Epazote (Chenopodium ambrosioides) is an aromatic perennial herb with an assertive, somewhat bitter taste.  It grows wild in many parts of the United States  —  even Central Park, New York  —  and has various English names of which my favorite is “stinkweed.”  I think epazote is perhaps the characteristic Mexican herb.  It is especially used in cooking huitlacoche, squash blossoms, beans, and various stews and soups.
Epazote.

In my native part of Mexico, the north, this ubiquitous herb  was most often used dried.  But for most Oaxacan cooking fresh epazote is definitely preferable.  The slightly bitter taste and unique medicinal odor mellow in cooking.  Epazote is always used in cooking dried beans, and goes into various green moles as well as other stews and soups.  It can be gathered as a roadside weed in may parts of the country (even New York City) and is sometimes sold fresh in farmers’ markets.  Dried epazote is available in most Latin American groceries, usually in 1/4-ounce packets equivalent to about 5 – 6 tablespoons.  To substitute dried for fresh epazote in recipes, allow 1 tablespoon of the dried herb to the leaves of about four 8-inch sprigs of fresh epazote.  (The stems are tough unless the plant is gathered young, so discard them after stripping the fresh leaves.)