I checked on my hardy-ish begonias this weekend and pushed away the mulch to see how they fared through the winter. B. sizemoreae, a B. rex relative from northern Vietnam, is quite dead--it just might surprise me from some deeply buried piece of rhizome but I'm not holding my breath. Here was the plant last fall:

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However, B. emeiensis (a Chinese species) has a firm green rhizome right on the soil surface and even had a couple of small curled-up leaves just below the surface of the mulch, quite alive and ready to grow (I know because I broke one off by mistake). Last fall:

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And a plant grown indoors:

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I also confirmed that the lower stems of 'Little Brother Montgomery' are still crisp and firm and green (or rather bright red-purple)--here's the plant last fall:

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I didn't check on 'Metallic Mist' but this one has been hardy through the last 3 winters, including one low of 8 degrees, so I fully expect it to come back. What's interesting is that, as with B. emeiensis, the rhizomes are not buried deeply and have survived with only a light mulch so they must have some actual resistance to freezing. Growing with the Taiwanese species B. chitoensis:

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B. chitoensis, a Taiwanese species that survived 2 previous winters in my garden, also has a surface-creeping rhizome but these tend to freeze and the plant regenerates from deeply buried rhizomes. The second winter nearly killed it so I dug it up and brought it in last fall, and it has regenerated very nicely. This is one of the more tropical-looking of the hardy(ish) begonias, with very large, glossy deep green glossy leaves:

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All of my begonias were mulched lightly with about an inch of leaf compost, and on top of that a fairly thin layer of fallen leaves from the japanese maple shading them overhead, which produces nice curly and fluffy dead leaves that

don't mat down.

I should note that it wasn't a terribly cold winter for Washington, DC. The lowest temperature in my yard was around 16 degrees with a few other nights in the high teens, and with very few days that did not go above freezing. Most winters we get low teens, and sometimes single digits but we also had virtually no snow this winter, so through several of the coldest periods we had no snow cover to provide additional insulation. This makes the survival of B. emeiensis, B. 'Metallic Mist', and B. 'Little Brother Montgomery' all the more impressive.