Guest mg -- the word you're looking for is BROOM (a hardy shrub whose yellow flowers brighten many a Scottish hillside -- but not to be confused with the fearsomely prickly gorse!).

I think that you'll find it if you search for plant genista (it was the symbol of the Plantagenet royal family in England in the early Middle Ages).

When Adam McNaughtan wrote the song, it was in response to a request to review the book of the same name by the late Betsy Whyte, a Scottish traveller who was a fine singer and story-teller. The focus of the book is Betsy's mother, who belonged to a travelling family accustomed to being on the road throughout the year -- but she married a traveller whose habit was to rest up over the winter, and she hated the confinement of a house, always yearning for the sign that it was time to take to the road again "When the yellow's on the broom."

Hope this helps.