At the Sanctuary Springs neighborhood go to the Desert Bloom lot in the and follow the path to the canyon to the Mine Cavern. Along the rock walls of the canyon is a little pond that feeds a superficial stream. Fish at the pond till you get the Cow Plant seed and go back to your home lot, open your stock/inventory, and drag the seed to where you wish to grow it. Once grown, water it daily till it is fully grown. It requires to be watered if the dirt at the base of plant turns light brown. You should feed the Cow Plant meat to keep it alive once completely grown.