Fertilizing plants in an urban planter box is quite different from fertilizing those in the ground. The main reason is that planter boxes are confined environments with limited soil volume, which affects how nutrients are available to your plants.
For plants in the ground, the soil ecosystem is larger and more stable. Microbes and earthworms help break down organic matter, and the soil holds nutrients for longer periods. You can typically apply a balanced, slow-release granular fertilizer (like a 10-10-10 NPK) once at the beginning of the growing season, and then a light side-dressing of compost or a liquid feed mid-season if needed. Watering will naturally distribute the nutrients downward to the roots.
For urban planter boxes, the situation is more intensive. Because the container volume is small, nutrients leach out quickly every time you water. This means you need to fertilize more frequently, but with a lighter hand. I recommend using a liquid fertilizer diluted to half strength every two to three weeks during the active growing season. For example, a water-soluble 5-5-5 or 10-10-10 formula works well. Alternatively, you can mix a slow-release granular fertilizer into the potting mix at planting time, which will feed the plant for several weeks, and then supplement with liquid feeds later. Avoid over-fertilizing in planter boxes, as salts can build up and burn the roots—always water thoroughly after applying liquid fertilizer to flush out excess.
Another key point: in-ground plants often benefit from organic fertilizers like compost or well-rotted manure, which improve soil structure. In planter boxes, these organics can decompose too quickly and become anaerobic, so it is safer to stick with high-quality potting mixes and controlled-release or liquid fertilizers designed for containers.
In summary, be consistent and gentle with your urban planter box plants, while in-ground plants can handle a more relaxed, seasonal fertilizing schedule.