Creating an M-theory-themed outdoor plant box is a fascinating way to merge theoretical physics with gardening. Inspired by the 11-dimensional framework of M-theory, this unique garden concept calls for plants that evoke cosmic wonder, symmetry, and multidimensional beauty. Here are the best plants to bring your M-theory vision to life:

1. String of Pearls (Senecio rowleyanus) – Mimicking the "strings" in string theory, this trailing succulent’s bead-like leaves symbolize the interconnectedness of dimensions.

2. Cosmos Flowers (Cosmos bipinnatus) – With their delicate, star-like petals, these flowers embody the vastness and elegance of the cosmos, aligning perfectly with M-theory’s grand scale.

3. Fractal Ferns (Matteuccia struthiopteris) – Their self-repeating patterns reflect the complex, multidimensional structures theorized in M-theory.

4. Spiral Aloe (Aloe polyphylla) – Its mesmerizing spiral growth pattern echoes the hidden geometries of higher-dimensional spaces.

5. Black Hollyhock (Alcea rosea ‘Nigra’) – The deep, almost black blooms represent the mysterious "membranes" (or "branes") central to M-theory.

For added depth, incorporate metallic or geometric planters to emphasize the scientific theme. This blend of botany and physics will transform your outdoor space into a living tribute to the universe’s deepest mysteries.