Conradina verticillata

A small shrub of the Cumberland Plateau, found only on the flood-scoured cobble and sand bars of three river systems in eastern Tennessee and a sliver of Kentucky — the Big South Fork of the Cumberland, the Caney Fork, and the Obed. The rest of the Conradina clan keeps to the sand scrub of Florida and the Gulf Coast of Alabama, sun-baked and semitropical. This one took a different path, north up into the cooler uplands, and the cold-hardiness that came with it is the gift to gardens further north.

It lives by disturbance. The plants grow on bars that are scoured by spring floods and baked dry by August, and they need that rhythm to survive. Flooding strips out the taller competition that would otherwise shade them off the bar; the drought that follows is the condition their roots are tuned to. Extended floods snap off stem fragments and carry them downstream, where they root in fresh gravel and start new colonies. It is a plant that requires the river to behave like a river. Dam the system, stabilize the banks, and the species quietly disappears.

There are other oddnesses. Cumberland rosemary is the only triploid in its genus, which makes seed production unreliable and most reproduction clonal — what looks like a colony of separate plants is often a single sprawling individual rooting at the nodes. The genus honors Solomon White Conrad, an 18th-century Philadelphia botanist and Quaker schoolmaster whose name attached to this small clan of southeastern mints almost by accident. The leaves, crushed, smell so much like culinary rosemary that they have been used in the kitchen as a substitute, though we would rather you grew the plant than ate it.

Lavender-pink flowers with darker throat spotting in May and June, two-lipped and small but produced freely along the trailing stems. A low evergreen mat, six to twelve inches high and spreading to two or three feet, the foliage holding through winter with a faint bronze cast in cold. Excellent in a rock garden, tumbling over a stone wall, or anywhere the conditions of its native cobble bars can be approximated.

Federally listed as threatened since 1991, which means it cannot be sold across state lines. South Carolina and in-person purchase only.

Photos courtesy of Sally and Andy Wasowski

Learn more about the Conradina genus here.