The honest answer is: it depends on the box’s width, depth, and what you are growing. A 24-inch planter box is usually 24 inches long, but the width is often 8–12 inches. That gives you about 1.3–2 square feet of soil surface. If your box is a full 24 x 24 inches, you have 4 square feet, so you can roughly double the numbers below.
For a typical 24 x 10–12 inch box, use these realistic amounts:
- Leafy greens: 4–6 full-size lettuce, spinach, or chard plants; 8–12 if you harvest baby leaves.
- Herbs: 2 basil, 2–3 parsley or cilantro, 4–6 thyme or oregano, or 6–8 chives.
- Root crops: 8–12 radishes, 6–8 beets, or 16–24 carrots.
- Fruiting vegetables: 1–2 peppers, 1 determinate or dwarf tomato, or 2–3 bush beans.
- Flowers: 3–4 marigolds, 4–6 pansies, or 2 petunias.
Do not simply fill every inch. Plants need airflow, root room, and light. For mixed planting, combine similar needs. In a 24 x 12 box, a good mix is 1 pepper + 2 basil + 4 lettuce, or 1 dwarf tomato + 2 parsley + 3 marigolds. If the box is only 6 inches deep, stick to lettuce, herbs, radishes, and chives. For tomatoes, peppers, and carrots, choose at least 10–12 inches of depth.
Spacing is the real rule: small greens and herbs 4–6 inches apart, medium plants 8–12 inches apart, and large fruiting plants 18–24 inches apart. When in doubt, plant slightly fewer and harvest more. A crowded 24-inch urban planter box may look full for a few weeks, but it usually means smaller harvests, disease, and disappointment. Start with the lower number, then adjust next season.