A good example of both hardscaping and plant material is the above picture which shows a red twig dogwood in front of a hunter green split rail fence with a chain link fence to the left of the shrub. This is structure (the fences and the shrub) but then you look at the shape and feeling the fence and shrub present to the gardener and you get the feel for the form. The fence kind of gives a solid appearance while providing us a backdrop for the plant material. The plant material is a deciduous shrub which has a strange reaching spidery type of form. One might think the shrub is reaching out to you. The two together make the form of this particular winter vignette.
Form can also be the straight trunks of the deciduous trees. Not only are the trees structure, but they are in a straight and linear which is the form; which makes the viewer want to look up and down. Therefore, form is different from structure in that structure tells the gardener there is something there, and form tells the gardener how to feel or where to look. These tree trunks break up the horizon and keep the gardener's eyes within the garden. This is form. If these trees were all Christmas tree shaped fir trees, the gardener would have an entirely different feeling. It would seem form and structure are the same, but they truly are not. The structure is the backbone and basis for the garden, whereas the form tells us what kind of garden we designed.
Lastly, form can be just the plant and shape of the plant itself. This PeeGee hydrangea is a deciduous shrub just like the red twig dogwood above. But this hydrangea, while spidery and reaching like the red twig, has a totally different feel than the red twig dogwood. The red twig seems to be lighter and airier and gives the gardener a sense of space. The hydrangea would seem to give the gardener (me) a sense of messiness since the stems are all intertwined and seem in need of organizing. The form of the plants is what sets the mood of the garden. It is how the structure is presented which makes form important in all gardens, no matter what the season.
in the garden....