Absolutely, it is not only possible but surprisingly easy to over-fertilize a small, contained space like an urban planter box. In fact, the confined environment makes it a common issue for container gardeners. Unlike in-ground gardens where nutrients can leach and disperse, fertilizers in a planter box are concentrated in a limited volume of soil. Applying too much, or fertilizing too frequently, leads to a harmful buildup of mineral salts.
This excess creates a condition called "fertilizer burn." The high concentration of salts in the soil makes it difficult for plant roots to absorb water, effectively causing drought stress even when the soil is moist. Visible signs include yellowing or browning leaf tips and margins, stunted growth, leaf drop, and a white, crusty residue on the soil surface.
If you suspect over-fertilization, stop feeding immediately. The most effective remedy is to "flush" the soil by slowly running a large amount of clean water through the planter, allowing it to drain freely. This helps leach out the excess salts. For severe cases, you may need to repot the plant with fresh potting mix.
To prevent this, always follow label instructions for container plants, often using a half-strength dilution. Use a balanced, slow-release fertilizer formulated for pots, and fertilize only during the active growing season. Remember, in a small planter box, less is often more. Healthy soil, proper watering, and mindful feeding will keep your urban garden thriving without the risk of chemical overload.