Mary is a remedy like a healing herb

The rule of Divine Providence is that it provides for everything according to what is best suited. This is why, for man, because he is human, it provides remedies derived from the ground. Thus in Ben Sira: “The Lord has brought forth medicinal herbs from the ground” (Si 38:4). Two remedies extracted from the earth are offered: green herbs and fruit-bearing trees. The healing herb is the Blessed Virgin Mary, whose nativity the Church is presently celebrating. She is called herb, on account of her humility; green, on account of her virginity; and fruit-bearing, on account of (her) fertility. Saint Thomas Aquinas In Sermon 16, Germinet terra (Birth of the Virgin)