Variety Of Form And Shape
Mixing plant shapes together in one pot
A container of all round mounds would be boring - but pair a rounded shape with a spiked one and let a third plant trail below - now you've got some interest going! (Boy, we'll show you a good time!) Back that same mound with a fan shape and a couple of pyramids and you have a more formal look. Make the spiky plant the center of interest and your container will look bold and aggressive.
In some plants, like coreopsis the shape is most apparent in the leaves and the flowers fit right into the shape. The pelargonium, (zonal geranium) on the other hand, is rounded until it flowers, then it is more like two tiers of rounded shapes.
Other plants change shape dramatically when flowering, turning from rosettes to spires. Consider the entire plant, both in and out of bloom when making your choice.