Plants with rhizomes, also known as rootstocks (not to be confused with the rootstock in grafting, which refers to the roots lower part of the trunk or stem), can either be a blessing or a curse. A blessing because desirable plants with rhizomes, such as groundcovers, are often vigorous growers, forming a thick mat which means less weeds. Plants grown for their edible rhizomes include potatoes, ginger, and turmeric. Rhizomes can also be cursed when weeds and invasive plants spread by rhizomes.
Knowing the special characteristics of rhizomes helps you not only when caring for plants with rhizomes but also to make more informed plant choices for your landscape.
What Are Rhizomes?
Rhizomes are plant stems, but they differ significantly from upright-growing above-ground plant stems, which is why they are also described as modified stems. Unlike plant stems, rhizomes have more functions than mechanically supporting the plant and sending out new shoots. Unlike roots, rhizomes have nodes. The nodes of rhizomes send out roots into the soil to perform asexual reproduction by vegetative propagation. Often roots grow from the bottom part of the rhizome while shoots grow from the upper portion of the nodes. The thickness, size, and growth patterns of rhizomes vary greatly from plant to plant, yet the basic characteristics are the same.
What Rhizomes Do
Rhizomes are a type of protective mechanism. They help perennial plants survive in adverse environmental conditions such as winter cold, wildfires, and being trampled on by foot traffic and livestock, as well as aid its propagation.
A crucial function of rhizomes is the underground storage of nutrients to provide the plants with energy during the winter. Some plants with thick edible rhizomes, such as potatoes, which store starch and sugars (glucose, sucrose, and fructose), are grown just for those nutrients.
Because rhizomes are well-equipped by nature to protect themselves and reproduce, they spread fast—often too fast. Ten of the worst weeds have rhizomes. Grasses that spread via rhizomes, such as Bermudagrass, are often classified as invasive species.
Rhizomes grow underground or aboveground. Ginger, canna, and snake plants have underground rhizomes while bearded irises and ferns have aboveground rhizomes. Most rhizomes grow in a single layer, but a few are multi-layered or multi-tiered. Often, these are highly invasive plants such as field horsetail.
How to Identify a Rhizome
A rhizome growing vertically, either above the soil or below, makes it relatively easy to distinguish from a true root. In addition, there are two other identifiers. Rhizomes can be dense or running. Dense rhizomes such as ginger have short internodes and they form compact, dense clumps that don’t spread. Running rhizomes such as bamboo have long internodes, and they spread horizontally, which makes them more difficult to control.
Plants With Rhizomes
Knowing which plants spread by rhizomes helps you avoid plants that might be vigorous growers for your space. When the Latin botanical name of a plant contains the word “reptans” or “radicans”, it can be an indicator that the plant has rhizomes.
Landscape Plants With Rhizomes
Plants with rhizomes are a good choice if you want a plant to spread and fill in bare spots in problem areas where other plants will not grow well.
Below are a few popular perennials with rhizomes. With the exception of rhubarb, these plants can spread in a way that requires control if you don’t want them to grow beyond their designated areas.
- Bearded iris
- Canna lily
- Golden hops vine
- Heavenly bamboo
- Pachysandra, namely Allegheny spurge (Pachysandra procumbens) and not Japanese pachysandra (Pachysandra terminalis), which is an invasive groundcover
- Stinging nettle
- Virginia creeper
Weeds and Invasive Plants With Rhizomes
The list of invasive plants that spread by rhizomes is long. Rhizomes make these aggressive plants especially vigorous, as a new plant can emerge from a small piece of rhizome. Eradication is only successful when all the rhizomes are thoroughly removed, which can be challenging to impossible. Solarization is the only mechanical way to get rid of them.
How to Care for Plants With Rhizomes
Some plants with rhizomes require a little extra care, others don’t. The rhizomes of the bearded iris, for example, should not be covered completely with soil. Iris rhizomes are also special in the sense that the rhizome that sends out growth buds for a new iris will not rebloom. It should be removed to make room for new growth, as irises do not like overcrowding and will fail to bloom when they are too packed. Divide iris rhizomes when they become too dense.
To prevent a rhizomatous plant from overgrowing or overwhelming your other garden plants, install a physical barrier in the soil that the rhizome can't grow past, like a container buried underground. If your plant has shallow roots, you can take any plastic container and remove the bottom, which gives your plant more drainage.
Another option to contain them is to deeply spade around the plants to sever and remove any straying rhizomes. This should be done at least twice during the growing season, once in the spring and once in the late summer.
Understanding Underground Stems. University of Arizona, College of Agriculture and Life Sciences.
Rhizomes and Stolons. Oregon State University, Forage Information System.