Peta is right, you have to remove the nightshade so that you can attract medium goats.
You actually have to attract more medium sized goats in your garden because every 5th medium goat spotted in your garden will turn the buttercup into a big one.
You can also read the thread about this fairytale which explains the most important points in detail:
https://www.fairylandgame.com/fbfairy/forums.php?tid=74066
