About a year ago I started learning that a number of begonias, essentially of all major classes are often hardier than usually or commonly thought. I'm of course particularly interested in possible candidates for my own outside garden, which is near the Northern California coast; a particularly cool Zone 9b. I've seen a number of cane and fibrous begonias survive our winters and even a Rex or two.
One begonia species I decided to experiment with is B. maculata var. wightii as it is a beauty indoors. The coolest climate reported in the DG PlantFiles, is 10b. Logee's, from whom I originally obtained my plant, also states the limit at Zone 10.
I planted a cutting that I rooted in early September into a partially shaded and protected SSE location in early November. Our winter proved to be cooler and wetter than usual, though the lowest temperature I noted at my location was 29º. While the plant lost 3 of it's 4 leaves it survived and is now starting to grow again. Considering that the plant didn't have much of a root system, only spent about 2 weeks outside before planting and that it was planted in the ground so late, makes it's survival all the more significant. I would encourage others to try their luck at least in zone 8b or 9a.
In addition to any comments by others experience with this particular species and any hybrids derived from it, I would encourage others to discuss any other Begonias that can survive outdoors in climates cooler than normally reported.
The plant pictured here is the mother plant. It has been growing in a large unobstructed NNW window and is currently 30" tall. The leaves reach over 11 inches in length with beautiful metallic silver spots and are deep red on the leaf backs.
The soil is well-drained topsoil with some humus. Also the plant was pretty-well protected by a house wall and shrubs. From my limited experience with growing Begonias indoors, I believe the most important factors are soil with good drainage and some protection from the drying winter winds.