Drill 1/4-inch drainage holes spaced 4 to 6 inches apart in the bottom of the wagon, using a power drill and a 1/4-inch drill bit. These holes will provide essential drainage to prevent root rot and overwatering.
Cut a piece of wire mesh screen, like a window screen, to fit the bottom of the wagon. You can cut the screen with a pair of household scissors.
Line the bottom of the wagon with the screen to prevent soil from falling through the drainage holes. If the wagon requires multiple pieces of screen to cover the bottom, overlap each piece by 2 to 3 inches to ensure no soil escapes through the joints.
Cover the screen with a thin layer of pea gravel to provide additional drainage, if desired. Use only enough gravel to cover the bottom of the wagon to ensure enough room to add soil for your plants.
Fill the wagon with potting mix, using a bagged mix or your own mixture of ingredients such as peat, compost and sand. Leave about 1/2 inch to the container edge.
Plant small annual flowers, herbs or other small plants in the soil; plant each plant to the same depth as the original container. The wagon planter works well as a kitchen herb garden or an annual flower garden in which you can trade out plant species as the blooming periods end.
Park the wagon in a fixed location (like the front porch!). If you used a working wagon, you can move the wagon to different locations throughout the yard.