The best way to protect an urban planter box from salt spray near the coast is to use a combined strategy: block the wind, choose salt-tolerant plants, improve drainage, mulch the soil surface, and flush the soil with fresh water after salty weather. No single measure is enough in an exposed coastal setting, because salt spray affects both the foliage and the soil.
Start with a wind-permeable barrier on the side facing the prevailing coastal wind. A solid wall can create turbulence and may concentrate salt in unexpected places, so use a woven screen, reed fence, or similar semi-permeable barrier. It should be tall enough to shield the plants without completely enclosing the box. Secure it firmly so it does not become a hazard in strong winds. For occasional storms, a temporary cover over the planter can help, but remove it afterward so humidity does not build up and encourage disease.
Plant selection is equally important. Choose compact, wind-tolerant, salt-tolerant species suited to your climate rather than tender broad-leaved plants that burn easily. In many coastal urban settings, robust Mediterranean-style plants, coastal grasses, and other salt-tolerant ornamentals perform better. Keep the planting density reasonable so air can move through the canopy and leaves dry quickly.
Use a well-drained growing medium and make sure the planter box has clear drainage holes. Salt accumulates in soil when water evaporates, so good drainage and regular leaching are essential. After periods of salt spray, slowly apply fresh water until it runs out of the drainage holes. This flushes salts downward instead of leaving them around the roots. Do this early in the day and avoid wasteful overwatering.
Mulch the soil surface with gravel, coarse bark, or another stable material to reduce salt-laden dust and splash reaching the soil. Rinse plant leaves with fresh water after noticeable salt events, especially during dry, windy weather. Prune badly burned growth, but avoid excessive fertilizer, which can worsen salt stress. Also avoid using grey water or de-icing salts near the planter.
In an urban context, position the box away from the most exposed splash zone if possible, group planters for mutual shelter, and make sure any runoff drains responsibly. If the planter is highly exposed, monitor the soil and flush it more often. The most reliable approach is therefore an integrated one: a windbreak barrier, salt-tolerant planting, free-draining soil, surface mulch, and routine freshwater leaching. This protects both the visible plants and the root environment, giving the planter the best chance of staying healthy near the coast.