Height lines should be broken up by letting some tall plants extend into the medium height groups, with a few recessed clumps or drifts leading the eye back into the border. This gives a more natural effect than a step profile. Try to vary heights, but in general, keep taller plants in the back and shorter ones toward the front. The distance between plants in a flower border depends on the form of the individual plants and the effect which is desired in the landscape. Allow adequate space between plants.
Many gardeners crowd their plants too much.
As a rule, the tall, spired-type flowers such as holly hock, gladiolus, and rocket snapdragons which are trained to a very few stems, should be spaced about as far apart as their mature height. Tall bushy plants may be spaced about as far apart as their mature height. Rounded, bushy annuals and perennials should be spaced about as far apart as their mature height. Creeping, groundcover-type plants may be spaced about twice as far apart as their mature height. In all cases, if a solid mass of plants is desired, the spacing may be reduced.