Friday, December 02, 2011
Farm Bulletin: Roots and Tubers
Books on food take pains to point out that the potato (Solanum tuberosum) and the sweet potato (Ipomea batatas) are not closely related. In a true/false test, the books are correct. The potato belongs to the large, economically important family that includes tobacco, tomatoes, eggplants and peppers, and many important medicinal plants. The sweet potato is a member of the morning glory family, and is the only food plant in a family better known for its weedy and toxic members, a belle among ne'er-do-wells and brigands. Moreover, the potato is a tuber that develops from the stem, whereas the sweet potato's tuber is a root. But the lack of a relationship is a dull thought, a conversational dead end. Certainly, there are something that links them if, at least since the time of Gerarde's Herbal (1597), both have been called potatoes.
The Eurasian roots on our table evolved from temperate biennial plants—plants that produce vegetative growth during the growing season of the first year, and in the early spring of the following year produce a flowering stalk for seed production. The root stores the energy and minerals needed for flower and seed production, nothing more. When the seeds mature and disperse, the original plant is dead. All of these plants are perpetuated by seed alone and, because they cross pollinate, each generation has a new combination of genes.
In marked contrast, the American cultivated "roots" are perennial tuberous plants, mostly originating in the tropics, and they are scattered hither and yon among various plant families. The tubers allow the plant to enter complete dormancy during dry or cold periods, and resume growth when conditions are favorable. Although they will produce seed, cultivators perpetuate the variety by replanting the tubers, called clonal reproduction, and each generation is substantially identical to the previous ones.
Peace Seeds in Corvallis has done a fair amount of work promoting oca for the Pacific Northwest.
The cultivated perennial tuber that has its origin outside of the Tropics of Cancer and Capricorn is Helianthus tuberosus, the Jerusalem artichoke, a member of the Asteraeae. This is the one American tuber that grew in New England at the time of the Mayflower's arrival and most certainly graced the table of the ship's survivors, yet it is unlikely to find its way onto Thanksgiving menus today. The name is probably a corruption of Terneuzen, the Dutch port where vegetables from the fertile lowlands were shipped to England. (Why can't the English teach their children how to speak…)
So what do we make of these two clusters, the temperate biennials of Eurasian lands concentrated in four families, and the American perennials originating mostly between the tropics and all belonging to different families?
Eurasian agriculture developed simultaneously with iron and draft animals. These were essential for the plowing necessary for the preparation and maintenance of a seed bed, originally to plant small grains such as barley and wheat. Annual cultivation was conducive to the growth of small-seeded biennials, first as volunteers and then as cultivated crops. For the most part, plowing works against the growth of perennial plants.
Despite the attempts of aid agencies and modern agronomists to eliminate shifting cultivation or swidden agriculture, it persists in many places, including Central America, Asia, Africa and parts of Northern Japan. Although traditional agronomists cast swidden agriculture as wasteful and polluting, the modern farm field spews forth far more pollutants, albeit invisible, than the farmers tending their milpa in Oaxaca or yakihata in the mountains of Japan. Their external combustion methods consume but a few years of accumulated wood, while our internal combustion engines and synthetic fertilizers have burned through whole geologic epochs. And the plant remains we burn in fossil fuels return no minerals and nutrients to the land.
The roots of winter provide the wonderful illustration of how agricultural practices lend shape to our foods. And when someone tells you the sweet potato and the Andean potato are not related, you will know that the story has a more interesting wrinkle rarely discussed in general conversation.
Photo of burning milpa from Dr. Darlene Applegate of Western Kentucky University.