The seven principles of landscape design are unity, balance, proportion, rhythm, contrast, emphasis, and simplicity. Unity means the parts read as one garden. Balance distributes visual weight, either symmetrically or loosely. Proportion keeps plants and structures sized to the house and lot. Rhythm comes from repeating plants or shapes, and contrast keeps that repetition from going flat. Emphasis gives the eye a focal point, like a specimen tree or a fire pit, and simplicity limits the palette so nothing competes.
The principles matter most when something feels off. A bed that looks busy usually fails simplicity; a yard that feels random usually fails rhythm or unity.
An easy way to test them on your own lot is to generate a few styled concepts with AI landscape design and compare which ones hold together.