Geiriadur Prifysgol Cymru (The Uni. of Wales Dic.) notes that Welsh calaf 'reed' is a borrowing from Latin calamus. Latin dictionaries note that calamus refers to objects made of reed such as flutes etc., not to the unprocessed plant, while Welsh sources attest only the latter meaning.
Some dictionaries note that Lat. calamus it is a borrowing from Greek καλάμι.
Would it be possible that the Welsh form is a PIE cognate rather than a borrowing?