Corpus Scriptorum Ecclesiasticorum Latinorum, Vol. LVI: Augustini Epistulae Pars III

And in the volume of Jesus, it is called the torrent of Cane Reeds, that is, of 'reeds', which has turbid waters, which Israel chose. Contemning the most pure streams of the Jordan and returning in mind to Egypt and desiring the muddy and marshy region and the melons and onions and others and cucumbers and the pots of Egyptian meats, it is most rightly called by Isaiah a broken reed, whose hand will be pierced if anyone wishes to lean upon it. For whoever, after the coming of the Lord Savior, abandons the spirit of evangelical interpretation and rests in the death of the Jewish letter, all the works of that person are wounded. And he shall not extinguish smoking flax—the people gathered from the Gentiles, who, with the fire of the natural law extinguished, were wrapped in the errors of most bitter smoke and of the dark gloom which is harmful to the eyes. Which he not only did not extinguish and reduce to ashes, but on the contrary, from a small and nearly dying spark, he raised up the greatest conflagrations, so that the whole world might burn with the fire of the Lord Savior, which he came to send upon the earth and desires to burn in all things. According to tropology figurative/moral interpretation, what we think about this place, we have briefly noted in the small commentaries on Matthew. He, however, who did not break the bruised reed and did not extinguish the smoking flax, also brought judgment to victory, whose judgments are true, justified in themselves, so that he may be justified

Jerome continues his allegorical interpretation. He identifies the "Cane" (reeds) with the muddy, worldly desires of Israel, contrasting them with the "pure streams of the Jordan." He explains that "smoking flax" represents the Gentiles who, though lacking the "fire" of the law, are ignited by Christ’s message to spread the Gospel across the world.