A principle of all art is unity. This means that each piece of art has one meaning. Creating landscaping or a flower garden should also incorporate this principle of unity. Your landscaping or flower garden is truly a work of art as much as any painting or sculpture.
Each work of art, large or small, should have one and only one meaning. Every part of the work of art, or in our case flower garden or landscaping, should contribute to this central or unifying meaning. Stated in physical terms, all parts of the landscaping should make one body of work with each part being subordinate to the whole. Every part which does not support in this effort must be removed, no matter how beautiful or interesting it may be in itself or so the principle of unity declares.
Unity taken to the extreme is monotony. When only one color, only one tone, or only one figure is used, the result may be monotonous. A garden planted with nothing but hollyhocks or a park with only red cedars would certainly be monotonous however beautiful the hollyhocks or cedars. Even so some believe it is better to have monotony than to lose unity. Indeed, in particular cases, monotony may be considered the height of artistic achievement.
The elimination of diverse colors, and forms, would tend towards unity. At least it would tend towards simplicity, which can be desirable in itself. Nearly all landscaping or gardening, in the hands of amateurs, shows too many forms, too many colors, and too many different kinds of plants. There are so many different kinds of plants and colors, you want to try them all. Fact is, the instances in which unity goes too far towards monotony are so rare, that these instances can be doubly interesting as curiosities.
Unity is frequently found through the development of functionality. If every part of the landscaping or flower garden has a definite and obvious practical pupose, then that dominating purpose can unify all the parts and pieces. Thus you can achieve unity.
In actual practice, the most powerful way to create unity lies in having a clear-cut motive and sticking to it. Essentially we mean, find a theme or subject for your landscaping or flower garden. Stick to it however tempting it is to not do so. In this way you can incorporate the principle of unity.