The herbs parsley, sage, rosemary and thyme, recurring in the second line of each stanza, make up for a key motive in the song. Although meaningless to most people today, these herbs spoke to the imagination of medieval people as much as red roses do to us today. Without any connotation neccesary, they symbolize virtues the singer wishes his true love and himself to have, in order to make it possible for her to come back again.
Parsley (Petroselinum crispum)
Parsley is still prescribed by phytotherapists today to people who suffer from bad digestion. Eating a leaf of parsley with a meal makes the digestion of heavy vegetables such as spinach a lot easier. It was said to take away the bitterness, and medieval doctors took this in a spiritual sense as well.
Sage (Salvia officinalis)
Sage has been known to symbolize strength for thousands of years.
Rosemary (Rosmarinus officinalis)
Rosemary represents faithfulness, love and remembrance. Ancient Greek lovers used to give rosemary to their ladies, and the custom of a bride wearing twigs of rosemary in her hair is still practised in England and several other European countries today. The herb also stands for sensibility and prudence. Ancient Roman doctors recommended putting a small bag of rosemary leaves under the pillow of someone who had to perform a difficult mental task, such as an exam. Rosemary is associated with feminine love, because it's very strong and tough, although it grows slowly.
Thyme (Thymus vulgaris)
According to legend, the king of fairies dances in the wild thyme with all of the fairies on midsummernight; that's the best known legendary appearance of the herb. But the reason Thyme is mentioned here is that it symbolizes courage. At the time this song was written, knights used to wear images of thyme in their shields when they went to combat, which their ladies embroidered in them as a symbol of their courage.
This makes it clear what the disappointed lover means to say by mentioning these herbs. He wishes his true love mildness to soothe the bitterness which is between them, strength to stand firm in the time of their being apart from each other, faithfulness to stay with him during this period of loneliness and paradoxically courage to fulfill her impossible tasks and to come back to him by the time she can.