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[These are not the same vegetable as true "yams", which are rare in the Western hemisphere and which we do not describe.]
It is only in very recent years that a realization has dawned on home vegetable gardeners that sweet potatoes can be successfully--even easily--grown outside the deep south, even up to the far north. That said, though, sweet potatoes are, to our minds, a marginal crop. They are, much like winter squash, something of a one-trick pony in the culinary sense, yet they take up quite a bit of garden space that could be used for far more useful crops. But, because if one has space galore, one can essay a modest crop, because there are a few things one can do with them other than drizzle melted butter and brown sugar over them (they respond well to being paired with tart tastes, such as sorrel, or even--we saw such a recipe on the net--grapefruit).
Note that when we say they can be grown successfully in the North, that doesn't mean you can just plunk them into the ground and wait: you need to take measures. That will, for sweets, probably mean, in this climate, setting up a "growing tunnel" of at least row cover, or maybe even transparent plastic sheeting. You need to get the daytime temperatures up, but even more to keep the nighttime temperatures from dropping too much. Sweets apparently need about 1200 "heat units" to grow properly. A "heat unit" is degrees the 24-hour average temperature exceeds 55. If, for example, your daily high was 80° and your nightly low was 50°, your daily average was 65° (80+50 / 2), so your heat units for that day were 10 (65 - 55). To grow sweets, you'd want a daily average over their full growing season of 10 to 15 heat units, meaning a daily 24-hour average temperature of 65° to 70° throughout that growing period--and in the North, you aren't going to get that in the open air. Hence, the need for "grow tunnels" or something of that sort--and, of course, black plastic mulch and drip irrigation. (So are these guys really worth that much effort to you?).
Sweet potatoes have many cultivated forms, but in the United States just two types prevail: the dry, mealy, yellow sweet potato; and the more watery orange so-called "yam" (which is no such thing at all, but that's the idiocy we're stuck with for terminology). It seems a market rule that northern consumers prefer the so-called "dry-fleshed" types, while southerners prefer the "moist-fleshed" types. (Strange But True: the "dry-fleshed" ones have more water in them than the "moist-fleshed" ones.)
Sweet potatoes normally grow on trailing vines that quickly cover the soil, rooting at the nodes along the way--but there are "bush" varieties available for when space is limited. Though "sweets" are far and away more common and popular in the south of the U.S. than up north, there are, as noted earlier, now cultivars that do well even up here. Take a look at the list of "Northern Assortment" varieties on the Sand Hill Preservation Center sweet-potato offerings page--50 types! (And, again, that's just those they carry suited for short-season climates). From most seedsmen, you're lucky if they offer Georgia Jets and that's it (if you have to pick one type, the Georgia Jet is probably the one.).
Incidentally, we have had it pointed out to us that the leaves of the sweet-potato plant are also edible. That might be worth looking into.
Sweet potatoes are invariably grown from "starter" plants called slips, which you receive from your chosen seedsman shortly before planting time. Take care to obtain slips only from seedsmen with certified disease-free roots.
Despite the urgent need for growing time, it just doesn't do to get sweets into the ground before that ground (much less the air) temperatures get well into the warm-season range. Since you're only going to grow these successfully if you're using such augmentations as grow tunnels and plastic mulch, don't rush them out. Middle to late June, or even July 1, is probably about right, at least according to the experts. That pushes their harvest to the end of September, which seems late, so perhaps mid-June is better, for mid-September harvest. (If we just work back 45 days--hald their supposed 90-day growing period--from out peak-temperature day, July 22nd around here, we get June 7th, so the 15th seems reasonable.)
Sweet potatoes prefer well-drained, loamy to sandy soil. Heavy clay soils are not good them--the potatoes can end up small or misshapen.
Even using certified disease-free slips, it is wise to rotate their garden location annually.
In our climate, it is well to use a plastic mulch for sweets, setting it out a couple of weeks prior to the expected planting date. Using a bush-type cultivar on a deep-dug or raised bed, a plant spacing of 9 inches is about right according to Jeavons. It is probably as well to also add drip irrigation to one's plans. And then top things off--literally--with a dome or tunnel; that will be a suitably shaped frame covered with at least heavy row cover, though where heat is a scarce resource, clear plastic would probably be best. (Leave some openings for air flow.)
Use row cover to help with warmth.
Water well, but: do not water during the last 3 to 4 weeks before harvest, so as to protect the developing roots (the "potatoes"). Keep an close eye on the temperatures under your dome or tunnel: peaks over about 95° actually start to slow down sweet-potato growth. Try to keep the daily maximum as high as possible but not over than 95°.
Dig your crop at about the time of the first fall frost. As with true potatoes and other "root" crops, take great care not to cut, bruise, or otherwise damage the roots.
Besides any links presented above on this page, the following ought to be especially helpful.
The sweet potato is not at all the same thing as the yam, though "yam" is very commonly taken to just be another nickname for the true sweet potato. True sweet potatoes are a member of the Convolvulaceae, or convolvulus, family, which includes few other edibles, but does include the flower Morning Glory.
(The true yam--family Dioscoreaceae--is a large root vegetable, some up to a hundred pounds, grown in Africa and Asia, and rarely even seen in the western world.)
We try not to re-invent the wheel, so here is a link to a comprehensive history of the sweet potato.
"Sweet potato candies, ice cream, cookies, and related delicacies prepared from this vegetable are not yet widely known, but they are surprisingly good." Or so says one site.
If you find this site interesting or useful, please link to it on your site by cutting and pasting this HTML:
The <a href="http://growingtaste.com/"><b>Growing Taste</b></a> Vegetable-Gardening Site
In association with The Book Depository, we offer a library of books on vegetables, including books on growing, specialty cookbooks, plus a few related odds-and-ends books on the topic of vegetables, available for purchase from The Book Depository (never any shipping charges added).
Since you're growing your own vegetables and fruits, shouldn't you be cooking them in the best way possible?
Visit The Induction Site to find out what that best way is!
If you like good-tasing food, perhaps you are interested in good-tasting wines as well?
Visit That Useful Wine Site for advice and recommendations for both novices and experts.
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