Thank you all for the replies.
Fortunately for me, in the meantime, I managed to find at a local flower shop some artificial flowers (in cloth) which seemed to me just about the same of those of the planet Felucia (Star Wars - Episode 3).
I began working to transform the flowers in alien plants and when I finish, I will post here the pictures of the various making phases.
To answer to Major Gilbear: no, I don't use the models of these plants for martian or vesuvian scenarios, but for a scenario based on the planet Felucia of the Star Wars saga, for which I will use the 28 mm miniatures of the Wizards of the Coast, of the Star Wars serie.
Bye,