As the holiday season approaches, it seems appropriate to discuss the issue of sweet potatoes versus yams. Officially a sweet potato is never a yam, but sweet potatoes are often sometimes referred to as yams.
Sweet potatoes (Ipomoea batatas) are yellow or orange tubers that elongate into ends that taper to a point. There are two types of sweet potatoes, which creates the confusion over yam versus sweet potato. The paler-skinned sweet potato has a thin, light yellow skin with pale yellow flesh which is not sweet and has a dry, crumbly texture similar to a white baking potato. The darker-skinned variety (which is most often called "yam" in error) has a thicker, dark orange to reddish skin with a vivid orange, sweet flesh, and a moist texture.
Most of us have probably never seen a true yam. The true yam is the tuber of a tropical vine (Dioscorea sp) and is not even distantly related to the sweet potato. The yam is a popular vegetable in Latin American and Caribbean markets and is slowly being introduced here. The true yam is even sweeter than the sweet potato, and the tuber can grow over seven feet in length. The yam tuber has a brown or black skin which resembles the bark of a tree and off-white, purple or red flesh, depending on the variety.
Another root vegetable commonly used in Central America is called cassava or yuca (Manihot esculenta). It differs from the yucca plant we grow here. When we were in Costa Rica, the starchy cassava root was peeled and boiled like potatoes. And interestingly enough, when it is dried to a powdery (or pearly) extract this root makes tapioca.
On another note, sweet potato vines are popular for their ornamental value as ground covers, hanging baskets, in planters, and even in bottles of water in the kitchen. The ornamental vine's tubers are edible but are reported to have a bland taste.
In the end, it doesn't matter what you call it. Regardless of whether you call them yams or sweet potatoes, enjoy them often - they're healthy and tasty choices for your kitchen table.