For thriving plants in your urban planter boxes, proper drainage is non-negotiable. The best way to arrange drainage rocks is to create a layered system that prevents soil from clogging the drainage holes while allowing excess water to escape freely.
Start by ensuring your planter has adequate drainage holes. Cover these holes with a piece of broken pottery or a mesh screen to prevent soil loss. Next, add a layer of coarse drainage material like pebbles, lava rocks, or clay shards. The depth of this layer should be proportional to your planter's size; for most boxes, a 1- to 2-inch layer is sufficient.
A critical step many skip is adding a permeable barrier atop the rocks. Place a piece of landscape fabric or a single sheet of newspaper over the rock layer. This separator allows water to pass through but stops the potting soil from washing down and mixing with the rocks, which would negate the drainage benefit.
Finally, add your high-quality potting mix and plants. This layered approach—hole cover, rocks, fabric, then soil—creates an efficient drainage zone, preventing "wet feet" and root rot. Remember, the rocks themselves don't hold water; they create air pockets for water to flow. Avoid the old myth of using gravel alone without a separator, as soil will eventually settle into the gaps and impede drainage. For deeper boxes, this system is especially crucial for long-term plant health in an urban setting.