Growing a small nyssa tree in an outdoor plant box is entirely possible with the right conditions and care. Nyssa, commonly known as the tupelo tree, is a beautiful deciduous species prized for its vibrant fall foliage and adaptability. Here’s what you need to know to succeed:

1. Choose the Right Variety: Opt for dwarf or slow-growing nyssa varieties like Nyssa sylvatica 'Sheri's Cloud' or 'Wildfire,' which are better suited for confined spaces.

2. Plant Box Requirements: Ensure your outdoor plant box is large enough (at least 24 inches deep and wide) to accommodate the tree’s root system. Use well-draining soil mixed with organic compost to promote healthy growth.

3. Sunlight and Watering: Nyssa thrives in full sun to partial shade. Water regularly to keep the soil moist but not waterlogged, especially during dry spells.

4. Winter Care: In colder climates, protect the roots by insulating the plant box or moving it to a sheltered location to prevent freezing.

5. Pruning and Maintenance: Light pruning helps maintain shape and encourages airflow, reducing the risk of disease.

With proper attention, a small nyssa can flourish in an outdoor plant box, adding elegance and seasonal color to patios, balconies, or gardens. Happy gardening!