Flower Bed Dimensions
A bed that is exclusively a flower border containing only perennials and bulbs can be narrow -- perhaps only two to three feet wide, and as long as you want. This allows you to maintain the bed from the lawn or hardscape bordering the bed. If a bed becomes much wider, adding spaces of open mulch or stepping stones will make maintenance easier over the long run.
A planting bed that consists of trees, shrubs, perennials and ground covers can be much wider, but you still need to provide open spaces for maintenance. In this type of bed, plants typically are arranged with taller plants in the rear and lower growing plants in front.
If a deciduous tree is placed in a planting bed, the bed needs to extend only about two to four feet beyond the trunk. That protects the tree from lawn mowers and creates an interesting transition area between the tree and the lawn. Planting beds for evergreen trees such as spruce and some pines need to be large enough to encompass the mature diameter of lower branches.
It's best to place any tree or large shrub off-center in a bed. This creates a natural look. Larger or more vertical plants over six feet tall can be balanced by lower or more horizontal plants under four feet tall. A good choice for lower or more horizontal plants include shrubs with a rounded shape or masses of perennials.
Adapted from an article
Paintings by Pio Carlone