The mystery maidenhair fern

Adiantum sp., still green after 20° F

Winter has come a bit early this year, with a low temperature of 20° F (-6.6° C) a few nights ago, and several more nights in the low 20's.  It's very unusual to have temperatures this cold before mid-December.  Yet among all the brown and crispy frozen foliage, one bit of bright green still stands out: a maidenhair fern (Adiantum sp.).

Adiantum sp.

I love ferns, especially the genus Adiantum, commonly known as maidenhair ferns.  This is a largely tropical and subtropical genus, with just a few species that can survive freezing temperatures.  I've tried several of these over the years with mixed results.  The North American native Adiantum pedatum is quite hardy, but just doesn't have that tropical look I want.  Moreover, many of the plants sold under this name have a tight, ungraceful clumping habit that I don't care for (and may actually be the closely related A. aleuticum).

Adiantum pedatum (A. aleuticum?) growing with Raphiocarpus petelotii (Gesneriaceae) 

A. capillus-veneris, the southern maidenhair fern, has the look I want but has been only marginally hardy for me and never grew very large.  The hardy Himalayan species A. venustum wasn't very vigorous for me, and has a reputation for disliking heat.  I've finally had good luck with A. ×mairisii (often confused with A. mariesii; the two names are hopelessly confused in cultivation), reputedly an A. capillus-veneris hybrid, which has proven hardy and vigorous.  Like A. capillus-veneris , A. ×mairisii has running rhizomes that form a steadily growing colony, but even this hardy species is already showing frost damage.

Adiantum capillus-veneris, southern maidenhair fern (summer 2003)

A. ×mairisii with begonias, September 2014

A. ×mairisii, showing frost damage after 20° F

Then there's the plant that's still green in my garden, forming a tidy, well-behaved clump.  I picked up this plant several years ago in the houseplant section of a local garden center to use as an annual.  Such unlabeled plants are usually A. raddianum, a common subtropical species that's generally considered hardy only to zone 9.  Out of several that I planted, one surprised me by not only surviving the winter, but remaining green.  That was an unusually warm winter, with our lowest temperature all winter in the high teens (ca. -8° C), so while I found its survival interesting I didn't think much of it.

Adiantum sp., January 2013

The following winter was a bit colder with temperatures in the low teens (ca. -10° C).  Although it remained green well into the winter, by spring its foliage was brown and crispy and I assumed it was dead.  When it came back yet again, I started to think maybe I had something interesting, so I propagated it from spores last fall so I could plant it out in several places around my garden this spring for further testing.

Then January came, with our coldest temperatures in 20 years.  After a low of 5° F (-15° C) in early January, followed by several lows in the 9-11 F (-12 to -11 C) range along with extended periods below freezing, I was again certain it was dead... but again I was wrong.  Not only did it survive, but this spring it came back better than ever.  It formed a nice clump this year, and I now have several more planted around the garden.  Most of them have held up as well as the original plant, although one of them does show some slight frost damage.

Adiantum sp., November 2014 (note frost damage at upper right)

What the heck is this thing?  I have no idea.  It's fertile, so probably isn't a hybrid.  The tightly clumping habit rules out several hardy species.  Coming from a garden center, it shouldn't be anything terribly rare or unusual, but two different fern specialists have been unable to positively i.d. it from the photos I sent them.  One thought it looked like A. raddianum, the most commonly sold species, but if so it shouldn't be nearly so hardy.  Have I stumbled across an unusually hardy version of that species?  Or is it something else entirely?

Adiantum sp., single frond showing upper surface

Adiantum sp. showing leaflet undersides with indusia

So... who knows their ferns?  Can anybody tell me what this might be?