Tuesday, March 12, 2024

What is a rhizome?

A brown, horizontal rhizome bearing a pair of whitish nubs (incipient shoots) and clusters of long, white roots.
Mayapple (Podophyllum peltatum) spreads by rhizomes. The two whitish nubs at the node in the middle are the beginning of shoots. Clusters of roots also grow from the nodes.

A rhizome (RY-zome), also called a creeping rootstock, isn’t a root at all. It’s a stem that runs roughly  horizontal under or just above the soil, producing roots and shoots along its length. Slender, aboveground rhizomes, like those of strawberries, are also called stolons (STOW-lons). In either case, they're stems, and they serve many purposes.

Rhizomatous (rhizome-bearing) plants are colony-formers. Mayapple rhizomes, pictured above, grow moderately fast to produce a steadily expanding colony. The compact rhizomes of large-flowered trillium (Trillium grandiflorum) grow much slower, producing closely spaced clumps of plants.  On the opposite end of the spectrum, weedy quackgrass (Elymus repens) and Japanese knotweed (Fallopia japonica) have vigorous rhizomes that quickly give rise to large, rapidly expanding colonies. That’s why they’re hard to manage. Even if they’re pulled or dug up, they can regrow quickly from even small bits of rhizomes left behind.

A set of three photos showing mayapple with its umbrella-like leaves, a clump of large-flowered trillium with several white, three-petaled flowers, and wild strawberry leaves and runners clambering over rocks.
Left: A mayapple colony. Each plant is 12-16 inches (30-40 cm) tall. Center: Large-flowered trillium grows in clumps from slowly growing rhizomes. The flowers are about 2 inches (5 cm) wide. Right: The slender red rhizomes of wild strawberry (Fragaria vesca) are also called stolons. Each is about as wide as a pencil tip. 

Rhizomes have several benefits.

Rhizomes are a form of vegetative reproduction. Compared to flowers and seeds, they’re a faster and energetically less expensive way to grow a population. Rhizomes won’t spread a plant far and wide – seeds are often better at that – but if a plant is growing in a favorable place, rhizomes can increase its numbers quickly, and without the risk of losing fragile seedlings.

Except for stolons, rhizomes also serve as storage organs. As winter approaches, sugars and nutrients are moved underground, forming a protected reserve that can be tapped to begin next year’s growth. Some rhizomes end in tubers, swollen organs specialized for storage. Potatoes are a familiar example, but other plants also have tubers. The small tubers of native enchanter’s nightshade (Circaea lutetiana) detach from their rhizomes in fall and function much like seeds, and the tubers of yellow nutsedge (Cyperus esculentus), also called earth almonds, are the edible but maddening means by which this plant persists.

Two photos showing yellow nutsedge plants with grass-like leaves and branched, yellow flower clusters, and a root/rhizome system with several light to dark brown, pea-sized tubers.
Left: Yellow nutsedge plants. Photo by Howard F. Schwartz, Colorado State University, Bugwood.org. 
Right: The thin, white rhizomes of yellow sedge bear small tubers. Photo by Steve Dewey, Utah State University, Bugwood.org.

Rhizomes have potential drawbacks, too.

Plants that produce seeds or spores combine DNA from different individuals to make genetically unique offspring. The young plants aren’t exactly like their parents or even like each other. In contrast, rhizomes produce genetically identical offspring. All shoots from a common rhizome are the same as their parents and the same as each other. In other words, they are clones.

If that uniform gene combination is adaptive in a certain environment, it’s an advantage. It’s like using the same, tried-and-true recipe over and over again, with great success. If conditions change, though, uniformity can be a drawback. If the plants don’t have the genetic makeup to adapt, say, to warmer or drier weather or shadier or lighter conditions, the population may not survive. Their genetic recipe may not serve them well anymore. Especially in a rapidly or drastically changing environment, plants that reproduce primarily by rhizomes may decline, while plants that reproduce by seeds or spores may survive if a few individuals have the genetic ability to adapt.

How to recognize a rhizomatous plant

In the field, there are several ways to know that a plant has rhizomes. One is to look for spreading growth. The presence of colonies can indicate that rhizomes lie below, although some plants without rhizomes also grow in spreading patches. They may have sprawling stems, for example, or seeds that land close to the parent plant.

Another option is to look underground. If possible and permissible, pull or dig up a stem and look at the root system. Rhizomes, if present, will grow horizontally or almost so. They will also have nodes, places where small, scale-like leaves are or were attached. That’s how to tell rhizomes from roots, which also grow from rhizomes. Some rhizomatous plants also produce aboveground leaves -- see the last section for an example. Wear gloves when you handle rhizomes; some can irritate skin or even cause poisoning if ingested. 

A single, whitish rhizome and clusters of thin, white roots of Canada goldenrod. The rhizome has dark marks at regular intervals that indicate the position of nodes.
Canada goldenrod (Solidago canadensis) has pencil-thick rhizomes and much thinner and more numerous roots.

Two photos, the first showing bloodroot plants with closed, white flowers and lobed green leaves wrapped around the flower stalks; the second showing a thick, orange-red rhizome.
Left: Bloodroot (Sanguinaria canadensis) plants grow in slowly expanding colonies from their rhizomes. These plants, photographed in early spring, will eventually unfold their leaves and open their flowers. Right: A mature bloodroot rhizome is about as thick as a thumb. If cut it will "bleed" an orange-red latex. So will the aboveground parts. The latex is poisonous in large doses. Photo by Joseph O'Brien, USDA Forest Service, Bugwood.org.

A thorough plant guide will tell you if a plant has rhizomes. A plant’s name can be another clue. If the  common name includes “creeping” or “crawling,” it’s a good bet it has rhizomes. Creeping Charlie and creeping bellflower are good examples. Sometimes plants creep by other means, such as low-growing or arching stems that root at nodes where they touch the soil. This kind of creeping habit, though, can be easily spotted above ground.

Scientific names, too, can be revealing. Look at the specific epithet, the second word in a plant’s scientific name, which identifies the species. If you see repens or reptans, from Latin words meaning creeping or crawling, the plant likely has rhizomes. As mentioned above, the Eurasian import Elymus repens, or quackgrass, spreads aggressively by rhizomes. Native Polemonium reptans, or spreading Jacob’s ladder, also has rhizomes, but they grow slowly. The plant also spreads with its sprawling stems and self-seeding habit.

Looking for an easy rhizome to study? Try clover.

Introduced Dutch or white clover, Trifolium repens, is a convenient plant to see rhizomes. Its stem grows just above or below the soil, so it’s easy to pull up. This is the only stem the plant has. The vertical “shoots” are actually petioles, or leaf stalks, and scapes, structures that support clusters of flowers. Notice that the rhizome has nodes, but the petiole and scape do not. 

Two photos, the first showing a mass of clover with clusters of white flowers and the second showing a narrow, red clover rhizome.
A white clover colony spreads by rhizomes. They can grow quickly, forming patches. 

True roots will also come up, and they lack nodes, too. Some of them may have tiny nodules attached. These aren’t tubers, but rather small bodies containing nitrogen-fixing bacteria. The bacteria convert nitrogen gas in the air to a form the plant can use. For more information about that, see The Boon of Biological Nitrogen Fixation.

A white clover rhizome with roots bearing many small nodules.
A white clover rhizome and roots with nodules.

What is a rhizome?

Mayapple ( Podophyllum peltatum ) spreads by rhizomes. The two whitish nubs at the node in the middle are the beginning of shoots. Clusters ...