I think that's the hardest one! There's a few requirements:
It's the second spinach event. Crop events happen on odd numbered years based on what crops you plant.
Terra needs to be old enough to help on the farm (year 7+) and you need to have her helping at least a little bit. You need to choose to make snail traps, and choose to kill the snails. Terra needs to be independent - you increase this stat by having her do hard things on her own. Then, she will suggest an alternative to killing the snails that will lead to this achievement.
I hope this helps!