The first step for repurposing a birdbath into a planter is to decide whether you want to plant directly in the birdbath bowl or add a container or containers inside for planting. Whatever you choose, make sure there are drainage holes. Because the planting area is fairly shallow, the best plant options include single-season annuals or succulents. Perennials are unlikely to do well due to the shallow root space making them vulnerable to freezing in winter. Be sure to use potting mix made for the type of plants you choose.