A planting bed should always be made large enough to accommodate
plants when they reach mature size. A bed also should big enough so you
can get inside to do routine maintenance.
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.
