Sweet potatoes have been on my mind a lot lately. A few weeks ago, I was buried in some research for an article I was writing about some very close cousins of sweet potatoes; a couple of days ago, I came across some really lovely-looking California-grown sweet potatoes in my supermarket and I just had to bring them home. Sweet potatoes aren’t so much of a big deal here in Canada as they are in the United States, although we do consume our fair share of sweet potato fries (yum!) and as such, are one of the main importers of the total crop produced by our American neighbours. Strangely, it seems that sweet potato love isn’t what it used to be in the States, either: in 1943, consumption of sweet potatoes per person averaged 21.7 pounds per year, while in 2007, that number fell to just 4.6 lbs.¹ I’d hazard a guess that most of that is eaten at the Thanksgiving and Christmas dinner tables, am I right? 🙂
Part of the reason sweet potatoes aren’t all the rage in Canada is the fact that we’re such a cold country; we simply cannot grow them ourselves, except for the few short-season varieties now available. Most sweet potatoes need a long period of hot, dry weather to produce tasty tubers, but short-season cultivars such as ‘Frazier White’ and ‘B-18’ can be grown in locations with a minimum frost-free period of 90 – 100 days (which means we might…just MIGHT… be able to grow them here in Calgary, as we sit at 114 frost-free days. The only foreseeable problem would be soil temperature, as sweet potatoes like consistently warm soil and really, we don’t get that until the middle of August, and then only for a week or two. I joke, but barely). Apparently growing sweet potatoes in large containers can help extend the season a bit – you can plant them in barrels or sacks as you would potatoes. Bear in mind, though, that sweet potatoes grow from “slips,” tiny shoots that sprout up out of the tubers and must be transplanted. Sweet potatoes like their soil a little on the acidic side, so those of us living on the alkaline Prairies may need to adjust our soil’s pH; otherwise, the nice thing about sweet potatoes is how completely undemanding they are. They don’t even need much in the way of fertilizer – in fact, too much nitrogen can cause them to concentrate their efforts on their foliage instead of their tubers.
So, what about the name-game? What is the difference between a sweet potato and a yam? (And, for that matter, a potato?).
A lot, actually. Sweet potatoes (Ipomoea batatas) are from the same genus as morning glories, the beautiful ornamental vines that I was researching a few weeks back. They’re not even remotely related to potatoes (from the genus Solanum) or yams (genus Dioscorea). True yams are primarily grown in Africa, Asia, Mexico and the Caribbean, but North Americans tend to toss around the words “yam” and “sweet potato” interchangeably, to refer to the same veggie: sweet potatoes. The reason for this is all about marketing: in the 1930s, growers of a new kind of orange-fleshed sweet potato wanted to differentiate their product from the white varieties available, so they appropriated the “yam” moniker to refer to the orange types.
A few fun facts about true yams:
- Some yam species will grow up to 6 feet long and weigh well over 100 lbs. Can you imagine digging up THOSE tubers? (“Yeah, honey, just bring the backhoe over this way a little…”).
- Not only do some yams grow to incredible size, but some, such as D. opposita (Chinese yam) and D. batas (cinnamon yam) have tubers that grow vertically down into the soil, which make them ever-so-challenging to harvest.
- Some yams, such as D. bulbifera and D. pentaphylla, have aerial tubers, small protuberances that grow above ground on the stems of the plants. Some aerial tubers are poisonous and may not be edible; others may be edible only if specially prepared, by boiling and changing the water several times to remove the toxin; still others may grow in conjunction with edible underground tubers. You seriously have to know how to ID these things.
- Nearly all yams contain a toxin called dioscorine (and some contain other poisons as well) and cannot be eaten raw.
For some excellent information on how to grow sweet potatoes in Canada, see How Sweet It Is in the Canadian Organic Grower and Learn to Grow a Sweet Potato in Canadian Gardening. Organic Gardening provides some additional growing tips – and recipes! Finally, if ever you’re in a region that grows yams and you don’t know what varieties you’re looking at, consult IRETA’s publication A Practical Guide to Identifying Yams.