By Larry Hodgson
Question: A few years ago, I planted this climbing plant in my garden in Montreal. To my surprise, it comes back every year. This year it produced berries for the first time, but since I don’t remember the name of the plant, I’m not sure it’s edible.
Answer: Yes, it is edible and it’s even a popular vegetable in some parts of the world. Here’s a portrait of a plant some people consider a little-known climbing vegetable and others, a noxious weed.
Your plant is a Chinese yam (Dioscorea polystachya, formerly D. batatas). The small “fruits” borne at the leaf axils along the stem are actually bulbils, small aerial tubers that resemble tiny potatoes. They’re sometimes called yam berries, but, of course, are not berries at all. There are Chinese yam cultivars with bulbils of varying size in the Orient, but the varieties available in the West mostly seem to produce round bulbils of rarely more than 3/8 inch (1 cm) in diameter.
Don’t confuse the Chinese yam with the sweet potato (Ipomoea batatas), a tropical tuber many Americans call yam. True yams are in no way related to the sweet potato, in the morning glory family, Convolvulaceae, but belong to the genus Dioscorea in their own family, the Dioscoreaceae.
Bulbils are only produced in significant numbers when the summer is hot. Thus, Northern gardeners may not see them very often, while in the South, the plant can produce hundreds of them every year.
A Perennial Vegetable
The bulbils are not the main crop, however. In fact, they’re usually considered too small to be of much culinary interest. Instead, it’s the main tuber, found underground, that is so popular in China, Japan, Korea and other Asian countries and is now being promoted as a vegetable for permaculture elsewhere.
It’s a long, spindle-shaped tuber reaching up to 3 ½ feet (1 m) in length and weighing up to 66 pounds (30 kg)! In a temperate climate garden, though, don’t expect anything that massive. There, an 8 inch (20 cm) tuber weighing about 2 to 2 ½ pounds (1 kg) would be considered an excellent harvest! The tuber generally forms at a considerable depth, 20 inches (50 cm) or more, so harvesting it requires a lot of effort, especially since it breaks readily if mishandled.
The tuber is normally harvested late in the fall when the aerial part of the plant begins to wither. In mild climates, it can also be harvested right through the winter. Again, gardeners in mild climates have a distinct advantage when it comes to harvesting tubers: they grow to a harvestable size in just one season, so can be collected annually, while it can take 2 to 4 years, even more, to get a tuber worth digging for in areas where summers are short.
The tubers of other yams (Dioscorea spp.) are poisonous until after they’re cooked, but the Chinese yam is an exception: it can be eaten both raw and cooked. When you peel off the pale brown skin, the floury white flesh, vaguely nutty in flavor, is served in different ways: grated, boiled, mashed, fried and even as yam chips. The tuber is rich in starch and vitamins C and B, but low in fat.
In its native lands, eastern Asia (China, Japan, Korea, Vietnam and the Philippines), Chinese yam is also used as a medicinal plant, although Western studies have not found it to have any therapeutic value.
The Chinese yam is an attractive climbing plant with twining stems that coil around their support such as a stake, a trellis, a shrub or a tree, reaching from 3 to 15 feet (1 to 5 m) in height. The shiny green leaves, about 2 to 4 inches (3–11 cm) long, are heart-shaped and have parallel veins, a feature quite unusual in a temperate plant. They create a charming effect and the plant can therefore be used as an ornamental climber.
Clusters of tiny yellowish flowers appear in the leaf axils in warmer years. They’re insignificant, although they do smell of cinnamon, giving this another common name: cinnamon vine.
This plant almost never produces seeds under garden conditions. It’s dioecious (male and female flowers are borne on separate plants), but usually only male plants are found in gardens, making fecundation impossible. If you see “Chinese yam seed” offered for sale, it will be bulbils, not actual seeds.
Indeed, the plant is most easily propagated by bulbils harvested in the fall and planted immediately or, if kept cool during the winter, in the spring. Also, at harvest, the upper end of the tuber can be cut off and replanted, then will root to form a new plant. It is also possible to take stem cuttings.
This plant is the hardiest of some 600 species of yam (Dioscorea spp.), adapted to temperate to subtropical climates (most others require tropical conditions). Chinese yam is said to be hardy down to 0 °F (-18 °C), and to be adapted to USDA hardiness zones 5 to 10 (AgCan zones 6 to 10), but it is obviously thriving in Montreal, Canada, hardiness zone USDA zone 4 (AgCan zone 5) and does fine in my own garden even further north in USDA zone 3 (AgCan zone 4), so it is clearly hardier than claimed.
However, I must admit that in my climate, I never see such vigorous growth as in the lead photo. Yearly, my 25-year-old plant grows to no more than 4 feet (1.4 m) tall and has never bloomed. Bulbils are rarely produced: at most, 1 or 2 per year. In most years, there are none. I’ve always assumed that my summers were simply not hot enough for this heat-loving plant to produce major crop of bulbils.
Chinese yam adapts to sun or partial shade, even shade, in rich, deep soil that is moist but well-drained. A good winter mulch will be vital in colder regions. Commercially, “seeds” are available in the spring or fall. Plant them about 3 to 5 inches (8–12 cm) deep, especially in cold climates.
If you plant bulbils in the fall in a cold climate, a good thick mulch over the planting area will be necessary. Otherwise, they can be killed by the cold, as happens to any bulbils that fall to the ground without frost protection.
This plant has been shown to be invasive in warm temperate climates because bulbils are easily carried out of gardens by small animals and flowing water (they float) and become established in natural environments. While this is less likely where winters are cold, given the poor frost tolerance of unburied bulbils, the fact remains that, with our warming climate, if you introduce this perennial vegetable to your permaculture bed, it would be wise to make sure you harvest all the bulbils each fall to avoid any risk of dispersal.
A Question of Legality
When researching this article, I was surprised to discover that distributing this vegetable in my country, Canada, is illegal. Yet I see Chinese yam “seeds” being offered for sale on the Internet in my country and the bulbils can also be found in fall in farmers markets and Asian grocery stores, so apparently the law is not being enforced.
Here’s what the Canadian Food Inspection Agency’s page says:
Chinese yam is regulated as a pest in Canada under the Plant Protection Act. Importation and domestic movement of regulated plants and their propagative parts is prohibited.
So, if you already own a plant, you’d be allowed to keep growing it, but you can’t move it or share it.
This plant has been shown to be very invasive in the southeastern United States, but only two states actually ban growing it: Alabama and Florida.
In Europe, growing Chinese yam seems to be legal everywhere and it is even part of the repertoire of permaculture vegetables recommended by several urban gardening organizations. Even Australia, which is generally so strict about invasive alien plants, permits its sale.
Chinese yam: both a very original and ornamental climbing plant and a nutritious vegetable, but its ability to become seriously invasive ought to make you think twice about planting it, even where its distribution is legal.
After the frost kills the vine, burn it & the bulbils is the best way to control the spread. A lot of survivalist grow this plant as a food supply after the Fall of the supply chain of North America. When & if this happen no one knows, but It is listed for off grid living with many other invasive edible plants, & Sun chokes, Figs, squash, okra, cucumbers, potatoes & other fast multiplying fruiting plants.
I am guessing you knew I would come up for this one! Wonderful examination of the “double-edged sword” nature many plants have. Is this a “good” plant, or a “bad” one? I would say it depends on where you are, how much you know about it, AND how realistic you are about your ability to keep from creating a problem with your selections.
Permaculture presents the idyllic possibility of feeding oneself in one’s back yard with very little effort. The fundamental philosophical idea is that plants and land were put here to feed us…and that is it. The biggest flaw of permaculture is that often the “easy to grow” part ignores the gardener’s presence in both a human community AND an ecology of complexly co-evolved (and in many cases utterly interdependent) plants and animals. Nurseries often commit the same error. Having the only criteria for plant selection and introduction be “is it pretty,” or “can I eat it” is also a potential rabbit hole in which you can discover that you are spending your waning years fighting with, and not enjoying your own garden.
“Edible” is very different form “palatable.” Ease of harvest and preparation are important factors too in selecting plants. This one sounds difficult to dig, and even more difficult to remove should I not like it. It is important to me to have the ability to grow many kinds of plants over time and often in one place rather than have one very difficult plant to remove dominate, because my tastes and abilities shift.
I am extremely cautious with any plant that has the ability to easily reproduce asexually, given that the vast majority of my time is already spent trying to control (when I realized eradicate was impossible) of invasive plants on my property introduced by the last owner. This dear little old lady fervently wished she had never planted a cute container plant called Moneywort (Lysimachia nummularia) in the ground. This plant is sold in about half of all containers where I live. It is everywhere on the property, invading my woods and stream banks, having been moved around by the lawn mower, and needing to be beat back four to five times a year…and I am still losing to it. I often fantasize about moving away but cannot afford to do that. My property looks like a battle zone. We have covered, dug, burned, and yes, sprayed. It shrugs. This is the nature of an invasive plant…people love it until they realize they are stuck with it. I write about them because I loved buying and planting plants…once upon a time, and I hope people might learn and make more careful choices.
Invasive plants dupe us because of a phenomenon called the Lag Time: The period of time where everyone thinks a plant is beautiful (and harmless) while the plant is adapting to its new home…before there are enough “escapees” in the environment to begin to reproduce exponentially. Babies are shared among friends and at church sales, and garden writers extoll their virtues. People tend to notice there is a problem…when there are already many seedlings in the wild…and by then it is usually too late and too expensive to do much about it. Most of our invasive plants are coming from gardens like yours and mine.
The plant you are profiling could be considered an “adventive invasive species” and has likely come out of its lag period in locations of the country where it is happiest or was planted earliest, and we should pay close attention when a state considers it a first class threat. Because most humans know (and care) little for the plant world that sustains them in virtually every way, it takes a while before government is made aware of the threats invasive species pose…and it grinds into movement only when the problems presented are impossible to ignore.
Funding for study has to compete with every other political squabble, and then it takes time and careful research for science to tell us there is actually a problem. By this point the plant is so widespread, no one wants to spend the money or effort to actually fight them…who wants to say their efforts were defeated by a plant? So, the plant is dubbed with a sweet name “naturalized” and we move on, and the stacking impacts of these plants then has cumulative impacts that are difficult to study.
Farmers see them, and are criticized for using herbicides in trying to make room to grow food for people who have little understanding of the serious issues they face. Both botanists and ecologists also see the profound impact invasive species have on our natural areas; that certain plants can create domino-like negative effects and acceleration of local (leading to permanent) extinctions of the plants and animals that make up the very identity of every country’s indigenous landscapes. This problem is not just introduced plants, but animals, fungi, etc. These impacts are profoundly impacting the viability and dependability of our living natural resources, such as our forests, and especially sunny grassland habitats whose existence is already heavily pressured by human habitation.
How invasive a plant is depends on a lot of factors: the number that can breed with each other, the ability to get itself around asexually, its capacity to live in a wide range of habitats, the numbers of seeds it makes, a short generation time, how long seeds remain viable in soil and, of course, the difficulty of removal/control. I have personally fought many kinds of invasive species, and the only battles I have won have been recognition before the first plant reproduces. This is why education about the issues and early recognition are so important.
Reading the Burkwood wiki is enlightening on just how difficult the Chinese yam is to remove. These are key phrases that make this a “oh hell no” plant for me: “can tolerate light levels ranging from full sun to full shade”, “can easily spread into nearby riparian swaths and undisturbed habitats.” When the people who have taken the time to deeply study this plant and gather what we know about it say, “due to its swift rate of vegetative growth and prolific rate of asexual reproduction via bulbils, it has the potential to become a major pest plant in the eastern and central United States….” And “The potential for large-scale restoration of wildlands where D. polystachya has become established is probably moderate.” I listen, because good scientists are ALWAYS careful about overstating an issue…but we have become a world of “gotta put my own hand on that hot stove” rather than comparing notes with and trusting our experts. Many assume that since we have Google and plant ID apps we are all experts and can make up our own minds.
If one is opposed to using “chemicals” to remove plants too hard to dig out, one really wants to investigate one’s plant introductions carefully to determine that one is not shooting oneself in the foot.
Are you so enthralled by your new plant that you are willing to have that be the ONLY plant in your garden or fight with your neighbor about it, or maybe see the bird and butterfly life around you decline as a result of your decision?
A lot of well-meaning gardeners will jump to a plant’s defense even if it is a well-known thug or invasive, either because they have not grown it for long, or believe it can be “kept in check” while failing to realize that their ability to “beat it back” wanes with inevitable age and health decline. Some don’t realize that many animals such as birds can transport plants long distances, dropping them into natural areas…or into the neighbors’ gardens and throughout the community.
Plants are clever and subtle about moving themselves around. Plants from somewhere else are carefully brought in without the natural predators that might keep them in check (and then be food for an evolved ecology which then keeps THEM in check). So every invasive starts with a HUGE advantage over plants that evolved over millennia in an environment of built-in “checks and balances”.
This is why it is vitally important to not run down to the nursery or onto the web without thoroughly investigating each and every plant you fall in love with…know thy plant…it is not the easy decision it once seemed to be. We are already in big trouble world-wide by having made a “stew” of novel ecological impacts that we are only beginning to understand. These include everything from driving up food costs, to the spread of human diseases, to extinctions.
As a gardener in my back yard, essentially entertaining myself, I don’t want to be contributing to these problems because of my own ignorance. The fact that no one had ever reported issues with the Chinese yam before 1970 and yet there are many reports of it potentially being the next Kudzu now should make everyone sit up and take notice. I really appreciate that you chose this adventive species to feature, because the most effective and economical response to invasive plants…is to consider their impact before they become widespread. Gardeners need more help with these decisions than they are getting.
Love your comments, as always!
Thank you…that means a lot coming from you!