For a plant combination to work, the arrangement of the colors, shapes, sizes, and textures must add up to a complete and satisfying unit. As with all good design, it’s a matter of blending similarities and differences in pleasing proportion to one another.
In human affairs as well as in art, harmony is the product of likeness—of shared qualities and mutual accord. Contrast, on the other hand, depends for its effect on differences, which produce drama and, if those differences are extreme, conflict. The secret of combining disparate ingredients is to preserve the peace but not at the cost of excitement.