There's a line of thought that this song is linked to the 'corn king' myth which appears in a lot of cultures, that a man was chosen by a country community, treated like a king for a year and then sacrificed at harvest and his body dragged through the harvest fields to put his blood on the land and give it life for the next year. The idea of making Kern dolls which were kept for a year and then ploughed into the fields at harvest or burned, and replaced with a doll from the new harvest is possibly a later version of the custom. Again it's based on the idea of death giving life, and tied in a lot with the Green Man mythology. The current Wicker Man remake is based on this. The words for the hymn 'now the green blade riseth' also links a lot between the Christian idea of death and renewal and the pagan harvest beliefs.