Start with drainage, not just the soil. In heavy rain, an urban planter box can quickly turn into a bathtub if water cannot escape. Check that the drainage holes are open, large enough, and not blocked by roots, compacted soil, or debris. If possible, add a few extra drainage holes and cover them with geotextile fabric or fine mesh. This lets water flow out while keeping most of the soil inside. If the planter sits flat on concrete or a balcony floor, raise it slightly on feet or battens so drained water can move away instead of pooling underneath.
Keep the soil level a little below the rim. Leaving about 2–5 cm of freeboard gives heavy rain somewhere to collect before it runs off. If your planter has no overflow route, add one near the top of the soil line or make sure the existing drainage path can handle a sudden downpour. Do not seal the box to stop soil loss; that traps water and makes the problem worse.
Use a soil mix that holds together without becoming muddy. A blend with compost, coarse sand, and aged pine bark or other coarse organic matter is usually more stable than fine topsoil, silt, or clay-heavy garden soil. These fine materials wash out easily and can clog drainage holes. Top the soil with a 5–8 cm layer of coarse mulch such as bark chips or straw. Mulch cushions raindrop impact, slows surface runoff, and keeps soil from splashing out. Keep mulch slightly away from plant stems to avoid rot.
Plants also help. Densely planted fibrous-rooted plants, grasses, and low groundcovers knit the soil surface together and reduce erosion. For a very exposed planter, add a temporary erosion-control mat, burlap, or a light layer of coarse mulch before a storm. If roof runoff or a downspout pours directly into the box, redirect it with a gutter extension, splash block, or temporary cover. After heavy rain, check for rills and low spots, repair them, and top up mulch. With good drainage, stable soil, mulch, and thoughtful overflow, your urban planter can survive heavy rain without losing its soil.