In landscape design terminology, "focal points" force the viewer’s perspective to a particular location. Such "focalization" gives a design a sense of purpose and order, without which a landscape is just a careless collection of plants and other objects (or empty space). Either plants or hardscape features (including decorations, yard art, etc.) can be used to serve this purpose.
The use of symmetry creates the effect in an obvious manner, while asymmetrical designs soften or even avoid focalization.
For example, it is common to create symmetry at a front entrance to a home, especially using shrubs. Such an attempt is not always, however, without its drawbacks. What happens when the growth of one of the shrubs outstrips that of the other (symmetry mismatches are one of the landscaping mistakes that beginners need to be warned about)? Even if you can keep them on an even footing via pruning, this may entail more landscape maintenance than you desire in the long run.
Note that, in a landscape design context, "balance" refers to the consistency of visual attraction, or lack thereof. Consistent visual attraction is achieved through symmetry; if the designer’s intention is to avoid the monotony of this kind of balance, asymmetrical plans will be implemented, instead. While it may seem a contradiction in terms to novices, landscape designers do speak of "asymmetrical balance," as well as of "symmetrical balance."
It is easy to achieve focalization on an uncluttered lawn. Simply adorn it with a suitable accent. There is nothing to compete with the selected feature for attention, so it automatically assumes focal-point status. Again, either a specimen plant or an inanimate object can do the trick (just ensure that it is large enough to be appreciated from a distance, especially on large properties).
While you may think of such focal points as thoroughly optional, sometimes they are, in fact, more or less necessary (from a designer's perspective). Take the scene in the picture above. The lawn here is so vast that it affords the viewer's gaze no rest. This lawn cries out for an accent -- any accent -- to furnish the eye with a resting place, a destination.
Indeed, supplying such visual resting places is one of the roles that a focal point can play. A focal point can also be used to highlight a particular location in your landscaping. For example, you might wish to highlight the intended entrance to your backyard garden with a garden arbor, under which visitors are invited to pass. The presence of such a structure makes it less likely that visitors will enter the garden where you do not want them to (for instance, through an area where you have small, fragile plants growing).
It is easy enough to remember the definition of "focal point" if you associate it with the word, "focus." An item that is distinguished from the other items in a group as being the "focus" (noun) is the center of attention.
Likewise, when you "focus" (verb) your thoughts on something, you are directing your attention to that particular thing, to the exclusion of others.
What Can Serve as a Focal Point?
With the movement and sound they provide, garden fountains are one of the best hardscape features to use in creating focal points, capturing one's attention on a number of levels. But on a purely visual level, many homeowners prefer yard art. Quality is obviously a consideration here. Tastes, however, will vary. You will see both tasteful garden art and far-out garden art used as focal points.
When using plants as focal points, color is a powerful weapon. The effect of so-called "warm" colors is explained in this article on color theory. But do not underestimate the value of working with plant form and texture, too (to compose an eye-popping contrast, for example). Combine all three properties, and you may get a scene that will turn heads.
Other examples of the concept are supplied in this photo gallery dealing with focal points and other visual cues in landscape design.
When Intended Focal Points Do Not Work
When a landscape plan is a jumble of accents (each installed with the intention of creating a focal point), the purpose of creating a focal point is defeated. By definition, focalization involves discrimination. You will not draw much attention to A if you have indiscriminately set up B, C, and D as competition. The human eye can dole out only so much attention at any one time.
Sometimes, an intended focal point does not work not because it fails to draw attention, but because it attracts too much (or the wrong kind of) attention. For example, a tree that is not in-scale with a home adjacent to it will attract excessive attention. We certainly do notice the tree, but -- in so doing -- we also notice how small the house is (which is not a property feature to which one normally wishes to call attention).