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.
Try to clear everything but trees and see if you get lucky! I went in and cleared the bushes (the ones that drop fiber and seeds) and grass and let the trees go crazy in this corner of my farm to stock up on wood for winter.