Oregano and Marjoram: 
An Herb Society of America Guide to the Genus Origanum

Description

Paniculate inflorescence of Origanum vulgare subsp. vulgare

Origanum is one of over 200 genera in the Lamiaceae (mint family), and the genus includes culinary, fragrant, medicinal and ornamental plants. Herbaceous perennials or subshrubs, origanums are native to the Mediterranean and Eurasia, and grow in mountainous areas with rocky, calcareous soil. Some species grow in mounds that are only 2-3 inches high (104) while others grow erect up to 39 inches tall (102). 

All members of the genus have flowers that occur in spikes; for most species these form a panicle with multiple branched stems growing from a central stalk. In O. onites, the spikes grow in a "false corymb" (56), forming a convex or flat-topped open inflorescence (102). Corollas may be purple, pink or white depending on the species. In some species flowers are arranged in whorls. The calyx, or small vase-like receptacle that supports and protects the corolla and reproductive organs of the flower, can be bell-shaped or tubular (97) with one or two lips (56). The shape of the calyx is the principal plant character used to distinguish between Origanum species (97). Flower stems can be erect or trailing/cascading (104). Trailing types like the ornamental O. rotundifolium and the cultivar 'Kent Beauty,' a hybrid of O. rotundifolium and another Origanum species, have a graceful, drooping appearance. The leaves (15) and flowering parts of the plant contain essential oil glands that secrete volatile oils responsible for the plant's fragrance (97). 

O. rotundifolium with prominent hop-like bracts

Plants in the genus Origanum have bracts, or non-typical leaves, that surround the calyx and corolla. In some instances the bracts are so beautiful and colorful that the casual observer might mistake them for the flower. In these cases, the flower is actually hidden within the bracts. In some species, like O. rotundifolium and O. dictamnus (dittany of Crete), bracts overlap and resemble hops (92). 

Both the stems and leaves of origanums are often covered with fine hairs. Leaves can be of various shapes including round, heart-shaped and oval and may be shiny/waxy or hairy-fuzzy in appearance. Stems may be woody or non-woody. All species also bear tiny brown fruits called nutlets.

Taxonomy

When most people think of oregano, pizza and pasta sauce come to mind. Oregano and marjoram may seem familiar and straightforward to the average cook, but in reality, these common herbs have a very complicated taxonomic history. Although Linnaeus first classified Origanum as a single genus, over the years, plants in the genus have been ordered under various botanical names including Amaracus, Origanum and Majorana (56, 96). 

According to Dr. Arthur O. Tucker, "it's best to think of oregano as a flavor rather than a genus or species" (100). Plants from several genera including Lippia and Plectranthus are also considered oreganos due primarily to the presence of the chemical carvacrol, which is largely responsible for oregano's signature scent and flavor. See the Chemistry and Culinary Uses sections of this guide for more information on this topic. 

According to current estimates there are 44 species, 6 subspecies, 3 varietates (botanical varieties) (95) and 18 naturally occurring hybrids (60). Because Origanum taxa are so variable and cross easily, there are hundreds of unclassified hybrids growing in gardens where close proximity encourages crossing that isn't likely in the wild (95). 

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