Form and line are closely related. Form can be defined as the external appearance of a clearly defined area, as distinguished from colour form or material. Line is considered as the outline or edge of the object while form is more encompassing. Line is by it’s nature two dimensional, however what it defines will have form which exists in three dimensions, height, width and depth.
Plant form represents the basic shape and structure of the plant. The main plant forms that you will come across are upright, oval, columnar, spreading, broad spreading, and weeping.
Different forms can create different effects within the garden. For example, columnar plants create a pillar effect, drawing the eye upwards to the sky, to a building or to high mountains in the wider landscape. Oval plants with their more rounded form are grounded and bring the eye downwards. They will help you to focus attention on whatever is right beside them. Spreading plants are a great way visually ground a scheme, whether using ground-cover or larger spreading shrubs or trees.
Structures within the garden also have form and should be considered when you are designing. Existing structures include garden sheds and greenhouses, walls, arbours, and trellises as well as the house itself. When looking at the form of a structure you should take into consideration the style of the building, its construction and the arrangement of parts within the structure.
The form of your house will sometimes help you to determine what you should be doing in your garden design. If, for instance your house is part of a terrace, you might like to distinguish it from its neighbours by planting a shrub that echos the height or width of the front elevation. If your house is very tall, then sometimes a somewhat taller, broad spreading tree can be a great way to integrated it with the local landscape.
Get yourself into the habit of looking around at how plant and other landscape forms behave in nature. You will soon develop an instinct for seeing which forms and shapes work well together.