Do you want the legal answer or the touchy feely answer? Legal answer... in the US...
States with practice acts (45) require a license to practice landscape architecture.
States with title acts (4) allow anyone to practice landscape architecture, regardless of their qualifications, but only those with a license may use the title “landscape architect” or advertise for “landscape architectural” services.
You'll be getting a lot of generalizations with this question, including this one. Landscape Archiects tend to have a different and much broader education beyond that of a landscape designer. You can be good at gardening with no formal education and be a landscape designer. If you become college educated in landscape design and especially the plants, you are an ornamental horticulturist. Landscape Architects are usually educated in a HUGE range of subjects; planning, urban design, architecture, horticulture and botany, ecology, soils and geology, and especially civil and structural engineering. There is also a difference in scale. Landscape design tends to be site specific, where LA can be regional or even super-regional in its scale, depening on the project and the focus of the practioner. Then there is the "backwards compatibility;" A Landscape Architect can be a landscape designer, but a landscape designer cannot be a Landscape Architect. Plus LAs charge more (jk).
I agree that much of the two allied professions overlap. I think many individual LDs are well qualified to approach topics of drainage, grading, sustainable planning, site design, specifying materials, lighting, planting, and many of things within in the LAs turf. In general it does seem that LD work focuses on residential and light commercial design excluding regulated site engineering.
So again I would have to agree that legally the license issue is the primary factor that defines what the two professions can and cannot do.
I've know of LD's that are extremely successful,however, they don't have the background in drainage, Grading and specifying materials, So what they do to compensate is hire Landscape Architects to remedy those areas they will or cannot do. My 2 Cents worth.
Boy I would disagree with this. They may have a strong knowledge of planting and micro drainage but not large scale at all. They have limited exposure to site grading and the other engineering elements that LA's are exposed too and really I consider them a subset of LA practice.
All landscape architects are landscape designers. Some landscape designers are landscape architects. Landscape architecture is an extremely diverse field and since landscape architecture is a portion of landscape design, landscape design is an even more diverse field.
Where does Residential Landscape Design fit into this mix, and is it viewed as a legitimate subset of the broader picture? Though many Landscape Architects do Residential Landscape Design, and some specialize in that market, where does the 1,000SF-5,000SF project fit in, and where do the practitioners who serve that market fit in to the definition of Landscape Architecture and Landscape Design?
And I'm a fan of non-excluders! Seems to me that the profession, however one defines it, has an essential mission ahead. Water Efficiency, Storm Water Management, appropriate (non-invasive) planting, prevention of light pollution, education of our clients... All of these are required by owners of smaller residential properties, and the exclusion of those who call themselves Landscape Designers would be a disservice to the public. My opinion, as well.