An unknown person dubbed The Midnight Knitter by residents of a US town is covering tree branches and lamp posts with little sweaters under cover of darkness.

Pam Kaithern, mayor of West Cape May, New Jersey said police are looking into the guerrilla needlework, which is against the law because it is being done on public property without permission, The Press of Atlantic City reported.

The mayor and many residents admit they are enthralled by the rainbow of colours that popped up.

Resident Susan Longacre takes a walk each morning in Wilbraham Park, where several tree branches and lamp posts got the treatment.