Yes, you can absolutely grow a small blackberry bush in an outdoor plant box! Blackberries are adaptable plants that thrive in containers, making them perfect for patios, balconies, or small gardens. Here’s how to ensure success:

1. Choose the Right Variety: Opt for compact or dwarf blackberry varieties like 'Baby Cakes' or 'Apache,' which are well-suited for container growth.

2. Select a Spacious Plant Box: Use a box at least 18-24 inches deep and wide to accommodate the roots and allow for proper growth.

3. Use Quality Soil: Blackberries prefer well-draining, slightly acidic soil. Mix potting soil with compost for nutrients.

4. Provide Sunlight: Place the plant box in a spot that receives 6-8 hours of sunlight daily for optimal berry production.

5. Water Regularly: Keep the soil consistently moist but not waterlogged. Mulching helps retain moisture.

6. Support the Plant: Install a small trellis or stakes to support the canes as they grow.

With proper care, your outdoor plant box can yield delicious blackberries in just a season or two. Happy gardening!