Interspecific crossing in the wild appears to be limited as well, at least partly due to geographic isolation. However, I. wallerana and I. usambarensis are known to have naturally hybridized in the wild (Grey-Wilson, 1980a; Grey-Wilson, 1980b). With the wonderful horticulture discovery of the New Guinea group of impatiens in the early 1970's, there was significant interest in attempting controlled interspecific pollinations.
Source: wiktionary