This is my 6th or so play-through and each time I would work steadily to completely clear my farm so I could maximize the amount of grass that spreads for future animal hay. This time I did four corners (single player), left one corner neglected, and this happened! I then read how it works on the wiki and I understand why I never got them before haha. I am sooooo excited.
Nice! I've been hoping for one myself, just finished first year playing sve. Maybe I'll try letting one of the animal areas go crazy with trees and see what happens.