Yes to Pycnanthemum, but I am not familiar with the one you named. According to the website below, it does not grow in NC. They are many Pycnanthemums, and they all spread as most mints. The ones I see are alongside the roads, in full sun, up in the mountains. They can compete very well with the hardpan soil and gravel there...and smell wonderful! Oops. Just previewed my posting, and this website has great photos but not the geographical distribution, which I saw on another site. Oh, well. Maybe it will help a little.

Here is a link that might be useful: Pycnanthemum

Thanks, ncrescue! The plants on the link you gave look just like my plants, so for now, I'm calling them Whiteleaf Mountainmint.

Pycnanthemums can be difficult to ID -- albescens/montanum/incanum/muticum/virginianum don't differ that much. Of the species I'm familiar with, your pics look most like Pycnanthemum incanum. Incanum is pretty widespread, and has the more ovate leaves as seen in your pics. Albescens leaves tend to be more lanceolate and more deeply toothed. But then, there's a fair bit of variation within all species, and certainly within this genus. I've wondered whether Pycnanthemums in that complex intergrade or hybridize, as there have been some I simply couldn't ID with certainty due to characteristics of more than one species on a single plant.