Growing a small crepe myrtle in an outdoor plant box is entirely possible, provided you follow a few key guidelines. Crepe myrtles are versatile flowering shrubs that can thrive in containers if given the right conditions.

1. Choose the Right Size: Opt for a large, sturdy outdoor plant box—at least 18-24 inches deep and wide—to accommodate the root system and ensure proper growth.

2. Soil and Drainage: Use well-draining soil mixed with compost to retain moisture without waterlogging. Ensure the box has drainage holes to prevent root rot.

3. Sunlight Needs: Crepe myrtles love full sun (6+ hours daily). Place the plant box in a sunny spot for optimal blooming.

4. Watering and Fertilizing: Water regularly, especially during dry spells, and use a balanced fertilizer in spring to promote healthy growth and vibrant flowers.

5. Pruning and Winter Care: Light pruning in late winter helps maintain shape. In colder climates, protect the roots by mulching or moving the box to a sheltered area.

With proper care, your small crepe myrtle can flourish in an outdoor plant box, adding beauty to patios, balconies, or gardens.