Vieni, ben mio: tra queste piante ascose... (Susanna in Nozze)
L'error d'un'alma amante fra quest'ombre e queste piante sempre ascoso... (Fiordiligi in Così)
Does the similarity of these two DaPontian quotes raise any suspicion, in you?
What the hell are these piante? What is with these girls, wanting to be ascose (in their error, furthermore) amidst them?
Ferrando comes in help: "Io tremo e palpito dalla testa alle piante" - so the piante are the feet! Piante is a synecdoche for piedi, and (drumroll, please)
piedi are the most elemental metric elements of the poetic Italian verse.
Piedi are the Lego bricks of Italian, hence of Music: be them oxytone (parole tronche: the 1, in Music), paroxytone (parole piane: the 2, in Music) or pro-paroxytone (parole sdrucciole: the 3, in Music).
As I have already said elsewhere, in the bohème of the poet Rodolfo la chiave (the rhyme) della stanza (of the verse) al buio (to the word buio) non si trova (cannot be found, for buio knows no rhyme, in Italian, in spite of the fact that trovare the rhyme is the very job of the expert trovatore, one who "conosce il mestier"), because it's poetry slang, before it being a plot twist.
And so are the piante ascose where Susanna and Fiordiligi are hiding: they are hardly plants of the green kind, they are poetry plants (of the tricolore kind)!
Tra queste piante ascose ("amidst these hidden poetic meters")
Ti vo' la fronte ("I want to make fronte": that is the name of the initial part of the stanza)
Incoronar di rose - ("rhyme with the word rose" in the sirma: the final part of the stanza).
The entire Susanna solo night scene in the garden is some sort of poetry speakeasy, and not by chance it is another Da Ponte invention from scratch, there is no trace of it whatsoever in the original Beaumarchais.
In this scene, as we all know, Susanna is impersonating the Countess. The two literate girlfriends had previously written a plan of such scene, which was of course delivered in verses (la Canzonetta sull'Aria), and which involved the piante (i pini del boschetto) from the beginning.
So now Susanna is there, dressed as the Countess and furthermore faking her voice by imitating her previously heard recitativo "E Susanna non vien...", simply changing the words into "Giunse alfine il momento...", but keeping the same notes and values in the same key of C major.
For that is how you rendered the making of an impression on a score, back in time: you are welcome (and yes, understanding all this dropped my, jaw, too).
But this scene doesn't serve the plot, it serves poetry.
Susanna is not only imitating Rosina in the clothing and in the sprezzatura speech, but also in the content: just as Cherubino's songs are copy-pasted by Da Ponte from Petrarch's, Susanna's nocturne is a Tasso madrigal:
Ecco mormorar l’onde [qui mormora il ruscel]
e tremolar le fronde a l’aura mattutina [qui scherza l'aura]
...
l’aura è tua messaggera, e tu de l’aura [che col dolce sussurro]
ch’ogni arso cor ristaura [il cor ristaura].
This scene was not contemplated in the original plot because not necessary: this is poets, speaking in poetry, to other poets, of Poetry itself, across the centuries, in secret.
Then of course, Susanna is a servant, so she is purposedly butchering a little the original madrigal and filling her mouth with nice-sounding words that she is parroting without knowing their meaning... but isn't that precisely what Opera is all about?
(Have you seen The Shawshank Redemption? That's exactly what the main character says, in the Canzonetta scene.)