- A rival farmer dams off his source of irrigation and makes other problems. Rancher wars.
- Any sort of burrowing monster
- Radical druids who believe agriculture is too unnatural attack the farm or otherwise cause problems. (maybe they're on the paleo diet or something

)
- Long winter causes a famine, and his crops are in great demand. Must find a way to grow more to feed the hungry.
- Learns of a new type of magical fertilizer or seed, and must adventure to retrieve it.
- He gets shrunk by a mischievous fey and must journey across his now huge garden to get to a potion/magic item/other fey that might cure him. On the way he contends with the pests he has been trying to exterminate, and while small finds out that he can communicate with both the helpful animals he meets and maybe discovers that there are small abominations infesting his garden that he never knew about.
- Hired hands/druid acolytes getting carried off by something, must go help them.
- Incredible success and the problems that go with it. Like maybe his green thumb is creating extremely large vegetables creating problems like "how do I transport this to market/to the soldiers/to the orphanage". Or he makes a fortune causes a glut on the market, putting other farmers out of business, and causing rampant unemployment leading to organized crime. Or the local baron increases taxes and starts claiming a personal tax on his goods. Or the farm gets so big that he can't manage it all by himself and need to hire a staff or acquire vassals, and must deal with politics and protecting these people; as a landowner maybe he acquires a title, whether he wants one or not!