When setting up a planter box, proper drainage is crucial for plant health. The simple answer is that you do not need to put special materials like rocks, gravel, or broken pottery at the bottom for drainage. Modern horticultural research shows that adding a separate "drainage layer" can actually create a perched water table, trapping moisture in the soil above and potentially harming roots.
The most important element is using a high-quality, well-draining potting mix designed for containers. This soil should fill the entire planter box. Ensure your box has sufficient drainage holes in the bottom. If holes are large, covering them with a piece of breathable landscape fabric can prevent soil from washing out while allowing water to escape freely.
For deep planters, you can fill the bottom third with lightweight, inexpensive filler materials like recycled plastic bottles or packing peanuts to save on soil cost, but this is for volume, not improved drainage. Always prioritize a good potting mix and adequate holes. Avoid compacting the soil, and water thoroughly until it runs freely from the drainage holes, ensuring a healthy environment for your plants' roots to grow.