Theory provides an understanding of what we do. When a person speaks or writes with the explanation or idea behind something, that idea is the ‘theory’. Freud thought that all human behavior could be explained in what he called the ‘pleasure principle’. That was his theory.
In landscape design you want someone who is both theoretical and practical.
Nineteenth century Philadelphia nurseryman Thomas Meehan said that in the January 1861 issue of his magazine Gardener’s Monthly: “The true landscape-gardener must be a gardener, practically and theoretically.” The term ‘landscape gardener’ was an early English expression for landscape designer.
This, of course, is an area of debate in many fields, where one questions an explanation from someone who has ‘no experience’ in the issue under consideration. He is simply an ‘armchair gardener’, or ‘gentleman farmer’, which referred to someone who had gardeners take care of the work of gardening, even though that person might lay out the garden.
Let me give an example closer to home.
Your garden’s overall design displays a certain inspiration, idea, style, or fashion. You express that design in your choice of plants and arrangement of other materials. You can discuss or explain that design to someone visiting your garden. Thus you could be a landscape gardener both practically and theoretically.
Finally, if you hire a garden designer, you want someone who knows about plants and the art of garden design.
Perhaps that is why when I visit a garden on a tour, I prefer to have the gardener there who can answer questions and explain issues of garden design.