Gardening has been messed up since EL update. This can happen with all plants unfortunately.
You can wait for it to grow up again... I'm pretty sure it will eventually, or you can cheat it back to normal. Which isn't really cheating because it's the games fault lol.
Turn testing cheats on
Shift click > Set gardening state > Growth stage > Mature
Yup, happened to me as well. Dont worry the plant will go to normal again. If you notice, you can still feed it, and interact as if it was an adult cowplant. Eventually it will revert to the adult state. In any case, i dont pay too much attention to it, its awful as sin. I wish we could have a regular carnivore plant instead of a parody joke.
@Valmont - one thing about using the cheat; in my experience if you set plants to the "blossoming" stage, it seems like they still need to wait until the next morning at the standard 5am to actually blossom. And all the stages between being planted and blossoming for the cowplant look the same (the little head sticking out like you have up there) so it might look like the cheat didn't work. However, when I used the cheat on my unfairly reduced cowplant it turned back into the big mamajamma at 5am the next day.
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You can wait for it to grow up again... I'm pretty sure it will eventually, or you can cheat it back to normal. Which isn't really cheating because it's the games fault lol.
Turn testing cheats on
Shift click > Set gardening state > Growth stage > Mature
https://answers.ea.com/t5/Bug-Reports/OPEN-Cow-Plant-Resets-to-previous-growing-stage/td-p/9782272