Begonia grisea has been known for a long time, but is not frequently cultivated. It was described by Alphonse De Candolle in 1859 from specimens collected in Brazil between 1816 and 1821. It was later ‘forgotten.’ When the American Begonia Society developed its system of assigning ‘U’ numbers as a way of tracking unidentified species, this plant was the first to enter that system as U001.
In 1986 Rudy Ziesenhenne published an article in The Begonian re-identifying it as B. grisea. That article provided the information for this handout. (It was the only information I could find about B. grisea.)
The leaves have a fuzzy gray appearance, stems are tan, hair on the leaf surface is clear and the leaves appear shiny. “New stems arise directly from the soil, do not branch (nonramified), and are self-supporting to a height of three feet.” I would classify the plant as a thick stem, based on this description. Flowers are white on long peduncles (flower stems).
Other plants that have similar characteristics to B. grisea are B. incana, B. kellermanii, B. peltata. These all have peltate leaves which B. grisea does not have. Peltate leaves are those that are shield shaped and where the stalk is attached inside the margin of the leaf, much like a water lily. Peltate does not refer to the fuzziness that is on B. grisea leaves. Another plant with similar characteristics is B. petasitifolia (which has heavy stems bending over to the ground. What separates this last look-alike from B. grisea is the structure of the flower parts, which is what taxonomists use to identify and group related plants.
Sources place this plant in Section Pritzelia. From information provided by Rudy, it would seem that this is not a very difficult plant to grow with few special requirements. It is probably be more sensitive to overwatering than most begonias.