Figuring out how many plants will thrive in your urban planter box is simpler than you might think. The key factors are the box's dimensions and the mature size of the plants you choose. A good rule of thumb is to follow the spacing recommendations found on your seed packets or plant tags, which are based on the plant's full-grown width.
For most vegetables and annual flowers, you can typically fit one plant per 6-12 inches of linear space. In a standard 24-inch long rectangular planter, this means 2-4 medium-sized plants like herbs, lettuces, or marigolds. For larger plants like tomatoes or peppers, one plant per 24-inch box is often ideal to prevent overcrowding.
To maximize a deep box, consider the "thriller, filler, spiller" technique: place one tall centerpiece plant, surround it with several mid-height plants, and allow trailing plants to spill over the edges. Always ensure your planter has adequate drainage holes and use high-quality potting mix to give each plant the resources it needs to flourish in its shared urban home.