The process of blending a number of flowering plants together to form a naturalistic tapestry that changes overtime is sometimes referred to as "intermingling."
This approach is similar to creating a prairie planting where the plants grow together without any defined edges or structure. It promotes a softer effect because as the plants grow they weave together. However, the heights also need to be similiar, otherwise short plants can be engulfed.
Growth habits need to be considered too, as a reluctant grower placed next to an overly aggressive one would quickly be overwhelmed.
Always start an intermingled garden by considering the characteristics of the site. Then be sure to choose plants that will thrive under those conditions. If, for example, water stands on your site after rain, you will need to research the types of plants that can endure wet feet.
For example, Physostegia, commonly called obedient plant, is one and it has either white or lavender summer blooms and likes moist sites where it will spread faster. Another is Siberian iris that has purple or white blooms in early spring.
While intermingled gardens are not high maintenance, some intervention on the part of the gardener is needed, especially in the early stages of the planting. Edit out all weeds and grass that detract from the plant groupings.