The "meaning" of flowers is sometimes tied in very closely to their pharmaceutical use, so "rosemary is for remembrance" because the ancient herbalists say so, as wellas becasue extract of rosemary today is sometimes recommended for memory improvement. Culpeper's nerbal (and many others I might add) were stil being published at about this time- the last Culpeper before the 20th century was 1813, IIRC, and his remedies and lore still in general circulation. So yes, The fact that flowers have poetic, and apothercarial meanings is quite valid.

Having said that, I don't have my Culpeper or Gerard at hand, so I really don't know if the hyacinth really meant "constancy" or "unobtrusive loveliness". At the back of my mind there's something niggling...about the roses in one of the gothic novels....if only I could remember which one...that makes me think that JA could well be just spoofing the genre yet again.