An examination of the first chapter will give you the answer that you are looking for. As the novel opens, we are told that Matt has just been left by his father in the middle of the forest to look after their plot of land and home whilst his father returns to civilisation to collect Matt's mother and sister, who will come and live with them. Matt and his father have been living in the forest for some time, making their house and clearing land for farming. They have also planted some crops. As Matt is left by himself, it is vitally important that he tends to the crops that they have left. Note what the first chapter tells us about these crops:
Behind the cabin were the mounds of corn he had helped to plant, the green blades already shooting up, and the pumpkin vines just showing between the stumps of trees.
Thus we know that the two crops that Matt has to look after and that he tends and waters are the corn and pumpkin vines that he planted with his father before his father left him to go and collect the rest of their family.