Creating an illusion garden in outdoor plant boxes is a fantastic way to add depth, mystery, and visual intrigue to your space. By carefully selecting plants that play with perception, you can transform a simple planter into a mesmerizing optical masterpiece. Here are some of the best plants to achieve this effect:

1. Dwarf Conifers: These small evergreen trees add height and structure, creating the illusion of a miniature forest. Varieties like 'Blue Star' juniper or 'Hog's Foot' pine work beautifully.

2. Trailing Plants: Ivy, creeping Jenny, or sweet potato vine spill over edges, making plant boxes appear larger than they are while softening hard lines.

3. Variegated Foliage: Plants like hostas or caladiums with striking leaf patterns trick the eye into seeing more space and movement than actually exists.

4. Ornamental Grasses: Feathery grasses like fountain grass or blue fescue add movement and depth, enhancing the illusion of space.

5. Color-Gradient Plants: Arrange plants in color gradients (dark to light) to create false perspective. Coleus varieties are perfect for this technique.

For maximum impact, combine vertical growers with cascading plants and use contrasting textures. Remember to consider growth habits - you want plants that maintain their proportions well in confined spaces. With these selections, your outdoor plant boxes will become captivating illusion gardens that delight and surprise viewers.