Transition is the incorporation into your garden design planning process of gradual change between one area and another. This is achieved by arranging design elements that have varying qualities; textures, form, or size in a defined way across a design scheme;
Columnar plants through globular and onto shrubs that are prostrate.
Dark green through mid green to light green while simultaneously moving from dense planting to intermediate to light planting.
Changing the colour of the scheme for instance from red via blue to purple as you move simultaneously around the colour wheel and along the herbaceous border.
Texture, colour, size and many other objects and qualities can be employed in this regard. The combinations are virtually endless.
Hard landscape elements such as enclosure and circulation surfaces will play an important role in achieving gradual change between one part of a space and another. Treat transition as a three dimensional skill, not simply something that happens on the visual surface of your scheme. In three dimensions you can employ it to good effect in the height, length and depth of your scheme.
This is used to avoid any dramatic changes in your landscape and it allows the eye to flow smoothly from one area to another. This gradual change can be created in the main using plant heights and colour. An example of a simple transition is known as the Stair Step, which progresses, from the back of your scheme to the front, from large trees to medium trees to shrubs, to bedding or ground-cover plants. This, like everything else, you can experiment with in order to be creative and not to rely on the old reliables.
The use of transitions helps to create illusions in your landscape. A transition from taller to shorter plants gives an illusion of distance and depth making the garden appear far larger than it actually is.
The photograph above taken in the gardens of the Impressionist Museum at Giverny, France shows the transition from the surrounding landscape to the garden.