The third phrase of Lúthien’s Song (LB/354). Three translations of this phrase are:
Patrick Wynne: “✱Here/now let flower and tree listen in silence/below” (NTTLS/11)
David Salo: “✱now flower and tree, listen silent” (GS/211)
Bertrand Bellet and Benjamin Babut: “✱then flower and tree, listen in silence” (GTLC)
The first word si resembles the Sindarin word sí “here”, though that word usually appears with a long vowel. All of Wynne, Salo, Bellet and Babut suggested instead that it might in this particular case be used temporally like its Quenya cognate Q. sí “now”. However, in notes published after all of their analyses, Tolkien stated that S. sí only meant “here”, and used hí for “now” (PE17/27). I think it is safer to assume that si means “here”.
The second word is loth “flower” joined by the conjunction a “and” to galadh “tree”. The fifth word lasto is the imperative form of the verb lasta- “to listen”. The last word is probably the noun dîn “silence”, though Wynne suggested it might be some form of di “beneath” (NTTLS/9).