Yes, you can absolutely grow a small spirea in an outdoor plant box! Spireas are versatile, hardy shrubs that adapt well to container gardening, making them a great choice for patios, balconies, or small gardens. Here’s how to ensure success:

1. Choose the Right Variety: Opt for compact spirea cultivars like 'Little Princess' or 'Goldflame', which thrive in confined spaces.

2. Select a Suitable Plant Box: Ensure the box is at least 18 inches deep with proper drainage holes to prevent waterlogging.

3. Use Quality Soil: A well-draining potting mix enriched with organic matter will support healthy root growth.

4. Sunlight Needs: Spireas love full sun (6+ hours daily) but tolerate partial shade, though flowering may reduce.

5. Watering & Fertilizing: Keep soil consistently moist but not soggy. Feed with a balanced fertilizer in spring.

6. Pruning: Trim after blooming to maintain shape and encourage new growth.

With these tips, your spirea will flourish in its outdoor plant box, adding vibrant color and texture to your space!