To keep soil from washing out of an urban planter box during rain, think about slowing the water down and giving it a clear way to leave. First, check the drainage holes at the bottom. Clear out roots, compacted soil, and debris, and make sure the planter is not sitting flat against pavement. Raise it slightly on feet or blocks so water can escape freely. If the box has no drainage holes, drill several before the next storm.
Leave a rim of about 1 to 2 inches between the soil surface and the top edge of the planter. Soil that sits flush with the rim splashes out easily. A lower soil line gives rain a place to collect and drain instead of overflowing.
Add a 1- to 2-inch layer of coarse mulch, such as shredded bark, straw, or chunky compost. Mulch cushions the impact of raindrops and keeps the soil surface from crusting or floating away. Avoid very fine, powdery compost or peat-based material, which can wash off easily. Keep mulch slightly away from plant stems so they do not stay too wet.
Use a soil mix that drains well and contains some coarse material, such as pumice, grit, or aged bark. Very fine, silty topsoil is more likely to run out with rainwater. Plant densely with herbs, groundcovers, trailing plants, or shallow-rooted flowers; their roots help hold the soil in place. If a section is bare, add a temporary cover crop or a light layer of mulch.
Manage the water coming into the planter. If a downspout, roof edge, or gutter drips directly into the box, redirect it with a downspout extension, diverter, or simple splash block. Move the planter out of the heaviest drip line when possible. For a large box, create a small overflow notch or spillway near the top and line it with coarse gravel or mesh so water leaves in a controlled place rather than cutting through the soil.
During an especially heavy storm, you can temporarily cover the soil with a breathable cloth or a tilted board that lets air move but breaks the force of the rain. Remove it afterward. Also check the planter after each heavy rain, top up any eroded soil, and adjust the mulch or drainage before the next storm.