It's not a Fenland custom but they've been Wassailing orchards to scare off evil spirits and encourage the apple trees into growth for hundreds (maybe thousands) of years in Devon, Somerset, Dorset, Gloucestershire and Herefordshire.
Here's to thee, old apple tree,
That blooms well, bears well.
Hats full, caps full,
Three bushel bags full,
An' all under one tree.