Little Miss Sam created a small mod to fix the gardening bug. I tested it in game and it works perfectly. I can now play my old saves that have mature gardens/farms.
The mod is on her Random Bug Fixes page and it is titled
Gardening Workaround Fix. I recommend installing both mods.
I still recommend you Me Too this bug report and submit saves to help the Sims team fix the bug in game:
https://answers.ea.com/t5/Bug-Reports/NEEDS-SAVES-Plants-reverting-to-dirt-pile-Patch-1-76-81/td-p/10475991
Comments
Maybe they're talking about the optional add-on.