With normal crops, you can plant them in rows along with fertilizer sticks to increase the star ranks of the crops grown. How do you fertilize trees if they take up a 3x3 grid and displace furrows?
Fertilizer can be applied to trees by planting them next to the space the tree takes up like so.
Fertilizer applies to every spot in a 3x3 grid centered on the stick, so in the previous example, the tree would gain 3 star points a day. This means that one stick can fertilize four trees if planted like this:
Warning: Do not plant fertilizer sticks in furrows/trenches when trying to fertilize trees, they will not work in this manner.