When it comes to growing vegetables in outdoor plant boxes, selecting deep-rooted varieties can significantly enhance your garden's productivity. These plants thrive in well-drained soil and often require less frequent watering due to their extensive root systems. Here are some of the best deep-rooted vegetables to consider:

1. Carrots: Known for their long taproots, carrots are perfect for deep plant boxes. They prefer loose, sandy soil to grow straight and strong.

2. Parsnips: Similar to carrots, parsnips develop deep roots and are excellent for cooler climates. They add a sweet, nutty flavor to dishes.

3. Beets: While beets have shallow foliage roots, their taproots can reach deep into the soil, making them suitable for deeper boxes.

4. Radishes: Fast-growing and easy to cultivate, radishes develop deep roots quickly, ideal for succession planting.

5. Turnips: Both the roots and greens are edible, and their deep roots help them access nutrients efficiently.

For optimal growth, ensure your plant boxes are at least 12-18 inches deep to accommodate these vegetables. Regularly loosen the soil to prevent compaction and promote healthy root development. Happy gardening!