|Osmunda regalis, known as royal fern in English.|
(Image from T. Moore, The ferns of Great Britain
and Ireland, 1856, public domain, via Wikipedia.)
You could capitalize the names (as 'Osmundas'), but you don't have to, and I think it is better if common names are generally not capitalized. We don't capitalize words like cat, dog, human, apple, and such common names. And if these scientific genus names are used as English common names, then they should not be in italics either. There are many examples where scientific genus names of plants are used in English, for example geranium, calla, petunia...