Determining how many plants can thrive in a three-foot (approximately 36-inch) long urban planter box depends on the mature size of the plants and your desired look. A good rule of thumb is to follow the spacing recommendations on seed packets or plant tags, which are for optimal growth. For a lush, full appearance, you can slightly reduce this spacing, but overcrowding will lead to competition for nutrients, water, and light.
As a general guide, you could fit about 3-4 large plants like tomatoes, peppers, or kale. For medium plants such as lettuce, herbs (basil, cilantro), or marigolds, aim for 6-8. For small, trailing, or compact plants like arugula, radishes, dwarf nasturtiums, or creeping thyme, you could plant 10-12 or more.
Consider the "Thriller, Filler, Spiller" method for visual appeal: one tall centerpiece plant (thriller), several mid-height plants around it (filler), and trailing plants at the edges (spiller). For a mixed planter, you might combine 1 tomato (thriller), 2 basil plants (filler), and 3 nasturtiums (spiller).
Key factors for success are using high-quality potting mix, ensuring adequate drainage holes, and consistent watering. Remember, a slightly underplanted box often yields healthier results than an overcrowded one. Choose plants with similar sun and water needs to create a harmonious and productive mini-garden on your balcony, patio, or windowsill.