Rhythm is usually understood to mean the variation of the length and accentuation of a series of sounds, or in this instance, visual events. A musical rhythm creates a constancy that unites a piece of music, and a visual rhythm in your garden design creates a visual constancy that unites your design scheme.
Components in you scheme lead to a visual motion being set up which instills in the viewer a sense of satisfaction with the ease with which certain elements of the design are repeated. When this repetition gives structure and lends a beat to the design, then a rhythmm has been established.
Colour, line, form and shape can all be employed as part of a repetitive pattern in order to establish an underlying rhythmic structure.
A good starting point for setting up your rhythm is to identify some elements which are in some way opposite to each other. Identify plants which are light and dark, colours which are opposite each other on the colour wheel, hard landscape elements which are large and small. Once you have done this then arrange some of these in you design plan in a way that they repeat at a frequency that you decide. Perhaps when you look along a herbaceous border you may decide to use different yellow flowering plants located at regular intervals to establish your rhythm.
If you are lucky enough to have a view of the wider landscape then simply repeating an element that occurs there, such as a type of tree, will unite your garden with the external view.
Like most of us you probably live in the city, or don’t have views of the external landscape. You can introduce rhythm to your garden in this event by using your planting, enclosure, hard landscape material or even your garden furniture. Simply by repeating larger elements of furniture with smaller elements, dining table and chairs with some bird feeders and rose arches, you can establish a pleasing rhythm.
Rhythm can be the guiding factor in terms of how you set out your design and once employed will prevent you from getting into all kinds of muddles as you work through what you want from your garden.