The short answer is yes for many urban planter boxes, but it is not a universal rule. Plants in planter boxes grow in a small, confined volume of growing medium, so their roots cannot spread widely in search of water and nutrients. That limited reservoir is also flushed more quickly by frequent watering, especially in hot, windy, exposed urban sites. As a result, planter box soil nutrients can decline faster than in a garden bed, where roots can access a much larger soil volume and organic matter cycling.
Urban planter boxes often hold annual flowers, herbs, vegetables, and compact shrubs, and many of these are heavy feeders during active growth. Dense planting increases competition. Fresh potting mix may contain starter fertilizer, but it usually runs out within weeks to a few months. Container plant feeding therefore tends to be more frequent, though it should usually be at a moderate rate rather than a heavy dose. A common approach is to mix a controlled-release fertilizer into the medium at planting and then use a liquid feed every two to four weeks during peak growth, following the product label. Compost topdressing can help support biological activity and moisture retention, but it is not a complete fertilizer substitute.
In-ground plants are not automatically self-sufficient. Compacted, sandy, polluted, or highly disturbed urban soils may also need organic matter and fertilizer. However, in-ground plants generally have deeper root zones, more stable moisture, and greater access to recycled nutrients, so they often need less frequent supplemental feeding than containerized plants.
The key is to avoid overfertilizing. Excess urban gardening fertilizer can cause salt buildup, runoff pollution, weak floppy growth, and reduced flowering or fruiting. When fertilizing container plants, check drainage, water deeply when needed, and use a soil test or professional advice for valuable plantings. For most urban planter boxes, the practical answer is: fertilize more often than in-ground plants, but in modest amounts and only during active growth.