Found this in a file of text from 1991, which gives:

This song started life as a ballad about a shepherdess who encountered a gentleman passing on horseback. The song became popular across Scotland and England towards the end of the reign of James l & VI, and the earliest publication date found is 1651. There are many versions of the ballad, indicating its great popularity, but the underlying story is consistent: she and the gentleman had an instant attraction to each other, and spent some time enjoying each other's company. The gentleman continued on his journey, leaving the shepherdess expecting a child. Just before the child was due to be born, the mystery man returned, declaring himself to be a wealthy Laird, and married her.

The lyrical version of the song is usually sung from the shepherdess' perspective. The Laird took the shepherdess far from her "ain country" and she became very homesick. The version given here swaps the gender, making a banished shepherd the subject, but the yearning for the "bonnie broom" remains.

The broom, a tall shrub which blooms with spikes of small golden flowers, once grew abundantly on hillsides of the Scottish Borders. Cowdenknowes is a Scottish estate on the east bank of the river Leader Water, 32 miles southeast of Edinburgh. The original tower house built by the Homes of Cowdenknowes in the 15th century is still occupied.