Rhizomes are a common plant feature that many gardeners may be unfamiliar with. If you have plants that spread aggressively, whether grasses, flowers, edibles or other plant types, rhizomes may be what is driving the growth. Understanding rhizomes is helpful whether managing existing beds or acquiring new plants. Let’s take a look at what they are, their issues, possible benefits, and how to manage them in our gardens.
What is a Rhizome?
Rhizomes are stem structures that grow horizontally from a plant’s stem tissue, at or just below ground level. As they grow, they form nodes that in turn grow roots that sprout vertical stems, forming new plants while continuing their horizontal growth. The photo above shows rhizomes growing from the roots of Liriope, often called lily turf, an aggressive ground cover originally from East Asia, that is commonly found in Hardiness Zones 4 to 7. The visible nodes on the rhizomes (note that the vertical roots have no nodes) will eventually sprout vertical stems allowing the plant to spread, making it a garden pest for many.
As if their horizontal growth wasn’t enough of a pain, typical management practices are not effective at controlling their spread. Plants that are mechanically weeded can leave pieces of the rhizomes in the soil that have enough stored energy to start new plants from relatively tiny pieces with growth nodes. Clearly, this characteristic makes careful management of rhizomatous plants an important consideration in garden plant choices and management.
Rhizomes are a growth and survival technique for many plants. Here are some examples:
- Grasses: There are many grasses that spread via rhizomes, as well as by seeds. These include Bermuda grass, zoysia grass, red fescue, quackgrass, western wheat grass, Japanese knotweed and others. While Bermuda grass for example, may be welcome as a lawn grass, its aggressive spreading via rhizome can make it an unwanted weed if it spreads into adjacent garden beds.
- Flowers: Some varieties of common garden plants that are rhizomatous include iris, Solomon’s seal, trillium, snake plant, ferns, hostas, certain begonia and milkweed varieties, and many others. If spreading is a desirable feature, these plants can be useful. However, they can also become a management challenge as they spread beyond planned spaces.
- Ground covers: Some examples of ground covers that may s spread via rhizome include lily of the valley, liriope (feature photo), chameleon plant (photo above), vinca, periwinkle and pachysandra.
- Edible plants: Edible plants that grow via rhizome include asparagus, mints, ginger and turmeric.
- Others: Bamboo, hops, lotus, nandina, oriental bittersweet, poison ivy, and many more.
In addition to rhizomes, many plants can also spread via seeds and stolons. Stolons are similar to rhizomes (strawberries spread via stolons for example). They are horizontal stems that grow at the soil surface and include nodes that root and grow vertical stems. With all these growth options, it becomes very clear that rhizomatous plants require care to avoid high management demands and serious garden disruption.
Managing Invasiveness
As mentioned above, managing the growth of invasive rhizomatous plants is difficult. For new plantings, it is a good idea to place them in contained areas like pots, raised beds (preferably with bottoms) or garden beds that are bounded by sidewalks, driveways or your house, unless you want them to spread. Bordering the beds with edging might help, but some rhizomatous plants can grow deeply into the soil if it is their only option for growth. Japanese knotweed for example, can grow several feet deep if its shallow growth path is blocked. Digging and sifting might reduce or even eliminate the problem plant material, but it is a lot of work and is only really practical for small spaces.
Hoeing, tilling, and other mechanical methods offer temporary benefit at best, since any nodes left in the soil will regrow, starting new plants. My Master Gardener group supports a local organization that grows crops for food-challenged families in the community. Their major garden inhabits a section of field that grew Bermuda grass for decades prior to becoming garden space. They tilled the area when they built the garden, and then sheet-mulched the soil a few inches thick. This treatment buried the Bermuda grass for a couple of years, but now, after 3 years of vegetable production, the rhizomes have climbed through the sheet mulch and also entered from nearby grass that still grows outside the garden fencing. The grass has reclaimed most of the garden. We tried to manage it with manual weeding this year, but within a couple of weeks of aggressive manual weed removal, the rhizome pieces left in the soil grew back and continued to disrupt garden growth.
The regrowth made it clear that we needed to change our tactics. The photo above illustrates our new attack. A test section of the garden was string-trimmed close to soil level, then covered with a silage tarp that is held in place with sandbags (this is called occultation). The tarp was put in place in late September and will remain there until spring, the goal being to starve the rhizomes of water and sunlight while heating up the soil, hopefully enough to weaken them further. While this is a new method for us, we are hopeful that it will have an impact on the tarped section next year. Stay tuned.
A final option that should be mentioned is the use of herbicides, including both conventional and organic products. Systemic herbicides, such as glyphosate, can be effective for some rhizomatous and aggressively spreading plants when used according to the label, because they move into underground plant tissues. Other herbicides, including many organic products, primarily affect above-ground growth and typically require repeated applications to weaken established plants. Because effectiveness varies by species, site conditions, and application timing, chemical control is often most successful when used as part of an integrated management approach that also includes physical and cultural methods. Gardeners who choose not to use herbicides can focus on techniques such as repeated cutting, smothering, or long-term exclusion of light, recognizing that these approaches may require significant time and persistence, especially for well-established invasive plants. The Clemson and NC State Extension resources linked below provide research-based guidance on selecting and combining management strategies for a range of weed and invasive plant situations.
Conclusions
The more I learn about horticulture, the more amazed I am at the versatility and adaptability of plant life. Rhizomatous plants that can survive and grow through root systems, rhizomes, stolons and seeds are examples of that adaptability. On the other hand, plant adaptability can be problematic in our gardens. For new plantings, it is wise to understand the growth habits of the plants you select. For areas already hosting aggressively spreading plants, I wish you the energy, patience and insight needed to manage their potentially aggressive expansion. I hope the information in this article and the links listed below are helpful whatever your situation.
Helpful Sources:
“Rhizome,” Master Gardeners of Northern Virginia: https://mgnv.org/plants/glossary/glossary-rhizome/
“Rhizomes,” University of Arkansas Extension: https://www.uaex.uada.edu/yard-garden/resource-library/plant-week/Rhizomes-12-06-2019-Ark.aspx
“Bulbs, Corms, Rhizomes and Tubers,” Penn State Extension Master Gardeners, https://extension.psu.edu/programs/master-gardener/counties/chester/how-to-gardening-brochures/bulbs-corms-rhizomes-and-tubers
“Planting bulbs, tubers and rhizomes,” University of Minnesota Extension, https://extension.umn.edu/how/planting-bulbs-tubers-and-rhizomes
“Vegetative Forms of Reproduction,” Ohio State Extension, https://ohiostate.pressbooks.pub/crpsoil2422t/chapter/3-3-vegetative-forms-of-reproduction/
“Spreading Ornamental Plants: Virtues and Vices,” Purdue Extension, https://www.extension.purdue.edu/extmedia/HO/HO-295-W.pdf