Since I'm always looking for dark-leaved perennials for accent in many different kinds of beds and pots, oxalis triangularis eventually popped up on
my radar. It's distinct advantage has been in those beds (or pots) where
during the height of summer it's getting full sun, but later in the season, the
sun's angle (or the shadow of a house or tree) slowly produces partial shade.
Not a problem for this plant - unfortunately, I have NEVER been able to get it
to winter over, and I'm in Zone 6b, as you are. . .my friends in Philadelphia,
however, who reside in Zone 7, usually get it to squeak through the winter.
With special protection of some sort, I'm sure you could get it to pull through
the winter, but I prefer not having to give any of my plants "special" treatment - there are plenty of other hardy substitutes. On the other hand,
I know some gardeners who LOVE pushing the envelope and consider it a
unique challenge to drag plants outside their zonal limits!

"To each his own.. ."

It doesn't winter over, but is very easy to carry forward from year to year. One quick movement with a trowel and you can dig up the pink tubers, corms or whatever they are. Let the foliage dry up, and simply store in the basement. You get many tubers produced from one plant and have the potential for many additional plants for the following year. So you only have to dig up a few plants to have all the tubers you need for the next year.

George, I have mine in a shallow hanging planter. They come in the house in the fall and bloom for me all winter. then back outside in the spring. /abi

Mine won't winter over either and I'm along the CT coastline.

Molie