Yes, you can absolutely grow a small viburnum in an outdoor plant box! Viburnums are versatile shrubs that adapt well to container gardening, provided you follow a few key guidelines.

Choosing the Right Variety: Opt for compact or dwarf viburnum species, such as Viburnum opulus 'Compactum' or Viburnum tinus 'Eve Price,' which thrive in confined spaces.

Plant Box Requirements: Ensure your plant box is large enough (at least 18-24 inches deep and wide) to accommodate root growth. Use a well-draining potting mix enriched with organic matter to promote healthy development.

Sunlight and Watering: Viburnums prefer full sun to partial shade. Water regularly to keep the soil moist but not waterlogged, especially during hot summers.

Winter Care: In colder climates, protect the roots by insulating the plant box or moving it to a sheltered location. Mulching helps retain soil warmth.

With proper care, your viburnum will flourish in an outdoor plant box, adding beauty and fragrance to your patio or balcony garden.