One really needn’t be a landscape gardener to lay out their home grounds with success. Let’s consider, for a moment, the similarities between indoor and outdoor decorative principles. Imagine a garden bed planted in bright colors — perhaps in reds and yellows — it’s cheerful, to be sure, but wouldn’t it be a little overpowering in the hot days of July or August? A room treated in the same way would give the same result. Again, doesn’t the tiny plot that is overpowered by a large statue have much in common with the small living room into which a set of overstuffed furniture has been crowded? And doesn’t a lawn, where a long, straight path is flanked on one side by grass and on the other by an elaborate arrangement of flower beds, statues, and fountains, have the top heavy effect of a room where most of the larger pieces of furniture are crowded together on one side? In these three instances, to look no further, are the very principles of color, scale, and balance that confront us indoors.
Balance
Balance is a familiar term in indoor decoration. It is equally important in the garden. It depends upon the axis. The axis, like the equator, is an imaginary line; it will probably run at right angles to the side of the house overlooking the garden, extending the length of your property. The design should work outwards from it, drawing the other parts of the garden to it as a center.
In the words of a leading landscape architect, it is a line of vision along which features of the design are centrally arranged. It is developed on the most important outlook from the house, perhaps from the terrace. The axis is often emphasized by a path, but this is not necessarily the case. In looking over the garden you will instinctively feel that the balance is correct if the plan is, roughly speaking, the same on both sides of the axis. Like the arrangement of the mantel shelf indoors, the balance may be symmetrical or asymmetrical.
That is, the design may be balanced exactly the same on both sides, as when a mantel is balanced on each end by two similar vases. Or — and this, while much more difficult to do, is also much more interesting — asymmetrical or occult balance may be used. In this case the features on one side of the axis occupy about the same space and have about the same decorative values as those on the other, though they do not resemble them in other respects. This would be the case when for the vase at one end of the mantel we substitute say, a figurine with a small framed picture behind it, which will give practically the same displacement as the vase which it balances.
Good balance in the garden is based upon constant use of the axis in design, and will go very far towards making the layout into an effective whole.
Scale
Scale is another term borrowed from interior decoration. That is, that the contents of a room should fit their surroundings. Outdoors, however, the scale is determined not only by the size of the plot, but also by its relation to human beings. The scale of a path, for instance, does not mean a width which would look well, and nothing more, in a certain area. It means a path wide enough (as, indeed, all paths should be) for two persons to walk side by side or to pass each other.
Color
There are first the primary hues — red, yellow, and blue, and the secondary ones — orange, green, and violet, which are known respectively as their complements — that is, the hues which harmonize best with them.
You’ll note that the complement of any hue is the secondary color that is made of the other two primary colors, for example, green (blue and yellow) being complementary to red, violet (red and blue) to yellow, and orange (red and yellow) to blue.
We may of course, have a garden in a single color (for example, a garden of red flowers, which could include all the varieties of red from pastel pink to dark rose). If we prefer to use two hues one color should be the complement of the other.
As a starting point, what is already growing, or what would look good with the house? Are plants already in the ground producing blooms of a certain color range, or every color known to plantdom? If the latter, consider doing some transplanting to place like colors (example: reds and pinks) together. This can be done without following a rigid geometric plan if irregularly-shaped masses of color would  be preferable.
For each group of flowers with similar color, identify the complementary color. Then use a few flowers of that complementary color as accents around that group. For example, with a group of red flowers, consider green foliage plants such as ferns, herbs, ornamental grass, even lawn grass. If adjoining color groups clash, consider dotting some white-flowered plants between the groups.
In a color combination one or the other hue should definitely preponderate. No room — or garden — looks well in which the color scheme is on a 50 – 50 basis. Another consideration in the garden, as it is in the house, is the fact that bright colors seem to push in upon you and to crowd. Pale, delicate tints give a feeling of distance. That’s the reason you should use pale colors in a small room. You’ll find that in the garden apparent distance may be added by the use of this same idea. The boundaries of your plot will seem further off if they are accented in dim, receding colors.
Color Design in Gardening Can Be Overdone
Though schemes based entirely on color combinations have been used extensively, their popularity is declining because they limit the garden to a few flowers of definite hues and exclude others which the owner would really like to include. It is also apt to result in bare spots in the beds when this flower or that is passed. It is, therefore, wisest to confine the use of color theory to harmonizing beds which are close together, or for some special corner of the garden, rather than to use it for the entire planting.
Courtesy of Amelia Leavitt Hill