Experts have described the artwork found at Lews Castle in Stornoway as an unusual example of interior design popular during Queen Victoria's reign.
Ferneries were painted on walls as a way of decorating rooms.
Extensive work has been done to Lews Castle to turn it into a museum and archive after being closed for more than 25 years.
The paintings of exotic ferns and foxgloves by an unknown artist were found beneath wallpaper in the Morning Room.
Other walls in the property may also have been painted, but the art has not survived repeated plastering over.
Fiona Allardyce, of Scottish Wall Paintings Conservators, said: "They are very unusual. In fact in 20 years experience I have never come across something like them before.
"Ferneries were very popular in Victorian times and we know the castle had a greenhouse and one was just for ferns.
"These works were meant to make you feel you had walked into a fernery, though in between the exotic plants there are things like foxgloves."
She added: "We don't know who painted them - there was no signature - but clearly they are the work of a skilled artist who has done this before."
Lews Castle was constructed in 1847 for James Matheson, who made his fortune from the opium trade. Opium exported from China and India was often mixed with tobacco by smokers.
Victorian wall paintings have been uncovered during work to revamp a 19th Century property in Lewis.