Absolutely, it is not only possible but quite common to over-fertilize plants in urban planter boxes. While nutrients are essential, the confined space of a planter box creates a unique environment where fertilizer salts can quickly build up in the soil, leading to a condition known as "fertilizer burn.
The primary risk is root damage. Excess salts in the fertilizer can draw water out of the plant's roots, effectively dehydrating them. Visually, this often manifests as yellowing or browning leaf edges and tips, wilting despite moist soil, stunted growth, or even leaf drop. In severe cases, it can kill the plant.
To address over-fertilization, immediate action is needed. The most effective remedy is leaching: slowly and thoroughly flushing the soil with plenty of clean water to help dissolve and wash away the excess salts. Ensure your planter box has excellent drainage holes for this process. For extreme cases, you may need to repot the plant with fresh potting mix.
Prevention is always best. For urban containers, use a balanced, water-soluble fertilizer and always apply it at half the strength recommended on the label. The "less is more" principle is crucial. Fertilize only during the active growing season (spring and summer) and avoid feeding stressed, dormant, or newly potted plants. Regularly incorporating organic compost into your potting mix can also provide slow-release nutrients and improve soil health, reducing dependency on chemical fertilizers.
By understanding the limits of your planter box ecosystem and adopting a cautious feeding approach, you can ensure your urban greenery thrives without the danger of nutrient overload.