Calla Lily in Genshin Impact is a flower that will grow near sources of water. Lake beds, ponds, and rivers can all be home to this green, white, and orange plant. You will need quite a few of them for cooking Calla Lily Seafood Soup, or for ascending characters like Kaeya.
While the plant can grow just about anywhere near water in Mondstadt, the best place to farm them is at the small pond in Springvale, just south of the city of Mondstadt itself.
This poind will have between 10 and 15 Calla Lily growing around it any any given time. It is a good idea to visit it every couple of days, grab all the plants, and then go do whatever other activities you need to do. Have a constant source of the plant means you won’t need to spend time specifically farming it up when you need it.
Calla Lily is also one of the plants that does not need a specific elemental interaction in order to pick it. You can farm it no matter what party members you are using.