[Updated below with variety of Rancho Gordo's Mexican oregano]
[Updated again, cleaning up some typos, 3/26/11]
This post is for Steve of Rancho Gordo, grower and seller of incredibly delicious heritage beans. He also sells Mexican oregano (update: the variety is Lippia graveolens), and when I met him a few weeks ago, we talked for a minute about the mysteries of Mexican oregano. Is it really oregano? Or just a misnomer by the Spanish? Neither of us knew, so I decided to spend some time in the library investigating the subject. I found a few answers, but left more confused than I arrived.
The herbs known as oregano are called marjoram in some countries. And for a good reason, both "oregano" and "marjoram" are in the Origanum genus, which contains 36 species. Three of the references that I consulted (numbers 4 - 6 in the reference list below) say that the genus name is derived from the Greek oros ganos, "joy of the mountains." One book (Ref. 3), however, claims that name derives from the Greek ori ganon ("bitter herb") a name supposedly used by Hippocrates in classical Greece. Perhaps both are right--the mountains of classical Greece were covered with bitter herbs that gave those who walked among them or ate them great joy. (do any Mental Masala readers know Greek?)
According to the USDA Germplasm Resources Information Network, many of the Origanum species are native to the "Old World." For example, O. vulgare is native to a wide span of Europe, Africa and Asia; O. syriacum is native to Western Asia.
The herb known as "Mexican oregano" is from the New World. It is not in the same family as Old World oregano and to confuse things, there are multiple herbs sold under the name, like Poliomentha longiflora, Monarda fistulosa var. menthifolia, and Lippia graveolens. Nancy Zaslavsky (Ref. 1) writes that "True Mexican oregano is sold by Indian women in weekly markets (there are about a dozen types of wild oregano in Mexico). What is sold in jars in supermercados is actually marjoram."
If Mexican oregano is not in the Origanum genus, why is it called oregano? I don't know--none of the books I consulted gave a reason. The explanation is probably simple: the Spanish found herbs in the New World that looked and tasted like the oregano/marjoram they knew back home, and so they called them oregano.
Oregano as a Flavor, Not an Herb
I'm not alone in being befuddled by oregano. The The Oxford Companion to Food devotes almost one-half of a page to oregano, with background on the plant and a list of species called oregano around the world. The Companion quotes from a scholar who says that we should think about oregano as a flavor instead of a plant: "Most of these [oregano] plants bear a unifying chemical signature: carvacrol and, to a lesser extent, thymol."
That's fine with me, but I'll still be careful about which dishes I season with each type of oregano. There is something about the flavor profile of Mexican oregano that makes it work in chile sauces, and something about Old World oregano that fits with tomatoes and pasta.
 A Cook's Tour of Mexico, by Nancy Zaslavsky
 Herbs and Spices, by Jill Norman
 Encyclopedia of Herbs, by Deni Brown
 Encylopedia of Herbs, Spices and Flavorings, by Elizabeth Lambert Ortiz (also wrote seveal Mexican cookbooks)
 Herbs by Lesley Bremness
 The on-line Food Lover's Companion
 The Oxford Companion to Food
 Several books by Diana Kennedy (a modern Mexican cooking legend)
Image credit: Java finch on an oregano branch from Eye of Einstein's flickr collection, subject to a Creative Commons License.
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