Absolutely, you can mix different plants in one urban planter box—it's not a bad idea at all! In fact, it's a fantastic gardening technique often called "companion planting" in containers. When done thoughtfully, it can create a lush, productive, and visually stunning mini-ecosystem on your balcony, patio, or windowsill.
The key to success lies in understanding a few simple principles. First, match plants with similar needs. Group together plants that require the same amount of sunlight, water, and soil type. For example, don't pair a thirsty tomato with a drought-tolerant lavender; both will struggle.
Second, consider the "Thriller, Filler, Spiller" design formula. Choose one tall, dramatic plant as the "thriller" (e.g., ornamental grass, dwarf sunflower). Add several mid-height "filler" plants for bulk (e.g., petunias, herbs like basil). Finally, include trailing "spiller" plants to cascade over the edges (e.g., ivy, sweet potato vine, nasturtiums).
Third, think about function. Mixing edible flowers like marigolds with vegetables can help deter pests. Fast-growing lettuces can be planted around slower-growing peppers. Herbs like rosemary can provide structure while thyme spreads as a living mulch.
The main pitfalls to avoid are overcrowding and incompatible pairings. Ensure your planter has adequate drainage and is large enough for the root systems of all your chosen plants. With careful selection, mixing plants transforms a single container into a vibrant, space-saving garden that maximizes your urban greenery.