miscanthus, and ferns.
Less common include hardy bamboo, geum (avens), ginger, pitcher plant, and thermopsis—among many others.
Tropical rhizomatous plants include rhizomatous begonia (e.g., Rex begonia), canna (“canna lily”), and sympodial orchid.
Look around the Demonstration Garden—and your yard—and you’ll probably find several rhizomatous plants!