Though sweet potatoes and yams are often advertised in the US as the same thing, they are two different vegetables. This confusion may stem from the US Department of Agriculture’s labeling guidelines, which require products identified as yams to also be labeled as sweet potatoes. No wonder Americans are confused!
Some of the major differences between sweet potatoes and yams:
1) Sweet potatoes and yams belong to two different plant classes. Sweet potatoes are a dicot and yams are a monocot. Characteristics that determine whether a plant is a monocot or a dicot include the number of embryonic seed leaves (monocots have one, dicots two), different types of root growth, and direction of leaf vein growth.
2) Sweet potatoes and yams are also from different plant families. Sweet potatoes are in the Convolvulacea family, while yams are from the Dioscoreaceae family.
3) Sweet potatoes originated in Central and South America, while yams originated in Asia and Africa. Sweet potatoes have also been around since prehistoric times, while yams are much newer, originating around 50,000 BC.
4) The sweet potato that we eat is a storage root, such as a carrot or parsnip. Yams
are tubers, which grow underground like roots but are actually stems.
5) Sweet potatoes are popular in the US for their sweet taste and high Vitamin A content. Yams have a much lower concentration of Vitamin A and are starchier.
Library of Congress Science Reference Services:
North Carolina Sweet Potato Commission
Monocots vs. Dicots UCMP Berkeley