Torrey’s mountain mint – an oddball species?

In a state whose flora has been studied for hundreds of years, grassland conservation and restoration are still hindered by a need for better understanding of basic plant ecology and systematics. Leighton Reid, Jordan Coscia, Jared Gorrell, and Bert Harris contributed to this post.

All ecologists deal with puzzling groups of plants. In eastern North America, sedges (genus Carex) and panic grasses (genus Dichanthelium) are notorious for having many species with similar characteristics. In Central America, tree seedlings in the avocado family (Lauraceae) can be tricky to separate.

Sometimes we also encounter oddballs – plant species that it’s hard to see where they fit into the contemporary landscape.

Torrey’s mountain mint (Pycnanthemum torreyi) is a bit of both – an oddball species whose relationships to other mountain mints is not yet worked out.

Late-season aspect of Torrey’s mountain mint. Photo credit: B. Harris.

Like others in its genus, Torrey’s mountain mint is an aromatic herb that grows (mostly) in more-or-less open areas. Its crushed leaves have a delightful minty smell. In summer, it produces clusters of small, white flowers that are visited by a variety of pollinators.

Unlike some other mountain mints, Torrey’s is also rare. NatureServe ranks it as a G2, meaning that it is imperiled throughout its range – which extends sporadically from New Hampshire to Kansas.

Virginia has more Torrey’s mountain mint populations than the other states. The Flora of Virginia describes its habit as “dry, rocky, or sandy woodlands and clearings.” In some places, like the Piedmont, it occurs mainly on basic soils, whereas in other places, like the Coastal Plain, it lives in sandy, acidic soils. In the mountains it has been found also in limestone seepages.

An oddball species

While restoring natural areas in Chicagoland in the 1980s, Stephen Packard described some of the plants he saw as “oddball species”. Species like purple milkweed (Aslcepias purparescens) and cream gentian (Gentiana alba) grew neither in closed forest nor in open prairie, so where did they belong? These species preferred intermediate levels of light, such as would be found beneath a spreading burr oak. Packard’s observation that these species preferred savanna conditions sparked his realization that savanna had once been a frequent component of the Chicago landscape.

Matthew Albrecht has considered a similar possibility in Tennessee for Pyne’s ground plum (Astragalus bibullatus). This species grows on so-called cedar glades around Nashville, but it does not grow right in the middle. It prefers the edges where there is intermediate light. This suggests that these cedar glades may once have had softer edges that tapered slowly from exposed, rocky glade into open woodland. With modern fire suppression, these edges have become hard; many glades are now bordered by dense forests of eastern red cedar.

Could our own Pycnanthemum torreyi fall into the same category? An “oddball species” with a preferred niche that is neither full sun nor full shade? In our fieldwork on the northern Virginia Piedmont, we encountered several populations of Torrey’s mountain mint, all of which were growing in edgy sites, like powerline right of ways, or the edge of an old apple orchard.

A small population of Torrey’s mountain mint grows along one edge of this field near the forest edge, not in the open center of the field. Is this typical of this species’ preferred light environment?

Last summer, one of us (Leighton) tested P. torreyi’s habitat affinities inadvertently and with a very small sample size. He planted three seedlings in his small, Blacksburg, Virginia yard – one in an exposed spot on the south side of the house and two in a partially-shaded spot on the north side of the house. The plant in the more open, southerly spot grew okay, but it was somewhat stunted – like a spider plant that has been left out in the sun. Its stem and leaves grew short and tough. In contrast, the two plants on the north side of the house grew full and spread out, both flowering and fruiting in their first season. They also remained green late into the season, even after nearby P. tenuifolium and P. incanum had senesced. If this was a species desirous of full sun, shouldn’t it be doing better in the exposed position in the back by the parking lot?

Two Torrey’s mountain mints growing well and flowering in partial shade on the north side of JL Reid’s home in Blacksburg, Virginia.

Clearly Leighton’s sample is way too small to draw any conclusions, but it does make us wonder if Torrey’s mountain mint prefers and intermediate level of light, such as would be found in a savanna or an open woodland. These disturbance-dependent habitats were once widespread but are excluded today in much of the eastern United States. Maybe Torrey’s mountain mint is an oddball species whose habitat preferences will eventually lead us to design new restoration targets in Virginia, but we’ll have to study its ecology in a bit more detail first.

A “Problematic Species”

The Flora of Virginia also highlights that Torrey’s mountain mint is a “problematic species”, whose interpretation is “confounded by its similarity to Pycnanthemum verticillatum and its hybridization with other species.”

During our fieldwork in 2020, we were able to positively identify all of the individuals that we encountered, differentiating P. torreyi from P. verticillatum by characteristics of their flowers and leaves. Still, the possibility that Torrey’s mountain mint is not a well-differentiated species is troubling. Several landowners in our area are conserving open habitat in part because this rare species occurs there, so it would be nice to know if it is a good species.

I asked Gary Fleming, a Vegetation Ecologist for the Virginia Natural Heritage Program, for his thoughts. “Well, the entire genus Pycnanthemum is a bit problematic!” Gary wrote me in an email. He explained that the problem is that nobody has studied this genus using molecular phylogenetics, that is, using DNA to reconstruct the evolutionary relationships between species. As a result, our understanding of how species in this genus relate to each other is pretty fuzzy.

“Personally, I think P. torreyi is a good species,” Gary continued, “Over the years, I’ve observed it in numerous places state-wide and it appears to be morphologically very consistent.”

In a state whose flora has been studied for hundreds of years, apparently the Pycnanthumum nut has not yet been cracked. Hopefully some enterprising botanist will take this up soon (and maybe Packera while they’re at it).

Torrey’s mountain mint flowers. Photo credit: JL Reid.

Note: Part of this blog post represents a USDA NIFA Hatch project.