As with another reply, I'd say that the bag isn't necessary, but I
don't know if it will do any harm.
My P. lusitanica grow all year round outside but under a shelter
in the U.K. Seeds germinate happily in these conditions, and attempt
to get into other nearby pots. The plants flower within a year.
The only problem I had was with an inquisitive bird, which was
attracted to the dead flower stems and flew off with a few of the
plants.
-- Clarke Brunt (clarke@brunt.demon.co.uk)