For decades, a common piece of gardening advice has been to place a layer of rocks or gravel at the bottom of a planter box to improve drainage. However, modern horticultural science shows this practice is not only unnecessary but can actually be detrimental to your plants.
The theory behind the rock layer is to prevent soil from washing out while allowing excess water to drain. In reality, this creates a problem known as a "perched water table." Water does not move easily from the fine-textured soil into the large air spaces of the gravel layer. Instead, it accumulates at the bottom of the soil layer, right above the rocks, keeping the lower roots soggy and potentially causing root rot.
The most critical factor for healthy container plants is having adequate drainage holes at the very bottom of the planter. For urban planters, especially in rainy climates, ensure you have several sizable holes. Instead of rocks, use a high-quality potting mix designed for containers. These mixes are formulated to be well-aerated and drain efficiently on their own.
For very deep pots, you can use a "filler" material to save on soil, but it's better to use something lightweight and porous, like upside-down plastic pots or recycled plastic bottles, placed at the bottom. Always ensure your filler does not block the drainage holes.
In summary, skip the gravel. Focus on using a proper potting mix and ensuring your planter has sufficient drainage holes. This simple change will lead to healthier roots and more thriving plants in your urban garden.