Just as there are standards that define a "classically" beautiful face, there are standards that define an aesthethically pleasing garden. Key among these is the requirement that a well-designed vista should have vegetation at at least 3 levels. In a flower border, this comes from having tall, medium-sized, and short plants. In a landscape, layers are typically provided by large trees at the top or canopy layer, small trees or shrubs at the middle layer, and by a ground layer composed of small shrubby groundcovers or other plants.
In the flower garden, structure comes from planting different sizes of annuals and perennials. For height and plain old cheerfulness, nothing beats sunflowers, as shown in this garden painting by Monet.
In the landscape, trees, shrubs, and groundcovers bring structure to the scene. This painting by Van Gogh shows a garden with at least four layers of vegetation (a large tree, many small trees and shrubs, a planting of perennials, and lawn). He's also added structure using man-made objects: benches, gates, and buildings. For instance, the church tower in the distance on the right echoes the tall tree on the left, creating a canopy layer for this design. The garden gate on the lower right side completes the middle layer.
Happily, multiple layers are also good for birds and other wildlife. Different animals "specialize" in different layers of the environment. Juncos, for instance, feed and nest near the ground, whereas other birds tend to hand out in the middle layer and some will be found only in the treetops. So what's aesthetically pleasing to many of us is also what creates the most hospitable environment for animals. (Personally, I don't think that's the least bit accidental.)
It's important to keep structure in mind when planning a landscape. Trees provide that essential top layer of landscape structure (even aside from all the other benefits they convey). Since trees take a long time to grow, we always want to hold onto and take advantage of any trees already present on the site, creating a design around them. If there are no trees on the site yet, we'll want to decide what we want in that top layer and get that planted before starting to worry about anything else.
While waiting for big trees to mature, consider planting fast-growing trees such as paper birch. Typically trees that grow quickly don't live long ("live fast, die young"), but some clumps of short-lived, fast-growing trees will give at least the illusion of an upper level while the slower growing, long-lived trees plod along to their ultimate size. In the meantime, those fast-living trees provide shelter for wildflowers and animals. And when they do die, they become excellent habitat for birds and other animals that nest in decaying trees.
This homeowoner created structure in a relatively new garden by planting fast-growing paper birch in a key location. Even though they probably won't live long, fast-growing trees create wildlife habitat and the shady conditions needed by many wildflowers that are native to wooded areas.
Sometimes you can get the effect of a canopy layer using man-made structures, especially if you grow vines on them. Some structures such as houses or pergolas can be covered in perennial vines. But you can also use fast-growing annual vines such as scarlet runner beans to mask structures such as telephone poles that wouldn't be suitable for permanent plantings. (Sure, there's a chance you'll lose the vines if the telephone company needs the pole, so that's why you want to plant easy annuals such as scarlet runners. But more likely the phone company will find other things to do, and you and the hummingbirds will get to enjoy the flowers.)
Whether you're gardening for wildlife or gardening for yourself, your environment will be richer and more aesthetically pleasing if you remember to plant the bones.
In this painting by Monet, layers come from low-growing flowering plants, flowering plants that have been raised above the ground in pots and window boxes, a small tree and large shrub, and -- at the top level -- vines that have been trained to grow to the roof of a two-story house. Both Monet paintings, the Van Gogh painting, and the Art Nouveau woman in the background of the image at the top of this post are all courtesy of Dover Publications.