Filling deep urban planter boxes entirely with premium potting mix is not only expensive but also often unnecessary for plant health. The most cost-effective strategy is to use a layered approach with filler materials at the bottom, reserving the nutrient-rich soil for the top where the roots will primarily grow.
Start by placing a layer of drainage material, such as broken clay pots or gravel, at the very base to prevent waterlogging. Then, add your bulk filler. Excellent, lightweight options include upside-down plastic pots, crushed and rinsed milk jugs, or reusable packing foam. For a more natural filler, you can use untreated wood chips, fallen leaves, or pine cones. These organic materials will slowly decompose over time.
Create a barrier between the filler layer and the soil above using a permeable landscape fabric or a thick layer of straw. This prevents the soil from washing down into the voids. Finally, fill the top one-third to one-half of the planter with a high-quality potting mix suitable for your plants. This method drastically reduces soil volume and cost, makes the planter lighter and easier to move, and still provides excellent drainage and a deep root zone for your plants to thrive.