Posted: 25/09/2015 at 18:50
Difficult and silly to give exact measurements really as it depends on garden size and compatability
however, narrow borders result in straight or formal dot plants. Plants do not grow as well either.
a wide border gives depth of planting, a sense of lushness and tiers of Plants. My borders range from 8' to more than 20'. Plants an be grown big and out of season varieties can be obscured.
( assuming you mean width of border rarher than depth of soil)