The trend is not to segregate flowers in their own beds, but to plant what's called a "mixed border" in which small flowering trees like ornamental pears or shrubs like rhododendrons join perennials in a planting that can look attractive all season. This style is practical for today's smaller properties, which don't have enough space for separate beds of shrubs and flowers - yet another reason not to stint on the width of your beds.
Trees and shrubs are a lower maintenance way to add all-season color and texture. If you choose carefully, they can be among your easiest-care plants. A garden populated with one of this and one of that tends to look jumbled and in need of something to pull it all together.
Garden designers recommend planting all except some of the largest stand-alone or specimen plants in odd-numbered groups of three or five. Depending on the space you have available, this is also a good idea with shrubs. The type of trees/plants should not vary from those you would plant on the ground. Depending on where you live and what season it is, different trees and plants will flourish. Just make sure the roof is strong enough to support the weight of the plants and the heavy dirt.