The stop signs are really a problem, because the number of gardens to hunt in is greatly diminished. This has particularly been true of the higher levels of bottom dwellers.
I hugely dislike reporting, I see it as snitching and it gives me the shivers, really.
It's not the responsibility of players to solve this problem. This problem should be addressed more systematically, and I think this should be done by the developers. The best possible solution at hand to me is that the developers would increase the frequency at which higher level bottom dwellers occur in gardens. This way they wouldn't be considered impossible to spot without "prior arrangements", they would be just what they were mean to be - difficult to spot.
P.S.: On the bright side, luckily, there still are quite a few gardens that are being kept open through the whole challenge, which is the reason why I managed to spot the required five of all the BD. I wanted to thank all these garden owners, but as there is no chat section anymore (the first three pages in that section are always just word games), my big thank you just got burried on page xy.
https://www.fairylandgame.com/fbfairy/forums.php?tid=104422