For generations, many gardeners have faithfully added a layer of gravel or rocks to the bottom of their planter boxes, believing it improves drainage. However, modern horticultural science has thoroughly debunked this practice. Contrary to popular belief, a gravel layer does not enhance drainage and can actually harm your plants.
The logic seems sound: rocks create space for excess water to pool away from the soil. The reality is more complex due to the physics of water movement. Water does not easily move from a fine-textured medium like potting soil into a coarse-textured one like gravel. Instead, it tends to "perch" or accumulate just above the boundary between the two layers, keeping the lower portion of your soil waterlogged. This creates a saturated zone precisely where you don't want it, right in the root zone of your plants.
This perched water table can lead to chronically wet soil, which is a primary cause of root rot, a fatal condition for plants. Healthy roots need both water and air; waterlogged soil suffocates them. The best practice for ensuring excellent drainage is to fill your entire planter box with a high-quality, well-draining potting mix. These mixes are specifically formulated with materials like perlite, vermiculite, or coarse sand to create air pockets and allow water to flow freely through the entire container.
The only essential feature for a planter box is adequate drainage holes at the very bottom. Ensure these holes are clear and unobstructed. If you are concerned about soil washing out, placing a single piece of landscape fabric or a coffee filter over the holes is a much more effective solution than a layer of gravel. By skipping the gravel and focusing on a uniform, quality soil mix, you provide your urban plants with the well-drained, aerated environment they need to thrive.