Yuzu

Citrus ichangensis × C. reticulata var. austera


UCR citrus pages
Wikipedia 
Citrus pages  

Yuzu or citrus junos.  This hybrid has been recreated more than once, Sudachi has the same parent species, Liudmila is from a cross with sweet mandarin.  The original Yuzu cross goes back centuries. Ichang papeda crossed with a sour mandarin.  It is used like a lemon, however the flavor is more complex with grapefruit overtones.  Apparently it comes mostly true from seed.  

After experiencing 8°F, -13°C, some stem dieback.