Yes, you can grow a small clethra (Clethra alnifolia) in an outdoor plant box! This charming shrub, also known as summersweet, thrives in containers with proper care. Choose a compact variety like 'Hummingbird' or 'Sixteen Candles' for best results. Ensure your plant box has drainage holes and use well-draining, acidic soil. Place it in partial shade to full sun, depending on your climate, and keep the soil consistently moist. Regular watering and occasional fertilizing will help your clethra flourish, rewarding you with fragrant blooms and vibrant foliage. Container-grown clethra may need winter protection in colder zones.