"Mexican Oregano" is just a common name for the two herbs listed below.

There is lots of discussion about "Mexican Oregano"!  Here is some information that I have compiled! 

Well, "Mexican Oregano" is just a common name for the two herbs listed below.  There is at least one other herb that is referred to as Mexican Oregano, but as far as I understand, it is not considered a culinary herb and it does not grow well in Texas! 

Poliomintha longiflora and Lippia graveolens are the ones most commonly grown as "Mexican Oregano" here in Texas with P. longiflora being slightly hardier than the L. graveolens.  Find information about both below.

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The P. longiflora is more attractive with the light mauve-pink, tubular flowers that hummingbirds love, and it stays smaller growing to about 3'. I have found that it can take a little more humidity and lower temperatures than the L. graveolens. It is considered to have the hotter taste of the two and is used in Mexican cooking as is the L. graveolens. It likes full sun but can tolerate partial sun during the afternoons.

Botanical Name	Poliomintha longiflora
Common Name	Mexican Oregano
Attribute	Perennial
Cultural Requirements	Full sun to part shade. Low water usage.
Mature size, Spacing	Ht: to 3' or so, 40" wide. Woody shrub.
Other Information	Light pink, tubular flowers cover this shrubby plant during the warm weather. Although not a true oregano, the narrow shiny green leaves are full of real oregano flavor. Dries well.

Courtesy of The Herb Cottage http://theherbcottage.com/

Mexican oregano (Poliomintha longiflora) is a strong-smelling plant popular in Mexico and Texas. Excellent for hot, humid areas, this woody shrub grows 3 feet tall. The small green leaves yield an essential oil similar to that of oregano and are used in cooking. Its tubular flowers of white to lavender blue attract hummingbirds.

From website: http://www.texasgardener.com/pastissues/janfeb07/SouthHerbs.html

South of the Border: Mexican Herbs for Texas

By Ann McCormick

Mexican Oregano
If you have room for just one native herb, then Mexican oregano (Poliomintha longiflora) is your best choice. The leaves of this shrubby herb are a somewhat spicy replacement for garden oregano. When substituting, reduce the amount in your recipe to about two-thirds of garden oregano.

Mexican oregano likes full sun but will also grow in partial shade. This graceful perennial provides lovely color through summer and into fall with tubular white, pink and lavender flowers. It generally reaches 3 feet. In my shade garden, however, it is prostrate, growing no higher than about 10 inches. Although native to the drier regions of Texas, it can adapt to the humid gulf area. It can also be grown in containers, where it will delight you with a cascade of showy flowers.

Mexican oregano (Poliomintha longiflora) is doing very well with minimal water, and it's covered in the pinky-purple flowers it's known for. This is an underused plant. It has great flavor, is evergreen during the winter in our part of the state, and flowers during the hottest part of the summer.

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The L. graveolens is not as pretty as the P. longiflora, with small yelow-white flowers and growing to a lanky 5', but in my humble opinion, it has a better spicy oregano flavor than the P. longiflora. It is closely related to Aloysia triphylla (lemon verbena) which was once classified as Lippia citriodora. It likes full sun and well-drained, sandy soil and hot climates. I finally lost mine after 5 years. I think it was because it wasn't planted in full sun.

Botanical Name	Lippia graveolens
Common Name	Mexican Oregano
Attribute	Tender Perennial
Cultural Requirements	Full sun to part shade. Low water usage.
Mature size, Spacing	Ht: 4' to 5'. Shrubby in form.
Other Information	Native of Mexico. Very pungent oregano flavor and aroma. Very tasty in salsa. Long lasting flavor when dried.

Courtesy of The Herb Cottage http://theherbcottage.com/

It's a slender aromatic shrub or small tree, whose pubescent (felty) branches bear rounded to obtuse, bluntly serrated leaves. Fragrant flowers are yellowish or white with a yellow eye and occur throughout the year, especially after rains.

Mexican Oregano (Lippia) lippia graveolens Oregano, Mexican (Lippia graveolens) This is probably the better known of the "Mexican Oreganos" in this country. Actually a relative of Lemon Verbena, this grows as a small shrub, reaching 3-5 feet in one season in Zone 5. Much of the oregano used commercially in the U.S. is actually this one. North of zone 8 it should be grown as a tender perennial

Both of these Mexican Oreganos are tender perennials, growing in zones 9-11, and can be propagated from cuttings and are well worth growing here in south Texas.