I've been reading a book by Linden Hawthorne called Gardening With Shape, Line and Texture. I like the book. There is a strong disconnect, though, between the first chapter of the book that talks about classic design concepts and her following chapters that talk about how she characterizes plants. It is the latter chapers that I like and find interesting. She characterizes plants based on whether their form is horizontal, vertical, arching, transparent or mounded. The assumption is that for good garden design you should try to vary the form of plants (i.e., not just put the tall plants in the back). I've attached two pictures from my yard. One includes alliums in the forefront and the other is cropped to not show the alliums. To the extent that the allium is a "vertical" and the hardy geraniums are a "mounded" plant the composition does appear more interesting to me when the alliums or verticals are included.
What's also interesting though is when plants are massed into horizontal bands, they lose their inherent structure that Hawthorne attributes to them as you can see in the pictures I've attached of the Lurie Garden in Chicago. The purple salvia, which in Hawthorne's book is a "vertical," takes on a horizontal structure because it is planted in bands.
