Whenplants contrast through one or more attributes, they emphasize their contrasting qualities, as for example, with opposite colors, or very different leaf shapes or overall form. When they do both-when plants relate to one another through both harmony and contrast those groupings have the greatest impact.
Creativescaping: Four Season Garden Design
TThroughthe principles of harmony and contrast plants form relationships with one another and it is the perception of these relationships which excite our emotions. When plants harmonize by some attribute - their form or texture, lets say-they reinforce that attribute by sharing it together. They relate to one another through that attribute.