I'd never heard of it, but I'm seeing suggestions that lodges of 'freegardeners' were outgrowths of the old Gardener's Guilds, turned into 'friendly societies'. That's pretty much what I would have suspected.

History Shelf draws the same conclusion:
http://www.historyshelf.org/shelf/free/index.php

The original craft guilds of gardeners were city organizations and were established between 1500 and 1700. At some point, apparently they grew beyond merely trade guilds... It's possible that either the masonic symbols were borrowed from the Freemasons, or crept in from the study of esoterica and symbolism that was especially popular in the 1700s.