Ah, the sweet potato

Can anyone tell me the garden plant most closely related to the sweet potato?

Nope, not the potato. The similar naming is actually a misnomer -- they're in different botanical families.

The potato, a plant with funky edible growths off the roots, family Nightshade (with tomatoes and eggplant)

The sweet potato, the root of a climbing vine, family to be revealed...

No, not the yam either. The yam is a starchy root grown in Africa. According to Amy Stewart's The Drunken Botanist, although we may call any soft, orange sweet potatoes yams, yams are "almost never sold in the United States." The yam is actually more closely related to grains and grasses (monocots) than either potatoes or sweet potatoes (dicots).

The yam, a tuberous root from the tropics which can weight 30lbs, family Yam

So the relative of the sweet potato-which-isn't-a-potato-or-a-yam? The morning glory.

Yep, sweet potatoes are in family Morning Glory. I guess this makes sense, since sweet potato vines, sold as decorative annuals in local nurseries, have leaves that look similar. Still, I find it hard to see sweet potatoes, which I adore eating, and morning glories, lovely vines which can kill a small evergreen if left unchecked, in the same grouping. (Not a fan of morning glories.)

Ok, so you've read this far, and you might be wondering why I'm nattering on about sweet potatoes. Well, for one, I just learned something new (the yam is not a sweet potato fact), and I like to share horticultural tidbits. 

And for two, we're growing sweet potatoes at Wagner Farm this year -- in the field closest to the sidewalk. I've only remembered seeing corn and hay in the fields, so seeing sweet potato slips* out there is a hoot. Our farmers planted them using a transplanter -- a very cool piece of equipment which creates a furrow, grabs a slip by the roots, plants it, covers the furrow and waters it all in the space of a minute. 

* Yep, another difference between potatoes and sweet potatoes. Potatoes are grown from seed potatoes --potatoes that have started to sprout from their "eyes" and which are then buried in several inches of soil. Sweet potatoes are grown from slips. Check out this site for more on sweet potato slips.