For a thriving urban garden display, fitting the right number of petunias in your three-foot-long planter box is key. Realistically, you can fit between 4 to 6 healthy petunia plants. The exact number depends on the petunia variety and your desired look.
For a lush, full appearance quickly, space plants about 6 to 8 inches apart. In a 36-inch box, this typically allows for 4 to 5 plants. If you choose vigorous trailing or spreading varieties like 'Wave' petunias, opt for 4 plants maximum, as they need more room—up to 12 inches—to spread beautifully without overcrowding. For compact, upright "multiflora" types, you can comfortably fit 5 or even 6 plants spaced 6 inches apart.
Overcrowding leads to competition for water, nutrients, and light, increasing disease risk. Ensure your planter is at least 8-12 inches deep with proper drainage holes. Use a high-quality potting mix, water regularly, and provide full sun (at least 6 hours) for continuous blooms all season. With correct spacing, your three-foot planter will become a vibrant, overflowing centerpiece.